McMurrich1914 Chapter 11
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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CHAPTER XI THE DEVELOPMENT OF THE PERICARDIUM, THE PLEURO-PERITONEUM AND THE DIAPHRAGM
It has been seen (p. 229) that the heart makes its appearance at a stage when the greater portion of the ventral surface of the intestine is still open to the yolk-sac. "The ventral mesoderm splits to form the somatic and splanchnic layers and the heart develops as a fold in the latter on each side of the median line, projecting into the ccelomic cavity enclosed by the two layers (Fig. 136, A). As the constriction of the anterior part of the embryo proceeds the two heart folds are brought nearer together and later meet, so that the heart becomes a cylindrical structure lying in the median line of the body and is suspended in the ccelom by a ventral band, the ventral tnesocardium, composed of two layers of splanchnic mesoderm which extend to it from the ventral wall of the body, and by a similar band, the dorsal tnesocardium, which unites it with the splanchnic mesoderm surrounding the digestive tract. The ventral mesocardium soon disappears (Fig. 136 C) and the dorsal one also vanishes somewhat later, so that the heart comes to lie freely in the ccelomic cavity, except for the connections which it makes with the body-walls by the vessels which enter and arise from it.
The ccelomic cavity of the embryo does not at first communicate with the extra-embryonic ccelom, which is formed at a very early period (see p. 67), but later when the splitting of the embryonic mesoderm takes place the two cavities become continuous behind the heart, but not anteriorly, since the ventral wall of the body is formed in the heart region before the union can take place. It is possible, therefore, to recognize two portions in the embryonic ccelom, an anterior one, the parietal cavity (His), which is never connected laterally with the extra-embryonic cavity, and a posterior one, the trunk cavity, which is so connected.^The heart is situated 316
THE PERICARDIUM AND PLEURO-PERITONEUM
in the parietal cavity, a considerable portion of which is destined to become the pericardial cavity.
Since the parietal cavity lies immediately anterior to the still wide yolk-stalk, as may be seen from the position of the heart in the embryo shown in Fig. 53, it is bounded posteriorly by the yolkstalk. This boundary is complete, however, only in the median line, the cavity being continuous on either side of the yolkstalk with the trunk-cavity by passages which have been termed the recessus parietales (Fig. 193, Bp and Rca). Passing forward toward the heart in the splanchnic mesoderm which surrounds the yolkstalk are the large vitelline veins, one on either side, and these shortly become so large as to bring the splanchnic mesoderm in which they lie in contact with the somatic mesoderm which forms the lateral wall of each recess. Fusion of the two layers of mesoderm along the course of the veins now takes place, and each recess thus becomes divided into two parallel passages, which have been termed the dorsal (Fig. 194, rpd) and ventral irpv) parietal recesses. Later the two veins fuse in the upper portion of their course to form the beginning of the sinus venosus, with the result that the ventral recesses become closed below and their continuity with the trunkcavity is interrupted, so that they form two blind pouches extending downward a short distance from the ventral portion of the floor of the parietal cavity. The dorsal recesses, however, retain their continuity with the trunk-cavity until a much later period.
Fig. 1 93 . - Reconstruction of a Rabbit Embryo of Eight Days, with the Pericardial Cavity Laid Open.
A, Auricle; Aob, aortic bulb; A. V., atrio- ventricular communication; Bp, ventral parietal recess; Om, vitelline vein; Pc, pericardial cavity; Rca, dorsal parietal recess; Sv, sinus venosus; V, ventricle. - (His.)
THE PERICARDIUM AND PLEURO-PERITONEUM
By the fusion of the vitelline veins mentioned above, there is formed a thick semilunar fold which projects horizontally into the ccelom from the ventral wall of the body and forms the floor of the ventral part of the parietal recess. This is known as the septum transversum, and besides containing the anterior portions of the vitelline veins, it also furnishes a passage by which the ductus Cuvieri, formed by the union of the jugular and cardinal veins, reach the heart. Its dorsal edge is continuous in the median line with the mesoderm surrounding the digestive tract just opposite the region where the liver outgrowth will form, but laterally this edge is free and forms the ventral walls of the dorsal parietal recess. An idea of the relations of the septum at this stage may be obtained
Fig. 194. - Transverse Sections of a Rabbit Embryo showing the Division of the Parietal Recesses by the Vitelline Veins.
am, Amnion; rp, parietal recess; rpd and rpv, dorsal and ventral divisions of the parietal recess; vom, vitelline vein. - (Ravn.) from Fig 195, which represents the anterior surface of the septum, together with the related parts, in a rabbit embryo of nine days.
The Separation of the Pericardial Cavity. - The septum transversum is at first almost horizontal, but later it becomes decidedly oblique in position, a change associated with the backward movement of the heart. As the closure of the ventral wall of the body extends posteriorly the ventral edge of the septum gradually slips downward upon it, while the dorsal edge is held in its former position by its attachment to the wall of the digestive tract and the ductus Cuvieri. The anterior surface of the septum thus comes to
THE PERICARDIUM AND PLEURO-PERITONEUM
3 J 9
look ventrally as well as forward, and the parietal cavity, having taken up into itself the blind pouches which represented the ventral recesses, comes to lie to a large extent ventral to the posterior recesses. As may be seen from Fig. 195, the ductus Cuvieri, as they bend from the lateral walls of the body into the free edges of the septum, form a marked projection which diminishes considerably the opening of the dorsal recesses into the parietal cavity. In later stages
Fig. 195. - Reconstruction from a Rabbit Embryo of Nine Days showing the Septum Transversum from Above.
am, Amnion; at, atrium; dc, ductus Cuvieri; rpd, dorsal parietal recess. - (Ravn.) this projection increases and from its dorsal edge a fold, which may be regarded as a continuation of the free edge of the septum, projects into the upper portions of the recesses and eventually fuses with the median portion of the septum attached to the wall of the gut. In this way the parietal cavity becomes a completely closed sac, and is henceforward known as the pericardial cavity, the original ccelom
3 2 Â°
being now divided into two portions, (i) the pericardial, and (2) the pleuro -peritoneal cavities, the latter consisting of the abdominal ccelom together with the two dorsal parietal recesses which have been separated from the pericardial (parietal) cavity and are destined to be converted into the pleural cavities.
The Formation of the Diaphragm. - It is to be remembered that the attachment of the transverse septum to the ventral wall of the digestive tract is opposite the point where the liver outgrowth develops. When, therefore, the outgrowth appears, it pushes its
Fig. 196, - Diagrams of (A) a Sagittal Section of an Embryo showing the Liver Enclosed within the Septum Transversum; (B) a Frontal Section of the Same; (C) a Frontal Section of a Later Stage when the Liver has Separated from the Diaphragm.
All, Allantois; CI, cloaca; D, diaphragm ;Li, liver;Ls, falciform ligament of the liver; M, mesentery; Mg, mesogastrium; Pc, pericardium; S, stomach;5T, septum transversum; U, umbilicus.
way into the substance of the septum, which thus acquires a very considerable thickness, especially toward its dorsal edge, and it furthermore becomes differentiated into two layers, an upper one, which forms the floor of the ventral portion of the pericardial cavity and encloses the Cuvierian ducts, and a lower one which contains the liver. The upper layer is comparatively thin, while the lower forms the greater part of the thickness of the septum, its posterior surface meeting the ventral wall of the abdomen at the level of the anterior margin of the umbilicus (Fig. 196, A).
THE DIAPHRAGM 32 1 In later stages of development the layer containing the liver becomes separated from the upper layer by two grooves which, appearing at the sides and ventrally immediately over the liver (Fig. 196, B), gradually deepen toward the median line and dorsally. These grooves do not, however, quite reach the median line, a portion of the lower layer of the septum being left in this region as a fold, situated in the sagittal plane of the body and attached above to the posterior surface of the upper layer and below to the anterior surface of the liver, beyond which it is continued down the ventral wall of the abdomen to the umbilicus (Fig. 196, C,Ls). This is the falciform ligament of the liver of adult anatomy, and in the free edge of its prolongation down the ventral wall of the abdomen the umbilical vein passes to the under surface of the liver, while the free edge of that portion which lies between the liver and the digestive tract contains the vitelline (portal) vein, the common bile-duct, and the hepatic artery. The diagram given in Fig. 196 will, it is hoped, make clear the mode of formation and the relation of this fold, which, in its entirety, constitutes what is sometimes termed the ventral mesentery.
And not only do the grooves fail to unite in the median line, but they also fail to completely separate the liver from the upper layer of the septum dorsally, the portion of the lower layer which persists in this region forming the coronary ligament of the liver. The portion of the lower layer which forms the roof of the grooves becomes the layer of peritoneum covering the posterior surface of the upper layer (which represents the diaphragm), while the portion which remains connected with the liver constitutes its peritoneal investment.
I In the meantime changes have been taking place in the upper layer of the septum. As the rotation of the heart occurs, so that its atrial portion comes to lie anterior to the ventricle, the Cuvierian ducts are drawn away from the septum and penetrate the posterior wall of the pericardium, the separation being assisted by the continued descent of the attachment of the edge of the septum to the ventral wall of the body. During the descent, when the upper
layer of the septum has reached the level of the fourth cervical segment, portions of the myotomes of that segment become prolonged into it and the layer assumes the characteristics of the diaphragm, the supply of whose musculature from the fourth cervical nerves is thus explained.
The Pleurce. - The diaphragm is as yet, however, incomplete dors ally, where the dorsal parietal recesses are still in continuity with the trunk-cavity. With the increase in thickness of the septum transversum, these recesses have acquired a considerable length antero-posteriorly, and into their upper portions the outgrowths from the lower part of the pharynx which form the lungs (see page 331) begin to project. The recesses thus become transformed into the pleural cavities, and as the diaphragm continues to descend, slipping down the ventral wall of the body and drawing with it the pericardial cavity, the latter comes to lie entirely ventral to the pleural cavities. The free borders of the diaphragm, which now form the ventral boundaries of the openings by which the pleural and peritoneal cavities communicate, begin to approach the dorsal wall of the body, with which they finally unite and so complete the separation of the cavities. The pleural cavities continue to enlarge after their separation and, extending laterally, pass between the pericardium and the lateral walls of the body until they finally almost completely surround the pericardium. The intervals between the two pleurae form what are termed the mediastina.
The downward movement of the septum transversum extends through a very considerable interval, which may be appreciated from the diagram shown in Fig. 197. From this it may be seen that in early embryos the septum is situated just in front of the first cervical segment and that it lies very obliquely, its free edge being decidedly posterior to its ventral attachment. When the downward displacement occurs, the ventral edge at first moves more rapidly than the dorsal, and soon comes to lie at a much lower level. The backward movement continues throughout the entire length of the cervical and thoracic regions, and when the level of the tenth thoracic segment is reached the separation of the pleural and peritoneal
3 2 3
cavities is completed, and then the dorsal edge begins to descend more rapidly than the ventral, so that the diaphragm again becomes oblique in the same sense as in the beginning, a position which it retains in the adult.
The Development of the Peritoneum. - The peritoneal cavity is developed from the trunk-cavity of early stages and is at first in free communication on all sides of theyolk-stalk with the extra-embryonic ccelom. As the ventral wall of the body develops the two cavities become more and more separated, and with the formation of the umbilical cord the separation is complete. Along the middorsal line of the body the archenteron forms a projection into the cavity and later moves further out from the body-wall into the cavity, pushing in front of it the peritoneum, which thus comes to surround the intestine, forming its serous coat, and from it is continued back to the dorsal body- wall forming the mesentery.
It has already been seen that on the separation of the liver from the septum transversum, the tissue of the latter gives rise to the peritoneal covering of the liver and of the posterior surface of the diaphragm, and also to the ventral mesentery. When the separation is taking place, the rotation of the stomach already described (p. 301) occurs, with the result that the portion of the ventral mesentery which stretches between the lesser curvature of the stomach and the liver shares in the rotation and comes to lie in a plane practically at right angles with that of the suspensory ligament, its surfaces looking dorsally and ventrally and its free edge being directed toward the right. This portion of the ventral mesentery forms
Fig. 197. - Diagram showing the Position of the Diaphragm in Embryos of Different Ages. - (M all.)
324 THE PERITONEUM what is termed the lesser omentum, and between it and the dorsal surface of the stomach as the ventral boundaries, and the dorsal wall of the abdominal cavity dorsally, there is a cavity, whose floor is formed by the dorsal mesentery of the stomach, the mesogastrium, the roof by the under surface of the left half of the liver, while to the right it communicates with the general peritoneal cavity dorsal to the free edge of the lesser omentum. This cavity is known as the bursa omentalis (lesser sac of the peritoneum), and the opening into it from the general cavity or greater sac is termed the epiploic foramen (foramen of Winslow). Later, the floor of the lesser sac is drawn downward to form a broad sheet of peritoneum lying ventral to the coils of the small intestine and consisting of four layers; this represents the great omentum of adult anatomy (Fig. 201).
Although the form assumed by the bursa omentalis is associated with the rotation of the stomach, it seems probable that its real origin is independent of that process (Broman). The subserous tissue of the transverse septum is at first thick and includes not only the liver, but also the pancreas and the portion of the digestive tract which becomes the stomach and the upper part of the duodenum (Fig. 196, A). The shrinkage of this tissue by which these organs become separated from the septum cannot take place evenly on account of the relations which the organs bear to one another, so that on the right side certain peritoneal recesses are formed, one between the right lung and the stomach, a second between the liver and the stomach, and a third between the pancreas and the same structure. In man these three recesses communicate with one another to form the primary bursa omentalis, and open by a common epiploic foramen into the general peritoneal cavity. The rotation of the stomach, which takes place later, merely serves to modify the original bursa.
In the human embryo a small recess also forms upon the left side between the left lung and the stomach. Later it separates from the rest of the bursa omentalis and passes up along the side of the oesophagus, coming to lie on its right side between it and the diaphragm. It gives rise to a small serous sac that lies beneath the infracardial lobe of the right
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lung, when this is present, and hence has been termed the infracardial bursa.
Below the level of the upper part of the duodenum the ventral mensentery is wanting; only the dorsal mesentery occurs. So long as the intestine is a straight tube the length of the intestinal edge of this mesentery is practically equal to that of its dorsal attached edge. The intestine, however, increasing in length much more rapidly than the abdominal walls, the intestinal edge of the mesentery soon becomes very much longer than the attached edge, and when the intestine grows out into the umbilical ccelom the mesentery accompanies it (Fig. 198). As the coils of the intestine develop, the intestinal edge of the mesentery is thrown, into corresponding folds, and on the return of the intestine to the abdominal cavity the mesentery is thrown into a somewhat funnel-like form by the twisting of the intestine to form its primary loop (Fig. 199). All that portion of the mesentery which is attached to the part of the intestine which will later become the jejunum, ileum, ascending and transverse colon, is attached to the body-wall at the apex of the funnel, at a point which bryo of Six Weeks. lies to the left of the duodenum. S p%^-VoMnÂ° m ^ Up to this stage or to about the middle of the fourth month the mesentery has retained its attachment to the median line of the dorsal wall of the abdomen throughout its entire length, but later fusions of certain portions occur, whereby the original condition is greatly modified. One of the earliest of these fusions takes place at the apex of the funnel, where the portion of the mesentery which passes to the tranverse colon and arches over the duodenum fuses with the ventral surface of the latter portion of the intestine and also with the peritoneum covering the dorsal wall of the abdomen both to the right and to the left of the duodenum. In this
Fig. 198. - Diagram showing the arrangement of the Mesentery and Visceral Branches of the Abdominal Aorta in an Em
way the attachment of the transverse mesocolon takes the form of a transverse line instead of a point, and this portion of the mesenterydivides the abdominal cavity into two portions, the upper (anterior) of which contains the liver and stomach, while the lower contains the remainder of the digestive tract with the exception of the duodenum. By passing across the ventral surface of the duodenum and fusing with it, the transverse mesocolon forces that portion of the intestine against the dorsal wall of the abdomen and fixes it in that position, and its mesentery thereupon degenerates, becoming
-Diagrams Illustrating the Development of the Great Omentum and the Transverse Mesocolon.
bid, Caecum; dd, small intestine; dg, yolk-stalk; di, colon; du, duodenum; gc, greater curvature of stomach; gg, bile duct; gn, mesogastrium; k, point where the loops of the intestine cross; mc, mesocolon; md, rectum; mes, mesentery; wf, vermiform appendix.
subserous areolar tissue, the duodenum assuming the retroperitoneal position which characterizes it in the adult.
The descending colon, which on account of the width of its mesentery is at first freely movable, lies well over to the left side of the abdominal cavity, and in consequence the left layer of its mesentery lies in contact with the parietal layer of the peritoneum. A fusion of these two layers, beginning near the middle line and thence extending outward, takes place, the fused layers becoming converted into
3 2 7
connective tissue, and this portion of the colon thus loses its mesentery and becomes fixed to the abdominal wall. The process by which the fixation is accomplished may be understood from the diagrams which constitute Fig. 200. When the ascending colon is formed, its mesentery undergoes a similar fusion, and it also becomes fixed to the abdominal wall.
The fusion of the mesentery of the ascending and descending colon remains incomplete in a considerable number of cases (one-fourth to onethird of all cases examined), and in these the colons are not perfectly fixed to the abdominal wall. It may also be pointed out that the caecum and appendix, being primarily a lateral outpouching of the intestine, do
Fig. 200 - Diagrams Illustrating the Manner in Which the Fixation of the Descending Colon (C) takes Place.
not possess any true mesentery, but are completely enclosed by peritoneum. Usually a falciform fold of peritoneum may be found extending along one surface of the appendix to become continuous with the left layer of the mesentery of the ileum. This, however, is not a true mesentery, and is better spoken of as a mesenteriole.
One other fusion is still necessary before the adult condition of the mesentery is acquired. The great omentum consists of two folds of peritoneum which start from the greater curvature of the stomach and pass downward to be reflected up again to the dorsal wall of the abdomen, which they reach just anterior to (above) the line of attachment of the transverse mesocolon (Fig. 201, A). At
first the attachment of the omentum is vertical, since it represents the mesogastrium, but later, by fusion with the parietal peritoneum, it assumes a transverse direction, while at the same time the pancreas, which originally lay between the two folds of the mesogastrium, is carried dorsally and comes to have a retroperitoneal position in the line of attachment of the omentum. By this change the lower layer of the omentum is brought in contact with the upper layer of the
Fig. 201. - Diagrams showing the Development of the Great Omentum and its Fusion with the Transverse Mesocolon.
B, Bladder; c, transverse colon; d, duodenum; Li, liver; p, pancreas; R, rectum; S, stomach; U, uterus. - (After Allen Thomson.) transverse mesocolon and a fusion and degeneration of the two results (Fig. 201 B), a condition which brings it about that the omentum seems to be attached to the transverse colon and that the pancreas seems to lie in the line of attachment of the transverse mesocolon. This mesentery, as is occurs in the adult, really consists partly of a portion of the original transverse mesocolon and partly of a layer of the great omentum.
LITERATURE 329 By these various changes the line of attachment of the mesentery to the dorsal wall of the body has become somewhat complicated and has departed to a very considerable extent from its original simple vertical arrangement. If all the viscera be removed from the body of an adult and the mesentery be cut close to the line of its attachment, the course of the latter will be seen to be as follows: Descending from the under surface of the diaphragm are the lines of attachment of the suspensory ligament, which on reaching the liver spread out to become the coronary and lateral ligaments of that organ. At about the mid-dorsal line these lines become continuous with those of the mesogastriumr which curve downward toward the left and are continued into the transverse lines of the transverse mesocolon. Between these last, in a slight prolongation, there may be seen to the right the cut end of the first portion of the duodenum as it passes back to the dorsal wall of the abdomen, and at about the mid-dorsal line the cut ends of its last part become visible as it passes ventrally again to become the jejunum. From the transverse mesocolon three lines of attachment pass downward; the two lateral broad ones represent the lines of fixation of the ascending and descending colons, while the narrower median one, which curves to the right, represents the attachment of the mesentery of the small intestine other than the duodenum. Finally, from the lower end of the fixation line of the descending colon the mesentery of the sigmoid is continued downward.
The special developments of the peritoneum in connection with the genito-urinary apparatuus will be considered in Chapter XIII.
I. Broman: "Ueber die Entwicklung und Bedeutung der Mesenterial und der Korperhohlen bei den Wirbeltieren," Ergebn. der Anat. u. Entw., XV, 1906. A. Bracket: "Die Entwickelung der grossen Korperhohlen und ihre Trennung von Einander," Ergebnisse der Anat. und Eniwickelungsgesch., vn, 1898. W. His: " Mittheilungen zur Embryologie der Saugethiere und des Menschen," Archiv fur Anat. und Physiol., Anat. Abth., 1881. F. P. Mall: "Development of the Human Ccelom," Journal of Morphol., xii, 1897. F. P. Mall: "On the Development of the Human Diaphragm," Johns Hopkins Hospital Bull., xii, 1901.
330 LITERATURE E. Ravn: "Ueber die Bildung der Scheidewand zwischen Brust- und Bauchhohle in Saugethierembryonen," Archiv fur Anat. und Physiol., Anat, Abth., 1889. A. Swaen: "Recherches sur le developpement du foie, du tube digestif, de l'arriere cavite du peritoine et du mesentere," Journ. de I' Anat. et de la Physiol., xxxii, 1896; xxxni, 1897. C. Toldt: "Bau und Wachstumsveranderungen der Gekrose des menschlichen Darmkanals," Denkschr. der kais. Akad. Wissensch. Wien, Math.-Naturwiss.
Classe, xli, 1879. C. Toldt: "Die Darmgekrose und Netze im gesetzmassigen und gesetzwidrigen Zustand," Denkschr. der kais. Akad. Wissensch. Wien. Math.-Naturwiss. Classe, lvt, 1889.
F. Treves: "Lectures on the Anatomy of the Intestinal Canal and Peritoneum," British Medical Journal, I, 1885.
CHAPTER XII. THE DEVELOPMENT OF THE ORGANS OF RESPIRATION
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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The Development of the Lungs. - The first indication of the lungs and trachea is found in embryos of about 3.2 mm. in the form of a groove on the ventral surface of the oesophagus, at first extending almost the entire length of that portion of the digestive tract. As the oesophagus lengthens the lung groove remains connected with its upper portion (Fig. 182, A), and furrows which appear along the line of junction of the groove and the oesophagus gradually deepen and separate the two structures (Fig. 182, B). The separation takes place earliest at the lower end of the groove and thence extends upward, so that the groove is transformed into a cylindrical pouch lying ventral to the oesophagus and dorsal to the heart and opening with the oesophagus into the terminal portion of the pharynx.
Soon after the separation of the groove from the oesophagus its lower end becomes enlarged and bilobed, and since this lower end lies, with the oesophagus, in the median attached portion of the dorsal edge of the septum transversum, the lobes, as they enlarge, project into the dorsal parietal recesses (Fig. 202), and so become enclosed within the peritoneal lining of the recesses which later become the pleural cavities.
The lobes, which represent the lungs, do not long remain simple, 33i
Fig. 202. - Portion of a Section THROUGH AN EMBRYO OF THE FOURTH Week.
A, Aorta; DC, ductus Cuvieri; L, lung; O, oesophagus; RP, parietal recess; VOm, vitelline vein. - (Toldt.)
but bud-like processes arise from their cavities, three appearing in the right lobe and two in the left (Fig. 203, A), and as these increase in size and give rise to additional outgrowths, the structure of the lobes rapidly becomes complicated (Fig. 203, B and C).
The lower primary process on each side may be regarded as a prolongation of the bronchus, while the remaining process or processes represent lateral outgrowths from it. Considerable difference of opinion has existed as to the nature of the further branching of the bronchi, some authors regarding it as a succession of dichotomies, one branch of each of these placing itself so as to be in the line of the
Fig. 203. - Reconstruction of the Lung Outgrowths of Embryos of (/I) 4.3, (5) 8.5, and (C) 10.5 MM.
Ap, Pulmonary artery; Ep, eparterial bronchus; Vp, pulmonary vein; 7, second lateral bronchus; II, main bronchi. - (His.) original main bronchus, while the other comes to resemble a lateral outgrowth, and other observers have held that the main bronchus has an uninterrupted growth, all other branches being lateral outgrowths from it, and the branching therefore a monopodial process. The recent thorough study by Flint of the development of the lung of the pig shows that, in that form at least, the branching is a monopodial one, and that from the main bronchus as it elongates four sets of secondary outgrowths develop, namely, a strong lateral, a dorsal, a ventral, and a weak and variable medial set.
There is a general tendency for the individual branches of the various sets to be arranged in regular succession and for their development to be symmetrical in the two lungs. But on account of the necessity under which the lungs are placed of adapting themselves to the neighboring structures and at the same time affording a respiratory surface as large as possible, an amount of asymmetry supervenes. Thus, it has already been noted that in the earliest branching a single lateral bronchus is formed in the left lung and two in the right. The uppermost of these latter, the first lateral bronchus, is unrepresented in the left lung, and is peculiar in that it lies behind the right pulmonary artery (Fig. 203, C), or in the adult, after the recession of the heart, above it, whence it is termed the eparterial bronchus. Its absence on the left side is perhaps due to its suppression to permit the normal recession of the aortic arch (Flint).
So, too, the inclination of the heart causes a suppression of the second ventral bronchus in the left lung, but at the same time it affords opportunity for an excessive development of the corresponding bronchus of the right lung, which pushes its way between the heart and the diaphragm and is known as the infra-cardiac bronchus.
As soon as the unpaired first lateral bronchus and the paired second lateral bronchi are formed mesenchyme begins to collect around each of them and also around the main bronchi, the lobes of the adult lung, three in the right lung and two in the left, being thus outlined. A development of mesenchyme also takes place around the excessively developed right second ventral bronchus, and sometimes produces a well-marked infra-cardiac lobe in the right lung.
Fig. 204. - Diagram of the Final Branches of the Mammalian Bronchi.
A, Atrium; B, bronchus; S, air-sac. - (Miller.)
In later stages the various bronchi of each lobe give rise to additional branches and these again to others, and the mesenchyme of each lobe grows in between the various branches. At first the amount of mesenchyme separating the branches is comparatively great, but as the branches continue, the growth of the mesenchyme fails to keep pace with it, so that in later stages the terminal enlargements are separated from one another by only very thin partitions of mesenchyme, in which the pulmonary vessels form a dense network. The final branching of each ultimate bronchus or bronchiole results in the formation at its extremity of from three to five enlargements, the atria (Fig. 204, A), from which arise a number of air-sacs (S) whose walls are pouched out into slight diverticula, the air-cells or alveoli. Such a combination of atria, air-sacs, and air-cells constitutes a lobule, and each lung is composed of a large number of such units.
The greater part of the original pulmonary groove becomes converted into the trachea, and in the mesenchyme surrounding it the incomplete cartilaginous rings develop at about the eighth or ninth week. The cells of the epithelial lining of the trachea and bronchi remain columnar or cubical in form and become ciliated at about the fourth month, but those of the epithelium of the airsacs become greatly flattened and constitute an exceedingly thin layer of pavement epithelium. The Development of the Larynx. - The opening of the upper end of the pulmonary groove into the pharynx is situated at first just behind the fourth branchial furrow and is surrounded anteriorly and laterally by the PI -shaped ridge already described (p. 294) as
Fig. 205. - Reconstruction of the Opening into the Larynx in an Embryo of Twenty-eight Days, Seen from Behind and Above, the Dorsal Wall of the Pharynx being Cut Away.
co, Cornicular, and cu, cuneiform tubercle; Ep, epiglottis; T, unpaired portion of the tongue. - (Kallius.)
THE LARYNX 335 the furcula, this separating it from the posterior portion of the tongue (Fig. 178). The anterior portion of this ridge, which is apparently derived from the ventral portions of the third branchial arch, gradually increases in height and forms the epiglottis, while the lateral portions, which pass posteriorly into the margins of the pulmonary groove, form the ary epiglottic folds. When the pulmonary groove separates from the oesophagus, the opening of the trachea into the pharynx is somewhat slit-like and is bounded laterally by the aryepiglottic folds, whose margins present two elevations which may be termed the comicular and cuneiform tubercles (Fig. 205, co and cu, and Fig. 175). The opening is, however, for a time almost obliterated by a thickening of the epithelium covering the ridges,
Fig. 206. - Reconstruction of the Mesenchyme Condensations which Represent the Hyoid and Thyreoid Carthages in an Embryo of Forty Days.
The darkly shaded areas represent centers of chondrification. c.ma, Greater cornu of hyoid; c.mi, lesser cornu; Th, thyreoid cartilage. - (Kallius.) and it is not until the tenth or eleventh week of development that it is re-established. Later than this, at the middle of the fourth month, a linear depression makes its appearance on the mesial surface of each ary-epiglottic fold, forming the beginning of the ventricle, and although at first the depression lies horizontally, its lateral edge later bends anteriorly, so that its surfaces look outward and inward. The lips which bound the opening of the ventricle into the laryngeal cavity give rise to the ventricular and vocal folds. The cartilages of the larynx can be distinguished during the seventh week as condensations of mesenchyme which are but indistinctly separated from one another. The thyreoid cartilage is represented at this stage by two lateral plates of mesenchyme,
336 THE LARYNX separated from one another both ventrally and dorsally, and each of these plates undergoes chondrification from two separate centers (Fig. 206) . These, as they increase in size, unite together and send prolongations ventrally which meet in the mid-ventral line with the corresponding prolongations of the plates of the opposite side, so as to enclose an area of mesenchyme into which the chondrification only extends at a later period, and occasionally fails to so extend, producing what is termed a foramen thyreoideum.
The mesenchymal condensations which represent the cricoid and arytenoid cartilages are continuous, but each arytenoid has a distinct center of chondrification, while the cartilage of the cricoid appears as a single ring which is at first open dorsally and only later becomes complete. The epiglottis cartilage resembles the thyreoid in being formed by the fusion of two originally distinct cartilages, from each of which a portion separates to form the cuneiform cartilages (cartilages of Wrisberg) which produce the tubercles of the same name on the ary-epiglottic fold, while the corniculate cartilages (cartilages of Santorini) are formed by the separation of a small portion of cartilage from each arytenoid.
The formation of the thyreoid cartilage by the fusion of two pairs of lateral elements finds an explanation from the study of the comparative anatomy of the larynx. In the lowest group of the mammalia, the Monotremata, the four cartilages do not fuse together and are very evidently serially homologous with the cartilages which form the cornua of the hyoid. In other words, the thyreoid results from the fusion of the fourth and fifth branchial cartilages. The cricoid, in its development, presents such striking similarities to the cartilaginous rings of the trachea that it is probably to be regarded as the uppermost cartilage of that series, but the epiglottis seems to be a secondary chondrification in the glossolaryngeal fold (Schaffer). The arytenoids possibly represent an additional pair of branchial cartilages, such as occur in the lower vertebrates (Gegenbaur).
These last arches have undergone almost complete reduction in the mammalia, the cartilages being their only representatives, but,
LITERATURE 337 in addition to the cartilages, the fourth and fifth arches have also preserved a portion of their musculature, part of which becomes transformed into the muscles of the larynx. Since the nerve which corresponds to these arches is the vagus, the supply of the larynx is derived from that nerve, the superior laryngeal nerve probably corresponding to the fourth arch, while the inferior (recurrent) answers to the fifth.
The course of the recurrent nerve finds its explanation in the relation of the nerve to the fourth branchial artery. When the heart occupies its primary position ventral to the floor of the pharynx, the inferior laryngeal nerve passes transversely inward to the larynx beneath the fourth branchial artery. As the heart recedes the nerve is caught by the vessel and is carried back with it, the portion of the vagus between it and the superior laryngeal nerve elongating until the origins of the two laryngeal nerves are separated by the entire length of the neck. Hence it is that the right recurrent nerve bends upward behind the right subclavian artery, while the left curves beneath the arch of the aorta (see Fig. 149).
J. M. Flint: "The Development of the Lungs," Amer. Journ. Anal., vi, 1906.
J. E. Frazer: "The Development of the Larynx," Journ. Anat. and Phys., xliv, 1910.
E. Goppert: "Ueber die Herkunft der Wrisbergschen Knorpels," Morphol. Jahrbuch, xxi, 1894. W. His: "Zur Bildungsgeschichte des Lungen beim menschlichen Embryo," Archiv fiir Anat. und Physiol., Anat. Abth., 1887. E. Kallius: "Beitrage zur Entwickelungsgeschichte des Kehlkopfes," Anat. Hefle, ix, 1897. E. Kallius: "Die Entwickelung des menschlichen Kehlkopfes," Verhandl. der Anat.
Gesellsch., xii, 1898. A. Lisser: "Studies on the Development of the Human Larynx," Amer. Journ.
Anat., xii, 191 1. A. Narath: "Der Bronchialbaum der Saugethiere und des Menschen," Bibliotheca Medica, Abth. A, Heft 3, 1901. J. Schaffer: "Zur Histologie Histogenese und phylogenetischen Bedeutung der Epiglottis," Anat. Hefte, xxxin, 1907. A. Sotjlie! and E. Bardier: "Recherches sur le developpement du larynx chez l'homme," Journ. de V Anat. et de la Physiol., xxiii, 1907.
THE DEVELOPMENT OF THE URINOGENITAL SYSTEM.
The excretory and reproductive systems of organs are so closely related in^their development that they must be considered together. They both owe their origin to the mesoderm which constitutes, the intermediate cell-mass (p. 77), this, at an early period of development, becoming thickened so as to form a ridge projecting into the dorsal portion of the ccelom and forming what is known as the Wolffian ridge (Fig. 207, wr). The greater portion of the substance
6 y L m y 7
Fig. 207. - Transverse Section through the Abdominal Region of a Rabbit Embryo of 12 mm.
a, Aorta; gl., glomerulus; gr, genital ridge; m, mesentery; nc, notochord; t, tubule of mesonephros; wd, Wolffian duct; wr, Wolffian ridge. - (Mihalkovicz.) of this ridge is concerned in the development of the primary and secondary excretory organs, but on its mesial surface a second ridge appears which is destined to give rise to the ovary or testis, and hence is termed the genital ridge (gr).
The development of the excretory organs is remarkable in that three sets of organs appear in succession. The first of these, the pronephros, exists only in a rudimentary condition in the human 338
embryo, although its duct, the pronephric or Wolffian duct, undergoes complete development and plays an important part in the development of the succeeding organs of excretion and also in that of the reproductive organs. The second set, the mesonephros or Wolffian body, reaches a considerable development during embryonic life, but later, on the development of the final set, the definite kidney or metanephros, undergoes degeneration, portions only persisting as rudimentary structures associated for the most part with the reproductive organs.
The Development of the Pronephros and the Pronephric Duct. - The first portions of thppjrrejl? r y system to make their appearance are the pronephric orWolffian ducts, which develop as
Fig.^2o8. - Transverse Section through Chick Embryo of about Thirty-six Hours.
en, Endoderm; im, intermediate cell mass; ms, mesodermic somite; nc, notochord; so, somatic, and sp, splanchnic mesoderm; wd, Wolffian duct. - (Waldeyer.) outgrowths of the dorsal wallsof_t he intermedi atecejljnasses ; At first ThT outgrowths are solid cords of cells (Fig. 208, wd), but" later a lumen appears in the center of each and the canal so formed from each intermediate cell mass, bending backward at its free end, comes into contact and fuses with the canal from the next succeeding segment. Two longitudinal canals, the pronephric or Wolffian ducts, are thus formed, with which the cavities of the intermediate cell masses communicate. The formation of the ducts begins in the anterior segments before the segmentation of the posterior portions of the mesoderm has taken place, and the further backward extension of the ducts takes place independently of the formation of excretory tubules, apparently by a process of terminal growth. The free end
340 THE PRONEPHROS of each duct comes into intimate relation with the ectoderm above it, so much so that its posterior portion has been held^by some observers to be formed from that layer, but it seems more probable that the relation to the ectoderm is a secondary process and that the ducts are entirely of mesodermal origin. They reach the cloaca in embryos of a little over 4 mm., and later they unite with that organ, so that their lumina open into its cavity.
The pronephric tubules make their appearance in embryos of about 1.7 mm., while as yet there are only nine or ten mesodermic somites, and they are formed from the intermediate cell masses of the seventh to the fourteenth segment, and perhaps from those situated
Fig. 209. - Diagram showing the Structure of a Fully Developed Pronephric tubule. Ao, Aorta; Coe, ccelom; ec, Ectoderm; eg, external glomerulus; en, endoderm; Ms, mesodermic somite; iV, nervous system; n, nephrostome; nc, notochord; pc, pronephric chamber; Wd, Wolffian duct. - (Modified from Felix.) still more anteriorly. The entire series, however, is never in existence at any one time, for before the more posterior tubules are formed, those of the anterior segments have undergone degeneration. Each pronephric tubule, when fully formed, consists of a portion which unites it to the Wolffian duct, and opens at its other end into an enlargement, the pronephric chamber, (Fig. 209, pc), which, on its part opens mto the ccelomic cavity by means of a nephrostome canal. In the neighborhood of the ccelomic opening, or nephrostome, an outgrowth of the ccelomic epithelium is formed, and a branch from the aorta penetrates into this to form a stalked external glomerulus lying free in the coelomic cavity (Fig. 209, e.g.). Internal
glomeruli, such as occur in connection with the mesonephric tubules do not occur in the pronephros of the human embryo, and this fact, together with the presence of external glomeruli and the participation of the tubules in the formation of the Wolffian duct, serve to distinguish the pronephros from the mesonephros.
The pronephric tubules, are, as has been stated, transitory structures and by the time the embryo has reached a length of about 5 mm. they have all disappeared. Before their disappearance is complete, however, a second series of tubules has commenced to develop, forming what is termed the mesonephros or Wolffian body.
The Development of the Mesonephros. - The pronephric duct does not disappear with the degeneration of the pronephric tubules, but persists to serve as the duct for the mesonephros and to play an important part in the development of the metanephros also. In the Wolffian ridge there appear in embryos of between 3 and 4 mm. a number of coiled tubules, which arise by some of the cells of the ridge aggregating to form solid cords, at first entirely unconnected with either the coelomic epithelium or the Wolffian duct. Later the cords become connected with the ccelomic epithelium and acquire a lumen, and near the coelomic end of the tubule, at a region corresponding to the chamber of a pronephric tubule, a condensation of the mesenchyme of the Wolffian ridge occurs to form a glomerulus into which a branch extends from the neighboring aorta. The tubules finally acquire connection with the Wolffian duct and at the same time lose their connections with the coelomic epithelium, their nephrostomes being accordingly but transitory structures. The tubules rapidly increase in length and
Fig. 210. - Transverse Section of the Wolffian Ridge of a Chick Embryo of Three Days.
ao, Aorta; gl, glomerulus; gr, genital ridge; mes, mesentery; ml, mesonephric tubule; vc, cardinal vein; Wd, Wolffian duct. - (Mihalkovicz.)
become coiled, and the glomeruli project into their cavities, pushing in front of them the wall of the tubule so that it has the appearance represented in Fig. 210.
In its anterior portion the Wolffian ridge is formed by distinct intermediate cell masses, but posterior to the tenth segment it becomes distinguishable from the rest of the mesoderm before this has become segmented, and, failing to undergo transverse division into segments, it forms a continuous column of cells, known as the nephrogenic cord. The anterior tubules of the mesonephros make their appearance in the intermediate cell masses belonging to the sixth cervical segment, its tubules thus overlapping those of the pronephros, and from this level they appear in all succeeding segments and in the nephrogenic cord as far back as the region of the third or fourth lumbar segment, where the cord is partially interrupted. This interruption marks the dividing line between the mesonephric and metanephric portions of the cord, the portions posterior to it being destined to give rise to the metanephros. But, as is the case with the pronephros, the entire series of mesonephric tubules is never in existence at any one time, a degeneration of the anterior ones supervening even before the posterior ones have differentiated, and the degeneration proceeds to such an extent that in an embryo of about 21 mm. all the tubules of the cervical and thoracic segments have disappeared, only those of the lumbar segments persisting.
This does not mean, however, that the number of persisting tubules corresponds with that of the segments in which they occur, for the tubules are not segmental in their arrangement, but are much more numerous than such an arrangement would allow. Two, three, or even as many as nine may correspond with the extent of a mesodermic somite and when the reduction is complete in an embryo of 21 mm., where only the tubules corresponding with four or five segments remain, they may number twenty-six in each mesonephros (Felix). This arrangement of the tubules together with the size which they assume when fully developed brings it about that the Wolffian ridges become somewhat voluminous structures in their mesonephric portions, projecting markedly into the ccelomic cavity
(Fig. 211). Each is attached to the dorsal wall of the body by a distinct mesentery and has in its lateral portion, embedded in its substance, the Wolffian duct, while on its mesial surface anteriorly is the but slightly developed genital ridge (/). This condition is reached in the human embryo at about the sixth or seventh week of development, and after that period the mesonephros again begins to undergo rapid degeneration, so that at about the sixteenth week
Fig. 211. - Urinogenital Apparatus of a Male Pig Embryo of 6 cm.
ao, Aorta; b, bladder; gh, gubernaculum testis; k, kidney; md, Mullerian duct; sr, suprarenal body; t, testis; w, Wolffian body; wd, Wolffian duct. - (Mihalkovicz.)
nothing remains of it except the duct and a few small rudiments whose history will be given later.
The Development of the Metanephros. - The first indication of the metanephros or permanent kidney is a tubular outgrowth from the dorsal surface of the Wolffian duct shortly before its entrance into the cloaca (Fig. 170). When first formed this outgrowth lies lateral to the posterior portion of the Wolffian ridge,
which, as has already been noted (p. 342), is separated from the portion that gives rise to the mesonephros. This terminal portion of the ridge forms what is termed the metanephric blastema and in embryos of 7 mm. it has come into relation with the outgrowth from the Wolffian duct and covers its free extremity as a cap. Since both the blastema and the outgrowth from the Wolffian duct take part in the formation of the uriniferous tubules, these have a double origin.
The outgrowth from the Wolffian duct as it continues to elongate comes to lie dorsal to the mesonephros, carrying the cap of blastema with it, and it soon assumes a somewhat club-shaped form, its terminal enlargement or ampulla forming what may be termed the primary renal pelvis, while the remainder represents theureter. The primary renal pelvis then gives rise to from three to six, usually four, tubular outgrowths, which may be termed primary collecting tubules, and with their formation the original cap of metanephric blastema undergoes a division into as many portions as there are tubules, so that each of the latter has its own cap of blastema. As soon as each tubule has reached a certain length it begins to enlarge at its free extremity to form an ampulla, just as did the primary renal pelvis, and from this ampulla there grow out from two to four secondary collecting tubules, a further corresponding division of the metanephric blastema taking place. In their turn these secondary tubules similarly enlarge at their extremities to form ampullae (Fig. 212, A) from which tertiary collecting tubules are budded out, accompanied by a third fragmentation of the blastema and so the process goes on until about the fifth fetal month, the number of generations of collecting tubules formed being between
Fig. 212. - Diagrams of Early Stages in the Development of the Metanephric Tubules.
t, Urinary tubule; Ur, ureter; v, renal ampulla. - (Haycrqft.)
eleven and thirteen, each tubule of the final generation having its cap of blastema.
In this way there is formed a complicated branching system of tubules all of which ultimately communicate with the primary renal pelvis, and all of which have, in the last analysis, had their origin from the Wolffian duct. They represent, however, only the collecting portions of the uriniferous tubules, their excreting por
Fig. 213. - Four Stages in the Development of a Uriniferous Tubule of a Cat. A, Arched collecting tubule, C, distal convoluted tubule; C, proximal convoluted tubule; H, loop of Henle; M, glomerulus; T, renal vesicle; V, ampulla (drawn from reconstructions prepared by G. C. Huber).
tions having yet to form, and these take their origin from the metanephric blastema.
When the terminal collecting tubules have been formed the blastemic cap in connection with each one condenses to form a renal vesicle (Fig. 213, A, T), which is at first solid, but later becomes hollow and proceeds to elongate to an S-shaped tubule, one end of which becomes continuous with the neighboring ampulla (Figs. 212, B, and 213, B), and in the space enclosed by_what may be termed the lower loop of the S a collection of mesenchyme cells
346 THE METANEPHROS appears, into which branches penetrate at an early stage from the renal artery to form a glomerulus, the neighboring walls of the tubule becoming exceedingly thin and being transformed into a capsule of Bowman. The upper loop of the S now begins to elongate (Fig. 213, C), growing toward the hilus of the kidney, parallel to the branch of the outgrowth from the Wolffian duct to which it is attached and between this and the glomerulus, and forms a loop of Henle. From the portion of the horizontal limb of the S which lies between the glomerulus and the descending limb of the loop of Henle the proximal convoluted tubule (C) arises, while the distal convoluted and the arched collecting tubules (C and A) are formed from the uppermost portion of the upper loop (Fig. 213, D). The entire length of each uriniferous tubule from Bowman's capsule to the arched collecting tubule inclusive is thus derived from a renal vesicle, that is to say, from the metanephric blastema.
Since the tubules of the kidney are formed by the union of two originally distinct structures it is conceivable that in the cases of certain tubules there may be a failure of the union. The blastemic portion of the tubules would, nevertheless, continue their development and become functional and, since there would be not means of escape for the secretion, the result would be a cystic kidney. Occasionally the two blastemata of opposite sides fuse across the middle line, the result being the formation of a single transverse or horse-shoe shaped kidney, or, what is much rarer, the blastema of one side may cross the middle line to fuse with that of the other, the result being an apparently single kidney with two ureters which open normally into the bladder.
The primary renal pelvis is the first formed ampulla and does not exactly represent the definitive pelvis. This is produced partly by the enlargement of the primary pelvis and partly by the enlargement of the collecting tubules of the first four generations, those of the third and fourth generations later being taken up or absorbed into those of the second generation, so that the tubules of the fifth generation appear to open directly into those of the second, which form the calices minores, while those of the first constitute the calices majores. In some kidneys the process of reduction of the earlier formed collecting tubules proceeds a step further, those of the first generation
THE MULLERIAN DUCT 347 being taken up into the primary renal pelvis, the secondaries then forming a series of short calices arising from a single pelvic cavity.
At about the tenth week of development the surface of the human kidney becomes marked by shallow depressions into lobes, of which there are about eighteen, one corresponding to each of the groups of tubules which arise from the same renal vesicle. This lobation persists until after birth and then disappears completely, the surface of the kidney becoming smooth.
The Development of the Mullerian Duct and of the Genital Ridge. - At the time when the Wolffian body has almost reached its greatest development the Wolffian ridge is distinctly divided into three portions (Fig. 214), a median or mesonephric portion attached to the body wall, a lateral or tubal portion containing the Wolffian duct and attached to the mesonephric portion, and a genital portion, formed by the genital ridge and also attached to the mesonephric portion, but to its medial surface. In the tubal portion a second longitudinal duct, known as the Mullerian duct (Fig. 214, Md), makes its appearance. Near the anterior end of each Wolffian ridge there is formed on the free edge of the tubal portion an invagination of the peritoneal covering, and by the proliferation of the cells at its tip this invagination gradually extends backward in the substance of the tubal portion and reaches the cloaca in embryos of about 22 mm. The primary peritoneal invagination becomes the abdominal ostium of the Mullerian duct, the backward prolongation forming the duct itself.
In Fig. 214 it will be seen that the tubal portion of the left Wolffian ridge is somewhat bent inward toward the median line and in the lower parts of their extent this becomes more pronounced in both tubal portions until finally their free edges come in contact and fuse in the median line, while at the same time their lower edges fuse with the floor of the ccelomic cavity. In this way a transverse partition is formed across what will eventually be the pelvis of the adult, this cavity being thus divided into two compartments, a posterior one containing the lower portion of the intestine and an anterior one containing the bladder. With the formation of this trans
THE GENITAL RIDGE
I ! UA
Fig. 214. - Transverse Section through the Abdominal Region oe an Embryo of 25 MM. _ Ao, Aorta; B, bladder; I, intestine; L, liver; M, muscle; Md, Miillerian duct; N, spinal cord; Ov, ovary; RA, rectus abdominis; Sg, spinal ganglion; UA, umbilical artery; Ur, ureter; V, vertebra; W, Wolffian body; Wd, Wolffian duct. - (Keibel.)
THE GENITAL RIDGE 349 verse fold, which is represented by the broad ligament in the female, the Miillerian ducts of opposite sides are brought into contact and finally fuse in the lower portions of their course to form an unpaired utero-vaginal canal.
Upon the lateral surface of the mesonephric portion of the Wolffian ridge a longitudinal elevation is formed at about this time. It is the inguinal fold and on the union of the transverse fold with the floor of the ccelomic cavity it comes into contact and fuses with the lower part of the anterior abdominal wall, just lateral to the lateral border of the rectus abdominis muscle. In the substance of the fold the mesenchyme condenses to form a ligament-like cord, the inguinal ligament, whose further history will be considered later on.
The genital ridge makes its apearance as a band-like thickening of the epithelium covering the mesial surface of the Wolffian ridge (Fig. 207, gr). Later columns of cells grow down from the thickening into the substance of the Wolffian ridge, displacing the mesonephric tubules to a greater or less extent. These columns are composed of two kinds of cells: (i) smaller epithelial cells with a relatively small amount of cytoplasm and (2) large, spherical cells with more abundant and clear cytoplasm known as sex-cells. The growth of the cell-columns down into the substance of the Wolffian body does not take place, however, to an equal extent in all portions of the length of the genital ridge. Indeed, three regions may be recognized in the ridge; an anterior one in which a relatively small number of cell-columns, extending deeply into the stroma, is formed; a middle one in which numerous columns are formed; and a posterior one in which practically none are formed. The first region has been termed the rete region and its cell-columns the retecords, the second region the sex-gland region and its columns the sex-cords, and the posterior region is the mesenteric region and plays no part in the actual formation of the ovary or testis.
In the human embryo all the sex-cells seem to have their origin from the epithelium of the genital ridge, but in the lower vertebrates and also in mammals (Allen, Rubaschkin) they have been found to make their appearance in the endoderm of the digestive tract. Thence they wander
into the mesentery and some of them eventually into the peritoneum covering the mesial surface of the Wolffian ridge, where they give rise to the sex-cells found in the epithelium of the genital ridge. This origin of the sex-cells has not yet been observed in the human embryo.
The various steps in the differentiation of the reproductive organs so far described occur in all embryos, no matter what their future sex may be. The later stages, however, differ according to sex, and consequently it will be necessary to follow the further development first of the testis and then of the ovary, the changes
Fig. 215. - Section through the Testis and the Broad Ligament of the Testis of an Embryo of 5.5 mm.
ep, Epithelium; md, Miillerian duct; mo, mesorchium; re, rete-cords; sc, sex-cords; wd, Wolffian duct. - (Mihalkovicz.) that take place in the ducts and other accessory structures being reserved for a special section.
The Development of the Testis. - At about the fourth or fifth week there appears in the sex-gland region of the genital ridge a structure which serves to characterize the region as a testis. This is a layer of somewhat dense connective tissue which grows in between the epithelial and stroma layers of the sex-gland region and gradually extends around almost the entire sex-gland to form the tunica albuginea. By its development the sex-cords are separated from the
epithelium, which later becomes much flattened and eventually almost disappears. Shortly after the appearance of the albuginea the sex-cords unite to from a complicated network and the rete-cords grow backward along the line of attachment of the testis to the mesonephric portion of the Wolffian ridge, coming to lie in the hilus
â– - R
Fig. 216. - Longitudinal Section of the Ovary of an Embryo Cat of 9.4 cm. cor, Cortical layer; ep, epoophoron; Mc, medullary cords; Mn, mesonephros; pf, peritoneal fold containing Fallopian tube; R, rete; T, Fallopian tube. - (Coert, from Bilhler.) of the testis (Fig. 215). They then develop a lumen and send off branches which connect with the sex-cord reticulum and they also make connection with the glomerular portions of the tubules belonging to the anterior part of the mesonephros. Since like the sexcords, they have by this time separated from the epithelium that
352 THE OVARY gave rise to them, they now extend between the sex-cord reticulum and the anterior mesonephric tubules. Certain portions of the sex-cords now begin to break down leaving other portions to form convoluted stems which eventually become the seminiferous tubules, while from the rete-cords are formed the tubuli recti and rete testis, by which the spermatozoa are transmitted to the mesonephric tubules and so to the Wolffian duct (see p. 355).
The development of the seminiferous tubules is not, however, completed until puberty. The stems derived from the sex-cords form cylindrical cords, between which lie stroma cells and interstitial cells derived from the stroma; but until puberty these cords remain solid, a lumen developing only at that period. The cords contain the same forms of cells as were described as occurring in the epithelium of the germinal ridge, and while in the early stages transitional forms seem to occur, in later periods the two varieties of cells are quite distinct, the sex-cells becoming spermatogonia (see p. 14) and being the mother cells of the spermatozoa, while the remaining epithelial cells perhaps become transformed into the connective-tissue walls of the tubules.
The Development of the Ovary. - In the case of the ovary, after the formation of the sex-cords, connective tissue grows in between these and the epithelium, forming a layer equivalent to the tunica albuginea of the testis. It is, however, a much looser tissue than its homologue in the male, and, indeed, does not completely isolate the sex-cords from the epithelium, although the majority of the cords are separated and sink into the deeper portions of the ovary where they form what have been termed the medullary cords. In the meantime the germinal epithelium has continued to bud off cords which unite to form a cortical layer of cells lying below the epithelium and separated from the medullary cords by the tunica albuginea (Fig. 216).
Later the cortical layer becomes broken up by the ingrowth of stroma tissue into spherical or cord-like masses, consisting of sexcells and epithelial cells (Fig. 217). The invasion of the stroma continuing, these spheres or cords (Pfluger's cords) become divided
into smaller masses, the primary ovarian follicles, each of which consists as a rule of a single sex-cell surrounded by a number of epithelial cells, the whole being enclosed by a- zone of condensed stroma tissue, which eventually becomes richly vascularized and forms a theca folliculi (Fig. 10). The epithelial cells in each follicle are at first comparatively few in number and closely surround the sex-cell (Fig. 217,/), which is destined to become an ovum, but in certain of the follicles they undergo an increase by mitosis, becoming extremely numerous, and later secrete a fluid, the liquor folliculi, which collects at one side of the follicle and eventually forms a considerable portion of its contents. The follicular cells are differentiated by its appearance into the stratum granulosum, which surrounds the wall of the follicle, and the discus froligerus, in which the ovum is embedded (Fig. 10, dp), and the cells which immediately surround the ovum, becoming cylindrical in shape, give rise to the corona radiata (Fig. 11, cr).
A somewhat similar fate is shared by the medullary cords, these also breaking up into a number of follicles, but sooner or later these follicles undergo degeneration so that shortly after birth practically no traces of the cords remain. It must be noted that degeneration of the follicles formed from the cortical layer also takes place even during fetal life and continues to occur throughout the entire periods of growth and functional activity, numerous atretic follicles being found in the ovary at all times. Indeed it would seem that degeneration is the fate of the great majority of the follicles and sex-cells of the ovary, but few ova coming to maturity during the life-time of any individual. 23
Fig. 217. - Section of the Ovary of a New-born Child.
a, Ovarial epithelium; b, proximal part of a Pfl tiger's cord; c, sex-cell in epithelium; d and e, spherical masses; /, primary follicle; g, blood-vessel. - (From Gegenbaur, after Waldeyer.)
354 THE GENITAL DUCTS Rete-cords developed from the rete portion of the germinal ridge occur in connection with the ovary as well as with the. testis and form a rete ovarii (Fig. 216, R). They do not, however, extend so deeply into the ovary, remaining in the neighborhood of the mesovarium, and they do not become tubular, but resemble closely the medullary cords with which they are serially homologous. They separate from the epithelium and make connections with the glomeruli of the anterior portion of the mesonephros, on the one hand, and on the other with medullary cords, and in later stages show a tendency to break up into primary follicles, which early degenerate and disappear like those of the medullary cords.
The Transformation of the Mesonephros and the Ducts. - At one period of development there are present, as representatives of the urinogenital apparatus, the Wolffian body (mesonephros) and duct, the Miillerian duct, and the developing ovary or testis. Such a condition forms an indifferent stage from which the development proceeds in one of two directions according as the genital ridge becomes a testis or an ovary, the Wolffian body in part undergoing degeneration and in part persisting to form organs which for the most part are rudimentary, while in the female the Wolffian duct also degenerates except for certain rudiments and in the male the Miillerian duct behaves similarly.
In the Male. - It has been seen that the Wolffian body, through the rete cords, enters into very intimate relations with the testis, and it may be regarded as divided into two portions, an upper genital and a lower excretory. In the male the genital portion persists in its entirety, serving as the efferent ducts of the testis, which, beginning in the spaces of the rete testis, already shown to be connected with the capsules of Bowman, open into the upper part of the Wolffian duct and form the globus major of the epididymis. The excretory portion undergoes extensive degeneration, a portion of it persisting as a mass of coiled tubules ending blindly at both ends, situated near the head of the epididymis and known as the paradidymis or organ of Giraldes, while a single elongated tubule, arising from the portion of the Wolffian duct which forms the
THE GENITAL DUCTS 355 globus minor of the epididymis, represents another portion of it and is known as the vas aberrans.
The Wolffian duct is retained complete, the portion of it nearest the testis becoming greatly elongated and thrown into numerous coils, forming the body and globus minor of the epididymis, while the remainder of it is converted into the vas deferens and the ductus ejaculatorius. A lateral outpouching of the wall of the duct to form a longitudinal fold appears at about the third month and gives rise to the vesicula seminalis, the lateral position of the outgrowth explaining the adult position of the vesiculse lateral to the vasa deferentia.
With the Mullerian ducts the case is very different, since they disappear completely throughout the greater part of their course, only their upper and lower ends persisting, the former giving rise to a small sac-like body, the sessile hydatid of Morgagni, attached to the upper end of each testis near the epididymis. It has been seen (p. 349) that the lower ends of the Mullerian ducts, in the male as well as the female, fuse to form the utero-vaginal canal, and the lower portion of this also persists to form what is termed the uterus masculinus, although it corresponds to the vagina of the female rather than to the uterus. It is a short cylindrical pouch of varying length, that opens into the urethra at the bottom of a depression known as the utriculus prostaticus (sinus pocularis).
The transverse pelvic partition, produced by the union of the two tubal portions of the Wolffian body, is formed in the male embryo, but at an early stage its anterior surface fuses with the posterior surface of the bladder and consequently there is in the male no pelvic compartment equivalent to the vesico-uterine pouch of the female. The male recto-vesical pouch is, however, the homologue of the rectouterine pouch of the female.
The formation of the inguinal ligament on the surface of the mesonephros has been described on p. 349. On the degeneration of the mesonephros the layer of peritoneum that covered it persists to form a mesorchium extending from the body wall to the hilus of the testis and the inguinal ligament now comes to have its origin from
356 THE GENITAL DUCTS the lower pole of that organ, whence it extends to the anterior abdominal wall. Owing to the rudimentary nature of the uterus masculinus and the slight development of its walls the inguinal ligament does not become involved with it, but remains independent and forms the gubemaculum testis of the adult, whose adult position is brought about by the descent of the testis into the scrotum (see p. 366).
In the Female. - In the female the transverse partition of the pelvis does not fuse w'th the bladder but remains distinct as the broad ligament. Consequently there is in the female both a vesicouterine and a recto-uterine pouch. Since the genital ridges form upon the mesial surfaces of the Wolffian ridges and the tubal portions are their lateral portions, when these latter unite to form the broad ligament the ovary will come to lie upon the posterior surface of that structure, projecting into the recto-vesical pouch. On the degeneration of the mesonephros the peritoneum that covered it becomes a part of the broad ligament, forming that part of it which contains the Fallopian tubes and hence is known as the mesosalpinx, while the lower part of the ligament, on account of its relation to the uterus,. is termed the mesometrium.
The genital portion of the mesonephros, though never functional as ducts in the female, persists as a group of ten to fifteen tubules, situated between the two layers of the broad ligament and in close proximity to the ovary; these constitute what is known as the epoophoron (parovarium or organ of Rosenmuller) . The tubules terminate blindly at the ends nearest the ovary, but at the other extremity, where they are somewhat coiled, they open into a collecting duct which represents the upper end of the Wolffian duct. Near this rudimentary body is another, also composed of tubules, representing the remains of the excretory portion of the mesonephros and termed the paroophoron which, however, degenerates during the early years of extra-uterine life. So far as the mesonephros is concerned, therefore, the persisting rudiments in the female are comparable to those occurring in the male.
As regards the ducts, however, the case is different, for in the
THE GENITAL DUCTS 357 female it is the Mtillerian ducts which persist, while the Wolmans undergo degeneration, a small portion of their upper ends persisting in connection with the epoophora, while their lower ends persist as straight tubules lying at the sides of the vagina and forming what are known as the canals of Gartner. The Mtillerian ducts, on the other hand, become converted into the Fallopian tubes (tubas uterince), and in their lower portions into the uterus and vagina. From the margins of the openings by which the Mullerian ducts communicate with the ccelom projections develop at an early period and give rise to the fimbria, with the exception of the one connected with the ovary, the fimbria ovarica, which is the persisting upper portion of the original genital ridge. From the utero-vaginal canal the two structures which give it its name are formed, the entire canal being transformed into the mucous membrane of the uterus and vagina. Indeed, the lower ends of the Fallopian tubes are also taken up into the uterus, for the condensation of mesenchyme that takes place around the mucosa to form the muscular wall of the uterus is so voluminous that it includes not only the utero-vaginal canal but also the adjacent portions of the Mullerian ducts. The histological differentiation of the uterus from the vagina begins to manifest itself at about the third month, and during the fourth month the vaginal portion of the duct becomes flattened and the epithelium lining its lumen fuses so as to completely occlude it and, a little later, there appears at its lower opening a distinct semicircular fold. This is the hymen, a structure which seems to be represented in the male by the colliculus seminalis. The obliteration of the lumen of the vagina persists until about the sixth month, when the cavity is re-established by the breaking down of the central epithelial cells.
The extent of the mesenchymal condensation to form the muscularis uteri also produces a modification of the relations of the inguinal ligament in the female. For the ligament becomes for a short portion of its length included in the condensation and thus attached to the upper portion of the uterus. It is consequently divided into two portions, one extending from the lower pole of
THE GENITAL DUCTS
the ovary to the uterus and forming the ligamentum ovarii proprium and the other extending from the uterus to the anterior abdominal wall and forming what is known in the adult as the round ligament of the uterus.
The diagram, Fig. 218, illustrates the transformation from the indifferent condition which occurs in the two sexes, and that the
female indifferent male Fig. 218. - Diagrams Illustrating the Transformation of the Mullerian and Wolffian Ducts. B, Bladder; C, clitoris; CG, canal of Gaertner; CI, cloaca; Eo, epoophoron; Ep, epididymis; F, Fallopian tube; G, genital gland; HE, hydatid of epididymis; HM, hydatid of Morgagni; K, kidney; MD, Mullerian duct; O, ovary; P, penis; Po, paroophron; Pr, prostate gland; R, rectum; T, testis; U, urethra; UM, uterus masculinus; Ur, ureter; US, urogenital sinus; Ut, uterus; V, vagina; Va, vas aberrans; VD, vas deferens; VS, vesicula seminalis; WB, Wolffian body; WD, Wolffian duct. - (Modified from Huxley.)
homologies of the various parts may be clearly understood they may also be stated in tabular form as on the next page.
Genital ridge J
Fimbria ovarica. Ovary.
Ovarian ligament. Round ligament.
Wolffian body <
Globus major of epididymis.
Wolffian ducts. . . . â€¢
Body and globus minor of epididymis. Vasa deferentia. Seminal vesicles.' Ejaculatory ducts.
Collecting tubules of epoophoron.
Canal of Gartner.
Miillerian ducts . . . <
Sessile hyatid. Uterus masculinus.
In addition to the sessile hydatid, a stalked hydatid also occurs in connection with the testis, and a similar structure is attached to the fimbriated opening of each Fallopian tube. The significance of these structures is uncertain, though it has been suggested that they are persisting rudiments of the pronephros.
A failure of the development of the various parts just described to be completed in the normal manner leads to various abnormalities in connection with the reproductive organs. Thus there may occur a failure in the fusion of the lower portions of the Miillerian ducts, a bihorned or bipartite uterus resulting, or the two ducts may come into contact and their adjacent walls fail to disappear, the result being a median partition separating the vagina or both the vagina and uterus into two compartments. The excessive development of the fold which gives rise to the hymen may lead to a complete closure of the lower opening of the vagina, while, on the other hand, a failure of the Miillerian ducts to fuse may produce a biperforate hymen.
The Development of the Urinary Bladder and the Urogenital Sinus. - So far the relations of the lower ends of the urinogenital ducts have not been considered in detail, although it has been
seen that in the early stages of development the Wolffian and Miillerian ducts open into the sides of the ventral portion of the cloaca; that the ureters communicate with the lower portions of the Wolffian ducts; that from the ventral anterior portion of the cloaca the allantoic duct extends outward into the belly-stalk; and, finally (p. 281), that the cloaca becomes divided into a dorsal portion, which forms the lower part of the rectum, and a ventral portion, which is continuous with the allantois and receives the urinogenital ducts
Fig. 219. - Reconstruction of the Cloacal Region op an Embryo of 14 mm.
al, Allantois; b, bladder; gt, genital tubercle; i, intestine; n, spinal cord; nc, notochord ; r, rectum; sg, urogenital sinus; ur, ureter; w, Wolffian duct. - (Keibel.) (Fig. 219). It is the history of this ventral portion of the cloaca which is now to be considered.
It may be regarded as consisting of two portions, an anterior and a posterior, the line of insertion of the urinogenital ducts marking the junction of the two. The anterior or upper portion is destined to give rise to the urinary bladder (Fig. 219, b), while the lower one forms what is known for a time as the urogenital ^inus (sg). The bladder, when first differentiated, is a tubular structure, whose lumen is continuous with that of the allantois, but after the second
month it enlarges to become more sac-like, while the intra-embryonic portion of the allantois degenerates to a solid cord extending from the apex of the bladder to the umbilicus and is known as the urachus. During the enlargement of the bladder the terminal portions of the urinogenital ducts are taken up into its walls, a process which continues until finally the ureters and Wolffian ducts open into it separately, the ureters opening to the sides of and a little anterior to the ducts. This condition is reached in embryos of about 14 mm.
Fig. 220. - Reconstruction of the Cloacal Structures of an Embryo of 25 mm.
bl, Bladder; m, Mullerian duct; r, rectum; sg, urogenital sinus; sy, symphysis pubis; u, ureter; ur, urethra; w. Wolffian duct. - (Adapted from Keibel.) (Fig. 219), and in later stages the interval between the two pairs of ducts is increased (Fig. 220), resulting in the formation of a short canal connecting the lower end of the bladder which receives the ureters with the upper end of the urogenital sinus, into which the Wolffian and Mullerian ducts open. This connecting canal represents the urethra (Fig. 220, ur), or rather the entire urethra of the female and the proximal part of that of the male, since a considerable portion of the latter canal is still undeveloped (see p. 364). From
362 THE UROGENITAL SINUS this urethra there is developed, at about the third month, a series of solid longitudinal folds which project upon the outer surface and separate from the urethra from above downward. These represent the tubules of the prostate gland and are developed in both sexes, although they remain in a somewhat rudimentary condition in the female. The muscular tissue, so characteristic of the gland in the adult male, is developed from the surrounding mesenchyme at a later stage.
The bladder is, accordingly, essentially a derivative of the cloaca and its mucous membrane is therefore largely of endodermal origin. Portions of the Wolffian ducts which are of mesodermal origin are, however, taken up into the wall of the bladder and form a portion of it. The extent of the portion so formed is indicated by the position of the orifices of the ureters above and of the ejaculatory ducts below, and it corresponds therefore with what is termed the trigonum vesica together with the floor of the urethra as far as the openings of the ejaculatory ducts. Throughout this region the mucous membrane is of mesodermal origin.
The urogenital sinus is in the early stages also tubular in its upper part, though it expands considerably below, where it is closed by the cloacal membrane. This, by the separation of the cloaca into rectum and sinus, has become divided into two portions, the more ventral of which closes the sinus and the dorsal the rectum, the interval between them having become considerably thickened to form the perineal body. In embryos of about 17 mm. the urogenital portion of the membrane has broken through, and in later stages the tubular portion of the sinus is gradually taken up into the more expanded lower portion, until finally the entire sinus forms a shallow depression, termed the vestibule, into the upper part of which the urethra opens, while below are the openings of the Wolffian (ejaculatory) ducts in the male or the orifice of the vagina in the female. From the sides of the lower part of the sinus a pair of evaginations arise toward the end of the fourth month and give rise to the bulbo-vestibular glands (Bartholin's) of the female or the corresponding bulbo-urethral glands (Cowper's) in the male.
THE EXTERNAL GENITALIA 363 The Development of the External Genitalia. - At about the fifth week, before the urogenital sinus has opened to the exterior, the mesenchyme on its ventral wall begins to thicken, producing a slight projection to the exterior. This eminence, which is known as the genital tubercle (Fig. 219, gt), rapidly increases in size, its extremity becomes somewhat bulbously enlarged (Fig. 221, gl) and a groove, extending to the base of the terminal enlargement, appears upon its vestibular surface, the lips of the groove forming two wellmarked genital folds (Fig. 221, gf). At about the tenth week there appears on either side of the tubercle an enlargement termed the genital swelling (Fig. 221, gs), which is due to a thickening of the mesenchyme of the lower part of the ventral^abdominal wall in the
Fig. 221. - The External Genitalia of an Embryo of 25 mm. a, Anus; gf, genital fold; gl, glans; gs, genital swelling; p, perineal body. - (Keibel.) region where the inguinal ligament is attached, and with the appearance of these structures the indifferent stage of the external genitals is completed.
In the female the growth of the genital tubercle proceeds rather slowly and it becomes transformed into the clitoris, the genital folds becoming prolonged to form the labia minora. The genital swellings increase in size, their mesenchyme becomes transformed into a mass of adipose and fibrous tissue and they become converted into the labia majora, the interval between them constituting the vulva.
In the male the early stages of development are closely similar to
3 6 4
THE EXTERNAL GENITALIA
those of the female; indeed, it has been well said that the external genitals of the adult female resemble those of the fetal male. In early stages the genital tubercle elongates to form the penis and the integument which covers the proximal part of it grows forward as a fold which encloses the bulbous enlargement or glans and forms the prepuce, whose epithelium fuses with that covering the glans and only separates from it later by a cornification of the cells along the plane of fusion. The genital folds meet together and fuse, converting the vestibule and the groove upon the vestibular surface of the penis into the terminal portion of the male urethra and bringing it about that the vasa deferentia and the uterus masculinus open upon the floor of that passage. The two genital swellings are at the same time brought closer together, so as to lie between the base of the penis and the perineal body and, eventually, they form the scrotum. The mesenchyme of which they were primarily composed differentiates into the same layers as are found in the wall of the abdomen and a peritoneal pouch is prolonged into them from the abdomen, so that they form sacs into which the testes descend toward the close of fetal life (p. 366).
The homologies of the portions of the reproductive apparatus derived from the cloaca and of the external genitalia in the two sexes may be perceived from the following table.
Proximal portion of urethra.
Urogenital sinus ....
The rest of the urethra.
Genital tubercle. . . .
Prepuce and integument of
Genital swellings... .
It is stated above that the layers which compose the walls of the scrotum are identical with those of the abdominal wall. This may be seen in detail from the following scheme:
THE DESCENT OF THE OVARIES 365 Abdominal Walls. Scrotum.
Superficial fascia. Dartos.
External oblique muscle. Intercolumnar fascia.
Internal oblique muscle. Cremasteric fascia.
Transverse muscle. Infundibuliform fascia.
Peritoneum. Tunica vaginalis.
Numerous anomalies, depending upon an inhibition or excess of the development of the parts, may occur in connection with the external genitalia. Should, for instance, the lips of the groove on the vestibular surface of the penis fail to fuse, the penial portion of the urethra remains incomplete, constituting a condition known as hypospadias, a condition whic,h offers a serious bar to the fulfilment of the sexual act. If the hypospadias is complete and there be at the same time an imperfect development of the penis, as frequently occurs in such cases, the male genitalia closely resemble those of the female and a condition is produced which is usually known as hermaphroditism. It is noteworthy that in such cases there is frequently a somewhat excessive development of the uterus masculinus, and a similar condition may be produced in the female by an excessive development of the clitoris. Such cases, however, which concern only the accessory organs of reproduction, are instances of what is more properly termed spurious hermaphroditism, true hermaphroditism being a term which should be reserved for possible cases in which the genital ridges give rise in the same individual to both ova and spermatozoa. Such cases are of exceeding rarity in the human species, although occasionally observed in the lower vertebrates, and the great majority of the examples of hermaphroditism hitherto observed are cases of the spurious variety.
The Descent of the Ovaries and Testes. - The positions finally occupied by the ovaries and testes are very different from those which they possess in the earlier stages of development, and this is especially true in the case of the testes. The change of position is partly due to the rate of growth of the inguinal ligaments being less than that of the abdominal walls, the reproductive organs being thereby drawn downward toward the inguinal regions where the ligaments are attached. The point of attachment is beneath the bottom of a slight pouch of peritoneum which projects a short distance into the substance of the genital swellings and is known as the canal of Nuck in the female, and in the male as the vaginal process.
In the female a second factor combines with that just mentioned.
THE DESCENT OF THE TESTES
The relative shortening of the inguinal ligaments acting alone would draw the ovaries toward the inguinal regions, but since they are united to the uterus by the ovarian ligaments movement in that direction is prevented and the ovaries come to lie in the recto-uterine compartment of the pelvic cavity.
With the testes the case is more complicated, since in addition to the relative shortening of the inguinal ligaments there is an elongation of the vaginal processes into the substance of the genital swellings, and it must be remembered that the testes, like the ovaries, are primarily connected with the peritoneum. Three stages may be recognized in the descent of the testes. The first of these depends
Fig. 222. - Diagrams Illustrating the Descent of the Testis. il, Inguinal ligament; m, muscular layer; s, skin and dartos of the scrotum; t, testis; tv, tunica vaginalis ; vd, vas deferens ; vp, vaginal process of peritoneum. - (After Hertwig.) on the slow rate of elongation of the inguinal ligaments or gubernacula. It lasts until about the fifth month of development, when the testes lie in the inguinal region of the abdomen, but during this month the elongation of the gubernaculum becomes more rapid and brings about the second stage, during which there is a slight ascent of the testes, so that they come to lie a little higher in the abdomen. This stage is, however, of short duration, and is succeeded by the stage of the final descent, which is characterized by the elongation of the vaginal processes of the peritoneum into the substance of the scrotum (Fig. 222, A). Since the gubernaculum is attached to the
THE DESCENT OF TBE TESTES 367 abdominal wall beneath this process, and since its growth has again diminished, the testes gradually assume again their inguinal position, and are finally drawn down into the scrotum with the vaginal processes.
The condition which is thus acquired persists for some time after birth, the testicles being readily pushed upward into the abdominal cavity along the cavity by which they descended. Later, however, the size of the openings of the vaginal processes into the general peritoneal cavity becomes greatly reduced, so that each process becomes converted into an upper narrow neck and a lower sac-like cavity (Fig. 222, B), and, still later, the walls of the neck portion fuse and become converted into a solid cord, while the lower portion, wrapping itself around the testis, becomes the tunica vaginalis (tv). By these changes the testes become permanently located in the scrotum. During the descent of the testes the remains of each Wolffian body, the epididymis, and the upper part of each vas deferens together with the spermatic vessels and nerves, are drawn down into the scrotum, and the mesenterial fold in which they were originally contained also practically disappears, becoming converted into a sheath of connective tissue which encloses the vas deferens and the vessels and nerves, binding them together into what is termed the spermatic cord. The mesorchium, which united the testis to the peritoneum enclosing the Wolffian body, does not share in the degeneration of the latter, but persists as a fold extending between the epididymis and the testis and forming the sinus epididymis.
In the text-books of anatomy the spermatic cord is usually described as lying in an inguinal canal which traverses the abdominal walls obliquely immediately above Poupart's ligament. So long as the lumen of the neck portion of the vaginal process of peritoneum remains patent there is such a canal, placing the cavity of the tunica vaginalis in communication with the general peritoneal cavity, but the cord does not traverse this canal, but lies outside it in the retroperitoneal connective tissue. When, however, the neck of the vaginal process disappears, a canal no longer exists, although the connective tissue which surrounds the spermatic cord and unites it with the tissues of the abdominal walls is less dense than the neighboring tissues, so that the cord may readily be separated from these and thus appear to He in a canal.
368 LITERATURE LITERATURE.
B. M. Allen: "The Embryonic Development of the Ovary and Testes in Mammals," Amer. Journ. of AnaL, in, 1904. J. L. Bremer: "Morphology of the Tubules of the Human Testis and Epididymis," Amer. Journ. Anat., xi, 1911. E. J. Evatt: "A Contribution to the Development of the Prostate in Man," Journ.
Anat. and Phys., xliii, 1909. E. J. Evatt: " A Contribution to the Development of the Prostate Gland in the Human Female," Journ. Anat. and Phys., xlv, 1911. W. Felix: " Entwickelungsgeschichte des Exkretions-sy stems," Ergebn. der Anat. und Entwicklungsgesch., xni, 1903. W. Felix: "Die Entwicklung der Ham- und Geschlechtsorgane," in Keibel-Mall Human Embryology, II, 1912. A. Fleischmann: " Morphologische Studien liber Kloake und Phallus der Amnioten, Morphol. Jarhbuch, xxx, xxxii und xxxvi, 1902, 1904, 1907. O. Frankl: "Beitrage zur Lehre vom Descensus testiculorum," Sitzungsber. der kais.
Akad. Wissensch. Wien, Math.-Naturwiss. Classe, cix, 1900. S. P. Gage: "A Three Weeks Human Embryo, with especial reference to the Brain and the Nephric System," Amer. Journ. of Anat., rv, 1905. D. B. Hart: " The Nature and Cause of the Physiological Descent of the Testes," Journ. Anat. and Phys., xliv, 1909.
D. B. Hart: " The Physiological Descent of the Ovaries in the HumanFoetus," Journ.
Anat. and Phys., xliv, 1909.
E. Hauch: "Ueber die Anatomie und Entwicklung der Nieren," Anat. Hefte, xxii, 1903. G. C. Huber: "On the Development and Shape of the Uriniferous Tubules of Certain of the Higher Mammals," Amer. Journ. of Anat., rv, Suppl. 1905. J. Janosik: "Histologisch-embryologische Untersuchungen uber das Urogenitalsystem," Sitzungsber. der kais. Akad. Wissensch. Wien, Math.-Naturwiss. Classe, xci, 1887 J. Janosik: "Ueber die Entwicklung der Nachniere bei den Amnioten," Arch, fur Anat. u. Phys., Anat. Abth., 1907. J. Janosik: "Entwicklung des Nierenbeckens beim Menschen," Arch, fitr mikrosk.
Anat., lxxviii, 191 1.
F. Keibel: "Zur Entwickelungsgeschichte des menschlichen Urogenital-apparatus," Archiv fiir Anat. und Physiol., Anat. Abth., 1896. J. B. Macallum: "Notes on the Wolffian Body of Higher Mammals," Amer. Journ.
Anat., 1, 1902. E. Martin: "Ueber die Anlage der Urniere beim Kaninchen," Archiv fiir Anat. und Physiol., Anat. Abth., 1888. H. Meyer: "Die Entwickelung der Urnieren beim Menschen," Archiv fiir mikrosk.
Anat., xxxvi, 1890. R. Meyer: "Zur Kenntnis des Gartner'schen Ganges besonders in der Vagina und dem Hymen des Menschen," Arch, fur mikrosk. Anat., lxxiii, 1909. R. Meyer: "Zur Entwicklungsgeschichte und Anatomie des utriculus prostaticus beim Menschen," Arch, fiir mikrosk. Anat., lxxtv, 1909
LITERATURE 369 G. VON Mihalkovicz : " Untersuchungen iiber die Entwickelung des Ham- und Geschlechtsapparates der Amnioten," Internat. Monatsschrift fiir Anat. und Physiol., 11, 1885. W. Nagel: "Ueber die Entwickelung des Urogenitalsystems des Menschen," Archiv fiir mikros. Anat., xxxiv, 1889. W. Nagel: "Ueber die Entwickelung des Uterus und der Vagina beim Menschen," Archiv fiir mikros k. Anat., xxxvn, 1891. W. Nagel: "Ueber die Entwickelung der innere und aussere Genitalien biem mensch lichen Weibes," Archiv fiir Gynakol., xlv, 1894. K. Peter: "Untersuchungen iiber Bau und Entwicklung der Niere. I. Die Nieren kanalchen des Menschen und einiger Saugetiere, Jena, 1909. A. G. Pohlman: "The Development of the Cloaca in Human Embryos." Amer. Journ.
of Anat., xii, 191 1. W. Rubaschkin: " Ueber die Urgeschlechtszellen bei Saugetiere,'Mwa<. Hefte, xxxix, 1909. K. E. Schrelner: "Ueber die Entwicklung der Amniotenniere," Zeit. fiir wissensch.
Zool., lxxi, 1902. O. Stoerk: "Beitrag zur Kenntnis des Aufbaues der menschlichen Niere," Anat.
Hefte, xxill, 1904. J. Tandler: "Ueber Vornieren-Rudimente beim menschliche Embryo," Anat. Hefte, xxvni, 1905. F. J. Taussig: "The Development of the Hymen," Amer. Journ. Anat., viii, 1908. F. Tourneux: " Sur le developpement et revolution du tubercule genital chez le foetus humain dans les deux sexes," Journ. de I' Anat. et de la Physiol., xxv, 1889. S. Weber: " Zur Entwickelungsgeschichte des uropoetischen Apparates bei Saugern, mit besonderer Beriicksichtigung der Urniere zur Zeit des Auftretens der blei benden Niere," Morphol. Arbeiten, vil, 1897.
CHAPTER XIV. THE SUPRARENAL SYSTEM OF ORGANS.
To the suprarenal system a number of bodies of peculiar structure, probably concerned with internal secretion, may be assigned. In the fishes they fall into two distinct groups, the one containing organs derived from the ccelomic epithelium and known as intervened organs, and the other consisting of organs derived from the sympathetic nervous system and which, on account of the characteristic affinity they possess for chromium salts, have been termed the chroma ffine organs. But in the amphibia and amniote vertebrates, while both the groups are represented by independent organs, yet they also become intimately associated to form the suprarenal bodies, so that, notwithstanding their distinctly different origins, it is convenient to consider them together.
The Development of the Suprarenal Bodies. - The suprarenal bodies make their appearance at an early stage, while the Wolffian bodies are still in a well-developed condition, and they are situated at first to the medial side of the upper ends of these structures (Fig. 211, sr). Their final relation to the metanephros is a secondary event, and is merely a topographic relation, there being no developmental relation between the two structures.
In the human embryo they make their appearance at about the beginning of the fourth week of development as a number of proliferations of the ccelomic epithelium, which project into the subjacent mesenchyme, and are situated on either side of the median line between the root of the mesentery and the upper portion of the Wolffian body. The various proliferations soon separate from the epithelium and unite to form two masses situated in the mesenchyme, one on either side of the upper portion of the abdominal aorta. In certain forms, such as the rabbit, the primary proliferations arise 37Â°
DEVELOPMENT OF THE SUPRARENAL BODIES
from the bottom of depressions of the ccelomic epithelium (Fig. 223), but in the human embryo these depressions do not form.
Up to this stage the structure is a pure interrenal organ, but during the fifth week of development masses of cells, derived from the abdominal portion of the sympathetic nervous system, begin to penetrate into each of the interrenal masses (Fig. 224), and form strands traversing them. At about the ninth or tenth week fatty granules begin to appear in the interrenal cells and somewhat later, about the fourth month, the sympathetic constituents begin to show their chromaffine characteristics. The two tissues, however, remain intermingled for a considerable time, and it is not until a much later
& Sr ns tern J â€ž-.- -
Fig. 223. - Section through a Portion of the Wolffian Ridge of a Rabbit Embryo of 6.5 mm.
Ao, Aorta; ns, nephrostome; Sr, suprarenal body; vc, cardinal vein; wc, tubule of Wolffian body; wd, Wolffian duct. - (Aichel.) period that they become definitely separated, the sympathetic elements gradually concentrating in the center of the compound organ to become its medullary substance, while the interrenal tissue forms the cortical substance. Indeed, it is not until after birth that the separation of the two tissues and their histological differentiation is complete, occasional masses of interrenal tissue remaining imbedded in the medullary substance and an immigration of sympathetic cells continuing until at least the tenth year (Wiesel).
A great deal of difference of opinion has existed in the past concerning the origin of the suprarenal glands. By several authors they have been regarded as derivatives in whole or in part of the excretory apparatus, some tracing their origin to the mesonephros and others even to the pronephros. The fact that in some mammals the cortical (interrenal) cells are
372 DEVELOPMENT OF THE SUPRARENAL BODIES formed from the bottom of depressions of the coelomic epithelium seemed to lend support to this view, but it is now pretty firmly established that the appearances thus presented do not warrant the interpretation placed upon them and that the interrenal tissue is derived from the ccelomic epithelium quite independently of the nephric tubules. That the chromaffine tissue is a derivative of the sympathetic nervous system has long been recognized.
During the development of the suprarenal glands portions of their tissue may be separated as the result of unequal growth and form what are commonly spoken of as accessory suprarenal glands, although, since they are usually composed solely of cortical sub
... â– / 'â€¢'Â§â€¢. ' . . .â– /.
Fig. 224. - Section through the Suprarenal Body of an Embryo of 17 mm.
A, Aorta; R, interrenal portion; S, sympathetic nervous system; SB, sympathetic cells penetrating the interrenal portion. - (Wiesel.) stance, the term accessory interrenal bodies would be more appropriate. They may be formed at different periods of development and occur in various situations, as for instance, in the vicinity of the kidneys or even actually imbedded in their substance, on the walls of neighboring blood-vessels, in the retroperitoneal tissue below the level of the kidneys, and in connection with the organs of reproduction, in the spermatic cord, epididymis or rete testis of the male and in the broad ligament of the female.
It seems probable that the bodies associated with the reproductive
DEVELOPMENT OF THE SUPRARENAL BODIES
apparatus are separated from the main mass of interrenal tissue before the immigration of the sympathetic tissue and before the descent of the ovaries or testes, while those which occur at higher levels are of later origin, and in some cases may contain some medullary substance, being then true accessory suprarenals. Such bodies are, however, comparatively rare, the great majority of the accessory bodies being composed of interrenal tissue alone.
Independent chromamne organs also occur, among them the
Fig. 225. - Section of a Cell Ball from the Intercarotid Ganglion of Man be, Blood capillaries; ev, efferent vein; S, connective-tissue septum; I, trabecular - (From Bohm and Davidoff, after Schaper.)
intercarotid ganglia and the organs of Zuckerkandl being especially deserving of note. It may also be pointed out, however, that the chromamne cells have the same origin as the cells of the sympathetic ganglia and may sometimes fail to separate from the latter, so that the sympathetic ganglia and plexuses frequently contain chromamne cells.
The Intercarotid Ganglia. - These structures, which are fre
374 T TTF. INTERCAROTID GANGLIA quently though incorrectly termed carotid glands, are small bodies about 5 mm. in length, which lie usually to the mesial side of the upper ends of the common carotid arteries. They possess a very rich arterial supply and stand in intimate relation with the branches of an intercarotid sympathetic plexus, and, furthermore, they are characterized by possessing as their specific constituents markedly chromamne cells, among which are scattered stellate cells resembling the cells of the sympathetic ganglia.
They have been found to arise in pig embryos of 44 mm. by the separation of cells from the ganglionic masses scattered throughout the carotid sympathetic plexuses. These cells, which become the chromamne cells, arrange themselves in round masses termed cell balls, many of which unite to form each ganglion, and in man each cell ball becomes broken up into trabecule by the blood-vessels (Fig. 225) which penetrate its substance, and the individual balls are separated from one another by considerable quantities of connective tissue.
Some confusion has existed in the past as to the origin of this structure. The mesial wall of the proximal part of the internal carotid artery becomes considerably thickened during the early stages of development and the thickening is traversed by numerous blood lacunae which communicate with the lumen of the vessel. This condition is perhaps a relic of the branchial capillaries which in the lower gill-breathing vertebrates represent the proximal portion of the internal carotid, and has nothing to do with the formation of the intercarotid ganglion, although it has been believed by some authors (Schaper) that the ganglion was derived from the thickening of the wall of the vessel. The fact that in some animals, such as the rat and the dog, the ganglion stands in relation with the external carotid and receives its blood- supply from that vessel is of importance in this connection.
The thickening of the internal carotid disappears in the higher vertebrates almost entirely, but in the Amphibia it persists throughout life, the lumen of the proximal part of the vessel being converted into a fine meshwork by the numerous trabecular which traverse it. This carotid labyrinth has been termed the carotid gland, a circumstance which has probably assisted in producing confusion as to the real significance of the intercarotid ganglion.
The Organs of Zuckerkandl. - In embryos of 14.5 mm. there have been found, in front of the abdominal aorta, closely packed
THE ORGANS OF ZUCKERKANDL
groups of cells which resemble in appearance the cells composing the ganglionated cord, two of these groups, which extend downward along the side of the aorta to below the point of origin of the inferior mesenteric artery, being especially distinct. These cell groups give rise to the ganglia of the prevertebral sympathetic plexuses and also
Fig. 226. - Organs of Zuckerkandl from a New-born Child. a, Aorta; ci, inferior vena cava; i.c, common iliac artery; mi, inferior mesenteric artery; n.l and n.r, left and right accessory organs; pl.a, aortic plexus; u, ureter; v.r.s, left renal vein. - (Zuckerkandl.) to peculiar bodies which, from their discoverer, may be termed the organs of Zuckerkandl. Each body stands in intimate relation with the fibers of the sympathetic plexuses and has a rich blood-supply, resembling in these respects the intercarotid ganglia, and the resem
376 LITERATURE blance is further increased by the fact that the specific cells of the organ are markedly chromamne.
i At birth the bodies situated in the upper portion of the abdominal cavity have broken up into small masses, but the two lower ones, mentioned above, are still well defined (Fig. 226). Even these, how- # ever, seem to disappear later on and no traces of them have as yet been found in the adult.
A. Kohn: "Ueber den Bau und die Entwickelung der sog. Carotisdruse," Archiv.
fur mikrosk. Anat., lvi, 1900. A. Kohn: "Das chromaffine Gewebe," Ergebn. der Anat. und Entwickelungsgesch., xii, 1902. H. Poll: "Die vergleichende Entwicklungsgeschichte der Nebennierensysteme der Wirbeltiere," Hertwig's Handb. der vergl. und exper. Entwicklungslehre der Wirbel tiere, in, 1906. A. Sotjlie: "Recherches sur le developpement des capsules surrenales chez les Vertebres," Journ. de V Anat. et de la Physiol., xxxix, 1903. J. Wiesel: "Beitrage zur Anatomie und Entwickelung der menschlichen Nebenniere," Anat. Heft., xix, 1902. E. Zuckerkandl: "Ueber Nebenorgane des Sympathicus im Retroperitonealraum des Menschen," Verhandl. Anat. Gesellsch., xv, 1901.
THE DEVELOPMENT OF THE NERVOUS SYSTEM.
The Histogenesis of the Nervous System. - The entire central nervous system is derived from the cells lining the medullary groove, whose formation and conversion into the medullary canal has already been described (p. 72). When the groove is first formed, the cells lining it are somewhat more columnar in shape than those on either side of it, though like them they are arranged in a single layer; later they increase by mitotic division and arrange themselves in several layers, so that the ectoderm of the groove becomes very much thicker than that of the general surface of the body. At the same time the cell boundaries, which were originally quite distinct, gradually disappear, the tissue becoming a syncytium. While its tissue is in this condition the lips of the medullary groove unite, and the subsequent differentiation of the canal so formed differs somewhat in different regions, although a fundamental plan may be recognized. This plan is most readily perceived in the region which becomes the spinal cord, and may be described as seen in that region.
Throughout the earlier stages, the cells lining the inner wall of the medullary tube are found in active proliferation, some of the cells so produced arranging themselves with their long axes at right angles to the central canal (Fig. 227), while others, whose destiny is for the most part not yet determinable, and which therefore may be termed indifferent cells are scattered throughout the syncytium. At this stage a transverse section of the medullary tube shows it to be composed of two well-defined zones, an inner one immediately surrounding the central canal and composed of the indifferent cells and the bodies of the inner or ependymal cells, and an outer one consisting of branched prolongations of the syncytial cytoplasm. This 377
378 THE HISTOGENESIS OF THE NERVOUS SYSTEM outer layer is termed the marginal velum (Randschleier) (Fig. 227, m). The indifferent cells now begin to wander outward to form a definite layer, termed the mantle layer, lying between the marginal velum and the bodies of the ependymal cells (Fig. 228), and when this layer has become well established the cells composing it begin to divide and to differentiate into (1) cells termed neuroblasts, destined to become nerve-cells, and (2) others which appear to be supportive in character and are termed neuroglia cells (Fig. 228, B).
6r Â° %
,'. I"' Â«9
Fig. 227. - Transverse Section through the Spinal Cord of a Pig Embryo of 30 mm., the Upper Part showing the Appearance produced by the Silver Method of Demonstrating the Neuroglia Fibers.
a, Ependyma of floor plate; b, boundary between mantle layer and marginal zone; cs, mesenchymal connective- tissue syncytium; ep, ependymal cells; i, ingrowth of connective tissue; m, marginal velum; mn, mantle layer; mv, mantle layer of floor plate; p, pia mater; r, neuroglia fibers. - (Hardesty.) The latter are for the" most part small and are scattered among the neuroblasts, these, on the other hand, being larger and each early developing a single strong process which grows out into the marginal velum and is known as an axis-cylinder. At a later period the
THE HISTOGENESIS OF THE NERVOUS SYSTEM
neuroblasts also give rise to other processes, termed dendrites, more slender and shorter than the axis-cylinders, branching repeatedly, and, as a rule, not extending beyond the limits of the mantle layer. In connection with the neuroglia cells peculiar neuroglia fibrils develop very much in the same way as the fibers are formed in mesenchymal connective tissue. That is to say, they are formed from the peripheral portions of the cytoplasm of the neuroglial and ependymal cells. But since these cells are connected i together to form a syncytium the fibrils are not confined to the territories of the indi
o ^i^r Ote%0Â«Â» OqQ q Â®Â», â– ooq.Â©^ u rtOO^*>^ a
o oÂ°b u o u o Â°
Fig. 228. - Diagrams showing the Development of the Mantle Layer in the Spinal Cord. The circles, indifferent cells; circles with dots, neuroglia cells; shaded cells, germinal cells; circles with cross, germinal cells in mitosis; black cells, nerve-cells. - (Schaper.) vidual cells, but may extend far beyond these, passing in the syncytium from the territory of one neuroglial cell to another, many of those, indeed, arising in connection with the ependymal cells extending throughout the entire thickness of the medullary wall (Fig. 227). The fibrils branch abundantly and form a supportive network extending through all portions of the central nervous system. The axis-cylinder processes of the majority of the neuroblasts on reaching the marginal velum bend upward or downward and, after
THE HISTOGENESIS OF THE NERVOUS SYSTEM
traversing a greater or less length of the cord, re-enter the mantle layer and terminate by dividing into numerous short branches which come into relation with the dendrites of adjacent neuroblasts. The processes of certain cells situated in the ventral region of the mantle zone pass, however, directly through the marginal velum out into the surrounding tissues and constitute the ventral nerveroots (Fig. 231).
The dorsal nerve-roots have a very different origin. In embryos of about 2.5 mm., in which the medullary canal is only partly closed (Fig. 53), the cells which lie along the line of transition between the lips of the groove and the general ectoderm form a distinct ridge readily recognized in sections and termed the neural crest (Fig. 229, A). When the lips of the groove fuse together the cells of the crest unite to form a wedge-shaped mass, completing the closure of the canal (Fig. 229, B), and later proliferate so as to extend outward over the surface of the canal (Fig. 229, C). Since this proliferation is most active in the regions of the crest which correspond to the mesodermic somites there is formed a series of cell masses, arranged segmentally and situated in the mesenchyme at the sides of the medullary canal (Fig. 214). These cell masses represent the dorsal root ganglia, and certain of their constituent cells, which may also be termed neuroblasts, early assume a fusiform shape and send out a process from each extremity. One of these processes, the axiscylinder, grows inward toward the medullary canal and penetrates its
Fig. 229. - Three Sections through the Medullary Canal of an Embryo of 2.5 mm. - (vonLenhossek.)
THE HISTOGENESIS OF THE NERVOUS SYSTEM 381 marginal velum, and, after a longer or shorter course in this zone, enters the mantle layer and comes into contact with the dendrites of some of the central neuroblasts. The other process extends peripherally and is to be regarded as an extremely elongated dendrite. The processes from the cells of each ganglion aggregate to form a nerve, that formed by the axis-cylinders being the posterior root of a spinal nerve, while that formed by the dendrites soon unites with the ventral nerve-root of the corresponding segment to form the main stem of a spinal nerve.
There is thus a very important difference in the mode of development of the two nerve-roots, the axis-cylinders of the ventral roots
Fig. 230. - Cells from the Gasserian Ganglion of a Guinea-pig Embryo. a, Bipolar cell; b and c, transitional stages to d, T-shaped cells. - (von Gehuchten.) arising from cells situated in the wall of the medullary canal and growing outward (centrif ugally) , while those of the dorsal root spring from cells situated peripherally and grow inward (centripetally) toward the medullary canal. In the majority of the dorsal root ganglia the points of origin of the two processes of each bi-polar cell gradually approach one another (Fig. 230, b) and eventually come to rise from a common stem, a process of the cell-body, which thus assumes a characteristic T form (Fig. 230, d).
From what has been said it will be seen that each axis-cylinder is an outgrowth from a single neuroblast and is part of its cell-body, as are also the dendrites. Another view has, however, been advanced to the
382 THE HISTOGENESIS OF THE NERVOUS SYSTEM effect that the nerve fibers first appear as chains of cells and that the axiscylinders, being differentiated from the cytoplasm of the chains, are really multicellular products. Many difficulties stand in the way of the acceptance of this view and recent observations, both histogenetic (Cajal) and experimental (Harrison), tend to confirm the unicellular origin of the axis-cylinders. The embryological evidence therefore goes to support the neurone theory, which regards the entire nervous system as composed of definite units, each of which corresponds to a single cell and is termed a neurone.
By the development of the axis-cylinders which occupy the meshes of the marginal velum, that zone increases in thickness and comes to consist principally of nerve-fibers, while the cell-bodies of the neurones of the cord are situated in the mantle zone. No such definite distinction of color in the two zones as exists in the adult is, however, noticeable until a late period of development, the medullary sheaths, which give to the nerve-fibers their white appearance not beginning to appear until the fifth month and continuing to form from that time onward until after birth. The origin of the myelin which composes the medullary sheaths is as yet uncertain, although the more recent observations tend to show that it is picked out from the blood and deposited around the axis-cylinders in some manner not yet understood. Its appearance is of importance as being associated with the beginning of the functional activity of the nerve-fibers.
In addition to the medullary sheaths the majority of the fibers of the peripheral nervous system are provided with primitive sheaths, which are lacking, however, to the fibers of the central system. They are formed by cells which wander out from the dorsal root-ganglia and are therefore of ectodermal origin. Frog larvae deprived of their neural crests at an early stage of development produce ventral nerve-fibers altogether destitute of primitive sheaths (Harrison).
Various theories have been advanced to account for the formation of the medullary sheaths. It has been held that the myelin is formed at the expense of the outermost portions of the axis-cylinders themselves (von Kolliker), and on the other hand, it has been regarded as an excretion of the cells which compose the primitive sheaths surrounding the fibers
THE SPINAL CORD 383 (Ranvier) , a theory which is, however, invalidated by the fact that myelin is formed around the fibers of the central nervous system which possess no primitive sheaths. As stated above, the more recent observations (Wlassak) indicate its exogenous origin.
It has been seen that the central canal is closed in the mid-dorsal line by a mass of cells derived from the neural crest. These cells do not take part in the formation of the mantle layer, but become completely converted into ependymal tissue, and the same is true of the cells situated in the mid-ventral line of the canal. In these two regions, known as the roof -plate and floor -plate respectively, the wall of the canal has a characteristic structure and does not share to any great extent in the increase of thickness which distinguishes the other regions (Fig. 231). In the lateral walls of the canal there is also noticeable a differentiation into two regions, a dorsal one standing in relation to the ingrowing fibers from the dorsal root ganglia and known as the dorsal zone, and a ventral one, the ventral zone, similarly related to the ventral nerve-roots. In different regions of the medullary tube these zones, as well as the roof- and floor-plates, undergo different degrees of development, producing peculiarities which may now be considered.
Trie Development of the Spinal Cord. - Even before the lips of the medullary groove have met a marked enlargement of the anterior portion of the canal is noticeable, the region which will become the brain being thus distinguished from the more posterior portion which will be converted into the spinal cord. When the formation of the mesodermic somites is completed, the spinal cord terminates at the level of the last somite, and in this region still retains its connection with the ectoderm of the dorsal surface of the body; but in that portion of the cord which is posterior to the first coccygeal segment the histological differentiation does not proceed beyond the stage when the walls consist of several layers of similar cells, the formation of neuroblasts and nerve-roots ceasing with the segment named. After the fourth month the more differentiated portion elongates at a much slower rate than the surrounding tissues and so appears to recede up the spinal canal, until its
384 THE SPINAL CORD termination is opposite the second lumber vertebra. The less differentiated portion, which retains its connection with the ectoderm until about the fifth month, is, on the other hand, drawn out into a slender filament whose cells degenerate during the sixth month, except in its uppermost part, so that it comes to be represented throughout the greater part of its extent by a thin cord composed of pia mater. This cord is the structure known in the adult as the filum terminate, and lies in the center of a leash of nerves occupying the lower part of the spinal canal and termed the cauda equina. The existence of the cauda is due to the recession of the cord which necessitates for the lower lumbar, sacral and coccygeal nerves, a descent through the spinal canal for a greater or less distance, before they can reach the intervertebral foramina through which they make their exit.
In the early stages of development the central canal of the cord is quite large and of an elongated oval form, but later it becomes somewhat rhomboidal in shape (Fig. 231, A), the lateral angles marking the boundaries between the dorsal and ventral zones. As development proceeds the sides of the canal in the dorsal region gradually approach one another and eventually fuse, so that this portion of the canal becomes obliterated (Fig. 231, B) and is indicated by the dorsal longitudinal fissure in the adult cord, the central canal of which corresponds to the ventral portion only of the embryonic cavity. While this process has been going on both the roofand the floor-plate have become depressed below the level of the general surface of the cord, and by a continuance of the depression of the floor-plate - a process really due to the enlargement and consequent bulging of the ventral zone - the anterior median fissure is produced, the difference between its shape and that of the dorsal fissure being due to the difference in its development.
The development of the mantle layer proceeds at first more rapidly in the ventral zone than in the dorsal, so that at an early stage (Fig. 231, A) the anterior column of gray matter is much more pronounced, but on the development of the dorsal nerve-roots the formation of neuroblasts in the dorsal zone proceeds apace, resulting
THE SPINAL CORD
in the formation of a dorsal column. A small portion of the zone, situated between the point of entrance of the dorsal nerve-roots and the roof-plate, fails, however, to give rise to neuroblasts and is entirely converted into ependyma. This represents the future funiculus gracilis (fasciculus of Goll) (Fig. 231, A, cG), and at the point of entrance of the dorsal roots into the cord a well-marked oval bundle of fibers is formed (Fig. 231, A, ob) which, as develop
Fig. 231. - Transverse Sections through the Spinal Cords of Embryos .of (A) about Four and a Half Weeks and (B) about Three Months'. cB, Fasciculus of Burdach; cG, fasciculus of Goll; dh, dorsal column; dz, dorsal zone; fp, floor-plate; ob, oval bundle; rp, roof-plate; vh, ventral column; vz, ventral zone. - (His.) ment proceeds, creeps dorsally over the surface of the dorsal horn until it meets the lateral surface of the fasciculus of Goll, and, its further progress toward the median line being thus impeded, it insinuates itself between that fasciculus and the posterior horn to form the funiculus cuneatus (fasciculus of Burdach) (Fig. 231, B, cB).
Little definite is as yet known concerning the development of the other fasciculi which are recognizable in the adult cord, but it seems 25
1 - H
certain that the lateral and anterior cerebro-spinal (pyramidal) fasciculi are composed of fibers which grow downward in the meshes of the marginal velum from neuroblasts situated in the cerebral cortex, while the cerebellospinal (direct cerebellar) fasciculi and the fibers of the ground-bundles have their origin from cells of the mantle layer of the cord.
The myelination of the fibers of the spinal cord begins between the fifth and sixth months and appears first in the funiculi cuneati, and about a month later in the funiculi graciles. The myelination of the great motor paths, the lateral and anterior cerebro-spinal fasciculi, is the last to develop, appearing toward the end of the ninth month of fetal life.
The Development of the Brain.
- The enlargement of the anterior portion of the medullary canal does not take place quite uniformly, but is less along two transverse lines than else where, so that the brain region early becomes divided into three primary vesicles which undergo further differentiation as follows. Upon each side of the anterior vesicle an evagination appears and becomes converted into a club-shaped structure attached to the ventral portion of the vesicle by a pedicle. These evaginations (Fig. 232, op) are known as the optic evaginations, and being concerned in the formation of the eye will be considered in the succeeding chapter. After their formation the antero-lateral portions of the vesicle become bulged out into two protuberances (h) which rapidly increase in size and give rise, eventually to the two cerebral hemispheres, which form, together with the portion of the vesicle which lies between them, what is termed the telencephalon or forebrain, the remainder of the vesicle giving rise to what is known as
Fig. 232. - Reconstruction of the Brain of an Embryo of 2.15 MM.
h, Hemisphere; i, isthmus; m, mesencephalon; mf, mid-brain flexure; mt, metencephalon ; myl, myelencephalon; nf, nape flexure; ot, otic capsule; op, optic evagination; t, diencephalon. - (His.)
the diencephalon or Hween-brain (Fig. 232, /). The middle vesicle is bodily converted into the mesencephalon or mid-brain (m), but the posterior vesicle differentiates so that three parts may be recognized : (1) a rather narrow portion which immediately succeeds the midbrain and is termed the isthmus (i); (2) a portion whose roof and floor give rise to the cerebellum and pons respectively, and which is termed the metencephalon or hind-brain (mi) ; and (3) a terminal portion which is known as the medulla oblongata, or, to retain a consistent nomenclature, the myelencephalon or after-brain (my). From each of these six divisions definite structures arise whose relations to the secondary divisions and to the primary vesicles may be understood from the following table and from the annexed figure (Fig. 233), which represents a median longitudinal section of the brain of a fetus of three months.
Medulla oblongata (I) .
/ Pons (II 1).
^ Cerebellum (II 2).
SBrachia conjunctiva (III). Cerebral peduncles (posterior portion) .
2nd Vesicle Mesencephalon
Cerebral peduncles (anterior portion) (IV 1). Corpora quadrigemina (IV 2).
1st Vesicle <
Pars mammillaris (V 1). Thalamus (V 2). Epiphysis (V 3).
Infundibulum (VI 1). Corpus striatum (VI 2). Olfactory bulb (VI 3). Hemispheres (VI 4).
But while the walls of the primary vesicles undergo this complex differentiation, their cavities retain much more perfectly their original relations, only that of the first vesicle sharing to any great extent the modifications of the walls.
388 THE BRAIN The cavity of the third vesicle persists in the adult as the fourth ventricle, traversing all the subdivisions of the vesicle; that of the second, increasing but little in height and breadth, constitutes the aquaductus cerebri; while that of the first vesicle is continued into the cerebral hemispheres to form the lateral ventricles, the remainder of it constituting the third ventricle, which includes the cavity of the median portion of the telencephalon as well as the entire cavity of the diencephalon.
During the differentiation of the various divisions of the brain certain flexures appear in the roof and floor, and to a certain extent
Fig. 233. - Median Longitudinal Section of the Brain of an Embryo of the Third Month. - (His.) correspond with those already described as occurring in the embryo. The first of these flexures to appear occurs in the region of the midbrain, the first vesicle being bent ventrally until it comes to lie at practically a right angle with the axis of the mid-brain. This may be termed the mid-brain flexure (Fig. 232, mf) and corresponds with the head-bend of the embryo. The second flexure occurs in the region of the medulla oblongata and is known as the nape flexure (Fig. 232, nf); it corresponds with the similarly named bend of the embryo and is produced by a bending ventrally of the entire head, so
THE MYELENCEPHALON 389 that the axis of the mid-brain comes to lie almost at right angles with that of the medulla and that of the first vesicle parallel with it. Finally, a third flexure occurs in the region of the metencephalon and is entirely peculiar to the nervous system; it consists of a bending ventrally of the floor of the hind-brain, the roof of this portion of the brain not being affected by it, and it may consequently be known as the pons flexure (Fig. 233).
In the later development the pons flexure practically disappears, owing to the development in this region of the transverse fibers and nuclei of the pons, but the mid-brain and nape flexures persist, though greatly reduced in acuteness, the axis of the anterior portion of the adult brain being inclined to that of the medulla at an angle of about 134 degrees.
The Development of the Myelencephalon. - In its posterior portion the myelencephalon closely resembles the spinal cord and has a very similar development. More anteriorly, however, the roof-plate (Fig. 234, rp) widens to form an exceedingly thin membrane, the posterior velum; with the broadening of the roof-plate there is associated a broadening of the dorsal portion of the brain cavity, the dorsal and ventral zones bending outward, until, in the anterior portion of the after-brain, the margins of the dorsal zone have a lateral position, and are, indeed, bent ventrally to form a reflected lip (Fig. 234, I). The portion of the fourth ventricle contained in this division of the brain becomes thus converted into a broad shallow cavity, whose floor is formed by the ventral zones separated in the median line by a deep groove, the floor of which is the somewhat thickened floor-plate. About the fourth month there appears in the roof-plate a transverse groove into which the surrounding mesenchyme dips, and, as the groove deepens in later stages, the mesenchyme contained within it becomes converted into blood-vessels, forming the chorioid plexus of the fourth ventricle, a structure which, as may be seen from its development, does not lie within the cavity of the ventricle, but is separated from it by the portion of the roofplate which forms the floor of the groove.
In embryos of about 9 mm. the differentiation of the dorsal
and ventral zones into ependymal and mantle layers is clearly visible (Fig. 234), and in the ventral zone the marginal velum is also well developed. Where the fibers from the sensory ganglion of the vagus nerve enter the dorsal zone an oval area (Fig. 234, fs) is to be seen which is evidently comparable to the oval bundle of the cord and consequently with the fasciculus of Burdach. It gives rise to the solitary fasciculus of adult anatomy, and in embryos of 11 to 13 mm. it becomes covered in by the fusion of the reflected lip of the dorsal zone with the sides of the myelencephalon, this fusion, at the same time, drawing the margins of the roof-plate ventrally to form a
Fig. 234. - Transverse Section through the Medulla Oblongata of an Embryo of 9.1 mm.
dz, Dorsal zone; fp, floor-plate; /s, fasciculus solitarius; I, lip; rp, roof-plate; vz, ventral zone; X and XII, tenth and twelfth nerves. - (His.) secondary lip (Fig. 235). Soon after this a remarkable migration ventrally of neuroblasts of the dorsal zone begins. Increasing rapidly in number the migrating cells pass on either side of the solitary fasciculus toward the territory of the ventral zone, and, passing ventrally to the ventral portion of the mantle layer, into which fibers have penetrated and which becomes the formatio reticularis (Fig. 235, fr), they differentiate to form the olivary body (ol).
The thickening of the floor-plate gives opportunity for fibers to pass across the median line from one side to the other, and this opportunity is taken advantage of at an early stage by the axis-cylin
ders of the neuroblasts of the ventral zone, and later, on the establishment of the olivary bodies, other fibers, descending from the cerebellum, decussate in this region to pass to the olivary body of the opposite side. In the lower part of the medulla fibers from the neuroblasts of the nuclei gracilis and cuneatus, which seem to be
ol z* Fig. 235. - Transverse Section through the Medulla Oblongata of an Embryo of about Eight Weeks.
av, Ascending root of the trigeminus ;fr, reticular formation; ol, olivary body; sf, solitary fasciculus; tr, restiform body; XII, hypoglossal nerve. - (His.) developments from the mantle layer of the dorsal zone, also decussate in the substance of the floor-plate; these fibers, known as the arcuate fibers, pass in part to the cerebellum, associating themselves with fibers ascending from the spinal cord and with the olivary fibers to form a round bundle situated in the dorsal portion of the marginal velum and known as the restiform body (Fig. 235, tr).
The principal differentiations of the zones of the myelencephalon may be stated in tabular form as follows: Roof-plate Posterior velum.
(Nuclei of termination of sensory roots of cranial nerves. Nuclei gracilis and cuneatus. The olivary bodies.
. ( Nuclei of origin of the motor roots of cranial nerves.
Ventral zones < _,, ...
I I he reticular formation.
Foor-plate The median raphe.
The Development of the Metencephalon and Isthmus. - Our knowledge of the development of the metencephalon, isthmus, and mesencephalon is by no means as complete as is that of the myelencephalon. The pons develops as a thickening of the portion of the brain floor which forms the anterior wall of the pons flexure, and its transverse fibers are well developed by the fourth month (Mihalkovicz), but all details regarding the origin of the pons nuclei are as yet wanting. If one may argue from what occurs in the myelencephalon, it seems probable that the reticular formation of the metencephalon is derived from the ventral zone, and that the median raphe represents the floor-plate. Furthermore, the relations of the pons nuclei to the reticular formation on the one hand, and its connection by means of
Fig. 236. - A, Dorsal View of the Brain or a Rabbit Embryo of 16 mm.; B, Median Longitudinal Section of a Calf Embryo of 3 cm.
c, Cerebellum; m, mid-brain. - (Mihalkovicz?) the transverse pons fibers with the cerebellum on the other, suggest the possibility that they may be the metencephalic representatives of the olivary bodies and are formed by a migration ventrally of neuroblasts from the dorsal zones, such a migration having been observed to occur (Essick).
The cerebellum is formed from the dorsal zones and roof-plate of the metencephalon and is a thickening of the tissue immediately anterior to the front edge of the posterior velum. This latter structure has in early stages a rhomboidal shape (Fig. 236, A) which causes the cerebellar thickening to appear at first as if composed of two lateral portions inclined obliquely toward one another. In reality, however, the thickening extends entirely across the roof of
the brain (Fig. 236, B), the roof-plate probably being invaded by cells from the dorsal zones and so giving rise to the vermis, while the lobes are formed directly from the dorsal zones. During the second month a groove appears on the ventral surface of each lobe, marking out an area which becomes the flocculus, and later, during the third month, transverse furrows appear upon the vermis dividing it into five lobes, and later still extend out upon the lobes and increase in number to produce the lamellate structure characteristic of the cerebellum.
The histogenetic development of the cerebellum at first proceeds along the lines which have already been described as typical, but after the development of the mantle layer the cells lining the greater portion of the cavity of the ventricle rease to rrmltinlv onlv those FlG - 237- - Diagram Representing the cease to multiply, oniy tnose DifferenT iation of the Cerebellar Cells.
which are situated in the roof- The circles, indifferent cells; circles with plate of the metencephalon d Â°f ' n< r ur . g lia c f s > shaded c ? lls : gâ„¢. al 1 r cells; circles with cross, germinal cells in and along the line of junction mitosis; black cells, nerve-cells. L, Lateral . , , ,, ,i â€¢ i â€¢ recess; M, median furrow, and R, floor of IV, of the cerebellar thickening fourth ven tricle. - (Schaper.) with the roof-plate continuing to divide. The indifferent cells formed in these regions migrate outward from the median line and forward in the marginal velum to form a superficial layer, known as the epithelioid layer, and cover the entire surface of the cerebellum (Fig. 237). The cells of this layer, like those of the mantle, differentiate into neuroglia cells and neuroblasts, the latter for the most part migrating centrally at a later stage to mingle with the cells of the mantle layer and to become transformed into the granular cells of the cerebellar cortex. The neuroglia cells remain at the surface, however, forming the principal constituent of the outer or, as it is now termed, the molecular layer of the cortex, and into this the dendrites of the Purkinje cells,
394 THE isthmus probably derived from the mantle layer, project. The migration of the neuroblasts of the epithelial layer is probably completed before birth, at which time but few remain in the molecular layer to form the stellate cells of the adult. The origin of the dentate and other nuclei of the cerebellum is at present unknown, but it seems probable that they arise from cells of the mantle layer.
The nerve-fibers which form the medullary substance of the cerebellum do not make their appearance until about the sixth month, when they are to be found in the ependymal tissue on the inner surface of the layer of granular cells. Those which are not commissural or associative in function converge to the line of junction of the cerebellum with the pons, and there pass into the marginal velum of the pons, myelencephalon, or isthmus as the case may be.
The dorsal surface of the isthmus is at first barely distinguishable from the cerebellum, but as development proceeds its roof-plate undergoes changes similar to those occurring in the medulla oblongata and becomes converted into the anterior velum. In the dorsal portion of its marginal velum fibers passing to and from the cerebellum appear and form the brachia conjunctiva, while ventrally fibers, descending from the more anterior portions of the brain, form the cerebral peduncles. Nothing is at present known as to the history of the gray matter of this division of the brain, although it may be presumed that its ventral zones take part in the formation of the tegmentum, while from its dorsal zones the nuclei of the brachia conjunctiva are possibly derived.
The following table gives the origin of the principal structures of the metencephalon and isthmus: Metencephalon. Isthmus.
/ Posterior velum. Anterior velum.
^ Vermis of cerebellum.
Lobes of cerebellum. Brachia conjunctiva.
Nuclei of termination of sensory roots of cranial nerves. Pons nuclei.
THE MESENCEPHALON 395 Metencephalon. Isthmus.
f Nuclei of origin of motor Posterior part of cerebral Ventral zones -j roots of cranial nerves. peduncles.
[ Reticular formation. Posterior part of tegmentum.
Floor-plate Median raphe. Median raphe.
The Development of the Mesencephalon. - Our knowledge of the development of this portion of the brain is again very imperfect. During the stages when the flexures of the brain are well marked (Figs. 232 and 233) it forms a very prominent structure and possesses for a time a capacious cavity. Later, however, it increases in size less rapidly than adjacent parts and its walls thicken, the roofand floor-plates as well as the zones, and, as a result, the cavity becomes the relatively smaller canal-like cerebral aquaeduct. In the marginal velum of its ventral zone fibers appear at about the third month, forming the anterior portion of the cerebral peduncles, and, at the same time, a median longitudinal furrow appears upon the dorsal surface, dividing it into two lateral elevations which, in the fifth month, are divided transversely by a second furrow and are thus converted from corpora bigemina (in which form they are found in the lower vertebrates) into corpora quadrigemina.
Nothing is known as to the differentiation of the gray matter of the dorsal and ventral zones of the mid-brain. From the relation of the parts in the adult it seems probable that in addition to the nuclei of origin of the oculomotor and trochlear nerves, the ventral zones give origin to the gray matter of the tegmentum, which is the forward continuation of the reticular formation. Similarly it may be supposed that the corpora quadrigemina are developments of the dorsal zones, as may also be the red nuclei, whose relations to the brachia conjunctiva suggest a comparison with the olivary bodies and the nuclei of the pons.
A tentative scheme representing the origin of the mid-brain structures may be stated thus: Roof -plate (?) J Corpora quadrigemina. .LJorsal zones. ...... \ .
^ Red nuclei.
[ Nuclei of origin of the third and fourth nerves. Ventral zones \ Anterior part of tegmentum.
[ Anterior part of cerebral peduncles. Floor-plate Median raphe.
The Development of the Diencephalon. - A transverse section through the diencephalon of an embryo of about five weeks (Fig. 238) shows clearly the differentiation of this portion of the brain into the typical zones, the roof-plate (rp) being represented by a thinwalled, somewhat folded area, the floor-plate (fp) by the tissue forming the floor of a well-marked ventral groove, while each lateral wall is divided into a dorsal and ventral zone by a groove known as the sulcus Monroi (Sm), which extends forward and ventrally toward the point of origin of the optic evagination (Fig. 240). At the posterior end of the ridge-like elevation which represents the roof-plate is a rounded elevation (Fig. 239, p) which, in later stages, elongates until it almost reaches the dermis, forming a hollow evagination of the brain roof known as the pineal process. The distal extremity of this process enlarges to a sac-like structure which later beFig. 238. - Transverse Section comes lobed, and, by an active pro of the Diencephalon of an Em- Hferation of the cells lining the cavibryo of Five Weeks. ^ dz, Dorsal zone; fp, floor-plate; tieS Â° f the various lobes, finally be
rp, roof-plate; Sm, sulcus Monroi vz, ventral zone. - (His.)
comes a solid structure, the pineal body.
The more proximal portion of the evagination, remaining hollow, forms the pineal stalk, and the entire structure, body and stalk, constitutes what is known as the epiphysis.
The significance of this organ in the Mammalia is doubtful. In the Reptilia and other lower forms the outgrowth is double, a secondary outgrowth arising from the base or from the anterior wall of the primary one. This anterior evagination elongates until it reaches the dorsal epidermis of the head, and, there expanding, develops into an unpaired eye, the epidermis which overlies it becoming converted into a transparent cornea. In the Mammalia this anterior process does not develop and the epiphysis in these forms is comparable only to the posterior process of the Reptilia.
In addition to the epiphysial evaginations, another evagination arises
from the roof-plate of the first brain vesicle, further forward, in the region which becomes the median portion of the telencephalon. This paraphysis as it has been called, has been observed in the lower vertebrates and in the Marsupials (Selenka), but up to the present has not been found in other groups of the Mammalia. It seems to be comparable to a chorioid plexus which is evaginated from the brain surface instead of being invaginated as is usually the case. There is no evidence that a paraphysis is developed in the human brain.
The portion of the roof-plate which lies in front of the epiphysis represents the velum interpositum of the adult brain, and it forms at first a distinct ridge (Fig. 239, rp). At an early stage, however, it becomes reduced to a thin membrane upon the surface of which bloodvessels, developing in the surrounding mesenchyme, arrange themselves at about the third month in two longitudinal plexuses, which, with the subjacent portions of the velum, become invaginated into the â€ž â€ž, riG. 239. - Dorsal View of the cavity of the third ventricle to form Brain, the Roof of the Lateral its chorioid Mexu* Ventricles being Removed, of an us cnomoia plexus. Embryo of 13.6 mm.
The dorsal zones thicken in b, Superior brachiuui; eg, lateral their more dorsal and anterior S eniculate . bod y; C P> chorioid plexus; tneir more aorsai ana anterior cqa> anterior corpu3 quadrigeminum; portions to form massive Structures, h > hippocampus; hf, hippocampal fis, 7 7 . , sure; ot, thalamus; p, pineal body; rp, the thatami [rigs. 233, V2, and roof-plate. - (Aw.) 239, ot), which, encroaching upon the cavity of the ventricle, transform it into a narrow slit-like space, so narrow, indeed, that at about the fifth month the inner surfaces of the two thalami come in contact in the median line, forming what is known as the intermediate mass. More ventrally
398 THE TELENCEPHALON and posteriorly another thickening of the dorsal zone occurs, giving rise on each side to the pulvinar of the thalamus and to a lateral geniculate body, and two ridges extending backward and dorsally from the latter structures to the thickenings in the roof of the midbrain which represent the anterior corpora quadrigemina, give a path along which the nerve-fibers which constitute the superior quadrigeminal brachia pass.
From the ventral zones what is known as the hypothalamic region develops, a mass of fibers and cells whose relations and development are not yet clearly understood, but which may be regarded as the forward continuation of the tegmentum and reticular formation. In the median line of the floor of the ventricle an unpaired thickening appears, representing the corpora mamillaria, which during the third month becomes divided by a median furrow into two rounded eminences; but whether these structures and the posterior portion of the tuber cinereum, which also develops from this region of the brain, are. derivatives of the ventral zones or of the floor-plate is as yet uncertain.
Assuming that the mamillaria and the tuber cinereum are derived from the ventral zones, the origins of the structures formed from the walls of the diencephalon may be tabulated as follows: ^ . , f Velum interpositum.
Roof-plate < ^ . . . ^ (_ Epiphysis.
(Thalami. Pulvinares. Lateral geniculate bodies. (Hypothalamic region. Corpora mamillaria. Tuber cinereum (in part) . Floor-plate Tissue of mid-ventral line.
The Development of the Telencephalon.- - For convenience of description the telencephalon may be regarded as consisting of a median portion, which contains the anterior part of the third ventricle, and two lateral outgrowths which constitute the cerebral hemispheres. The roof of the median portion undergoes the same transformation as does the greater portion of that of the diencephalon
THE TELENCEPHALON 399 and is converted into the anterior part of the velum interpositum (Fig. 240, vi). Anteriorly this passes into the anterior wall of the third ventricle, the lamina terminalis (It), a structure which is to be regarded as formed by the union of the dorsal zones of opposite sides, since it lies entirely dorsal to the anterior end of the sulcus Monroi. From the ventral part of the dorsal zones the optic evaginations are formed, a depression, the optic recess (or), marking their point of origin.
The ventral zones are but feebly developed, and form the anterior part of the hypothalamic region, while at the anterior extremity of the floor-plate an evagination occurs, the infundibular recess (ir), which elongates to form a funnel-shaped structure known as the hypophysis. At its extremity the hypophysis comes in contact during the fifth week with the enlarged extremity of Rathke's pouch formed by an invagination of the roof of the oral sinus (see p. 285), and applies itself closely to the posterior surface of this (Fig. 233) to form with it the pituitary body. The anterior lobe at an early stage separates from the mucous membrane of the oral sinus, the stalk by which it was attached completely disappearing, and toward the end of the second month it begins to send out processes from its walls into the surrounding mesenchyme and so becomes converted into a mass of solid epithelial cords embedded in a mesenchyme rich in blood and lymphatic vessels. The cords later on divide transversely to a greater or less extent to form alveoli, the entire structure coming to resemble somewhat the parathyreoid bodies (see p. 297), and, like these, having the function of producing an internal secretion. The posterior lobe, derived from the brain, retains its connection with that structure, its stalk being the injundibidum, but its terminal portion does not undergo such extensive modifications as does the anterior lobe, although it is claimed that it gives rise to a glandular epithelium which may become arranged so as to form alveoli.
The cerebral hemispheres are formed from the lateral portions of the dorsal zones, each possessing also a prolongation of the roofplate. From the more ventral portion of each dorsal zone there is
400 THE TELENCEPHALON formed a thickening, the corpus striatum (Figs. 240, cs, and 233, VI 2), a structure which is for the telencephalon what the optic thalamus is for the diencephalon, while from the more dorsal portion there is formed the remaining or mantle (pallial) portions of the hemispheres (Figs. 240, h, and 233, VI 4). When first formed, the hemispheres are slight evaginations from the median portion of the telencephalon, the openings by which their cavities communicate with the third ventricle, the interventricular foramina, being relatively very large (Fig. 240), but, in later stages (Fig. 233), the hemispheres increase more markedly and eventually surpass all the other portions of the
- â€¢â– /
Fig. 240. - Median Longitudinal Section of the Brain of an Embryo of 16.3 mm. br, Anterior brachium; eg, corpus geniculatum laterale; cs, corpus striatum; h, cerebral hemisphere; ir, infundibular recess; It, lamina terminalis; or, optic recess; ot, thalamus; p, pineal process; sm, sulcus Monroi; st, hypothalamic region; vi, velum interpositum. - (His.) brain in magnitude, overlapping and completely concealing the roof and sides of the diencephalon and mesencephalon and also the anterior surface of the cerebellum. In this enlargement, however, the interventricular foramina share only to a slight extent, and consequently become relatively smaller (Fig. 233), forming in the adult merely slit-like openings lying between the lamina terminalis and the thalami and having for their roof the anterior portion of the velum interpositum.
The velum Interpositum - that is to say, the roof-plate - where
it forms the roof of the interventricular foramen, is prolonged out upon the dorsal surface of each hemisphere, and, becoming invaginated, forms upon it a groove.' As the hemispheres, increasing in height, develop a mesial wall, the groove, which is the so-called chorioidal fissure, comes to lie along the ventral edge of this wall, and as the growth of the hemispheres continues it becomes more and more elongated, being carried at first backward (Fig. 241), then ventrally, and finally forward to end at the tip of the temporal lobe. After the establishment of the grooves the mesenchyme in their vicinity dips into them, and, developing blood-vessels, becomes the chorioid plexuses of the lateral ventricles, and at first these plexuses grow much more rapidly than the ventricles, and so fill them almost completely. Later, however, the walls of the hemispheres gain the ascendancy in rapidity of growth and the plexuses become relatively much smaller. Since the portions of the roof-plate which form the chorioidal fissures are continuous with the velum interpositum in the roofs of the interventricular foramina, the chorioid plexuses of the lateral and third ventricles become continuous also at that point.
-Median Longitudinal Section of the Brain of an Embryo Calf of 5 cm.
cb, Cerebellum; cp, chorioid plexus; cs, corpus striatum; JM, interventricular foramen; in, The mode of growth of the chorioid hypophysis; m, mid-brain; oc, , optic commissure; t, posterior fissures seems to indicate the mode of par t of the diencephalon - growth of the hemispheres. At first the Wihalkovicz.) growth is more or less equal in all directions, but later it becomes more extensive posteriorly, there being more room for expansion in that direction, and when further extension backward becomes difficult the posterior extremities of the hemispheres bend ventrally toward the base of the cranium, and reaching this, turn forward to form the temporal lobes. As a result the cavities of the hemispheres, the lateral ventricles, in addition to being carried forward to form an anterior horn, are also carried backward and ventrally to form the lateral or descending horn, and the corpus striatum likewise extends 26
402 THE TELENCEPHALON backward to the tip of each temporal lobe as a slender process known as" the tail of the caudate nucleus. In addition to the anterior and lateral horns, the ventricles of the human brain also possess posterior horns extending backward into the occipital portions of the hemispheres, these portions, on account of the greater persistence of the mid-brain flexure (see p. 388), being enabled to develop to a greater extent than in the lower mammals.
The scheme of the origin of parts in the telencephalon may be stated as follows: Median Part. Hemispheres.
â€ž , , f Anterior part of velum inter- f _ , n ..,,,.
Roof-plate < . < Moor of chonoidai nssure.
(^ positum. [ r , . ... Pallium.
-r. , Lamina terminahs. _ Dorsal zones â– (_... < Corpus striatum.
Optic evaginations. _,, , , . ..
> . , . Olfactory lobes (see p. 406) Anterior part of hypothalamic [ Ventral zones < region.
[ Anterior part of tuber cinereum.
The Convolutions of the Hemispheres. - The growth of the hemispheres to form the voluminous structures found in the adult depends mainly upon an increase of size of the pallium. The corpus striatum, although it takes part in the elongation of each hemisphere, nevertheless does not increase in other directions as rapidly and extensively as the pallium, and hence, even in very early stages, a depression appears upon the surface of the hemispheres where the corpus is situated (Fig. 242). This depression is the lateral cerebral fossa, and for a considerable period it is the only sign of inequality of growth on the outer surfaces of the hemispheres. Upon the mesial surfaces, however, at about the time that the choroid fissure appears, another linear depression is formed dorsal to the chorioid, and when fully formed extends from in front of the interventricular foramen to the tip of the temporal lobe (Fig. 244, h). It affects the entire thickness of the pallial wall and consequently produces an elevation upon the inner surface, a projection into the cavity of the ventricle which is known as the hippocampus, whence
THE CEREBRAL CONVOLUTIONS
the fissure may be termed the hippocampal fissure. The portion of the pallium which intervenes between this fissure and the chorioidal forms what is known as the dentate gyrus.
Toward the end of the third or the beginning of the fourth month two prolongations arise from the fissure just where it turns to be continued into the temporal lobe, and these, extending posteriorly, give rise to the parieto-occipital and calcarine fissures. Like the hippocampal, these fissures produce elevations upon the inner surface of the pallium, that formed by the parieto-occipital early disappearing, while that produced by the calcarine persists to form the calcar (hippocampus minor) of adult anatomy.
The three fissures just described, together with the chorioidal and the lateral cerebral fossa, are all formed by the beginning of the fourth month and all the fissures affect the entire thickness of the wall of the hemisphere, and hence have been termed the primary or total fissures. Until the beginning of the fifth month they are the only fissures present, but at that time secondary fissures, which, with one exception, are merely furrows of the surface of the pallium, make their appearance and continue to form until birth and possibly later. Before considering these, however, certain changes which occur in the neighborhood of the lateral cerebral fossa may be described.
The fossa is at first a triangular depression situated above the temporal lobe on the surface of the hemisphere. During the fourth month it deepens considerably, so that its upper and lower margins become more pronounced and form projecting folds, and, during the fifth month, these two folds approach one another and eventually
Fig. 242. - Brain of an Embryo of the Fourth Month. c, Cerebellum; p, pons; s, lateral cerebral fossa.
TEE CEREBRAL CONVOLUTIONS
cover in the floor of the fossa completely, the groove which marks the line of their contact forming the lateral cerebral fissure, while the floor of the fossa becomes known as the insula.
The first of the secondary fissures to appear is the sulcus cinguli, which is formed about the middle of the fifth month on the mesial surface of the hemispheres, lying parallel to the anterior portion of the hippocampal fissure and dividing the mesial surface into the gyri marginalis and fornicatus. A little later, at the beginning of the sixth month, several other fissures make their appearance upon
Fig. 243. - Cerebral Hemisphere oe an Embryo of about the Seventh Month. fs, Superior frontal sulcus; ip, interparietal; IR, insula; pet, inferior pre-central; pes, superior pre-central; ptc, post-central; R, central; S, lateral; t 1 , first temporal. - (Cunningham )
the outer surface of the pallium, the chief of these being the central sulcus, the inter-parietal, the pre- and post-central, and the temporal sulci, the most ventral of these last running parallel with the lower portion of the hippocampal fissure and differing from the others in forming a ridge on the wall of the ventricle termed the collateral eminence, whence the fissure is known as the collateral. The position of most of these fissures may be seen from Fig. 243, and for a more
THE CORPUS CALLOSUM
complete description of them reference may be had to text-books of descriptive anatomy.
In later stages numerous tertiary fissures make their appearance and mask more or less extensively the secondaries, than which they are, as a rule, much more inconstant in position and shallower. The Corpus Callosum and Fornix. - While these fissures have been forming, important structures have developed in connection with the lamina terminalis. Up to about the fourth month the lamina is thin and of nearly uniform thickness throughout, but at this time it begins to thicken near its dorsal edge and fibers appear in the thickening. These fibers belong to three sets. In the first place, certain of them arise in connection with the olfactory tracts (see p. 407) and from the region of the hippocampal gyrus, which is also associated with the olfactory sense, and, passing through tbe substance of the lamina terminalis, they extend across the median line to the corresponding regions of the opposite cerebral hemisphere. They are therefore commissural fibers and form what is termed the anterior commissure (Figs. 244, ca and 245, ac). Secondly, fibers, which have their origin from the cells of the hippocampus, develop along the chorioidal edge of that structure, forming what is termed the fimbria. They follow along the edge of the chorioidal fissure and, when this reaches the interventricular foramen, they enter as the pillars of the fornix (Figs. 244, cf; Fig. 245,/) the substance of the lamina terminalis and, passing ventrally in it, eventually reach the hypothalamic region, where they terminate in the corpora mammillaria.
Thirdly, as the mantle develops fibers radiate from all parts of it toward the dorsal portion of the lamina terminalis and traversing it are distributed to the corresponding portions of the mantle of the opposite side. There fibers are also commissural in character and form the corpus callosum (Figs. 244 and 245, cc). With the development of these three sets of fibers and especially those forming the corpus callosum, the dorsal portion of the lamina terminalis becomes enlarged so as to form a triangular area extending between the two cerebral hemispheres (Fig. 245), the corpus callosum form
THE CORPUS CALLOSUM
ing its dorsal portion and base, which is directed anteriorly, the pillars of the fornix its ventral portion, while the anterior commissure occupies its ventral anterior angle.
The portion of the triangle included between the callosum and the fornix remains thin and forms the septum pellucidum, and a split occurring in the center of this gives rise to the so-called^///* ventricle, which, from its mode of formation, is a completely closed cavity and is not lined with ependymal tissue of the same nature as that found in the other ventricles.
Owing to the very considerable size reached by the triangular area whose history has just been described, important changes are wrought in the adjoining portions of the mesial surface of the hemispheres. Before the development of the area the gyrus dentatus and the hippocampus extend forward into the anterior portion of the hemispheres (Fig. 244), but on account of their position they become encroached upon by the enlargement of the corpus callosum, with the result that the hippocampus becomes practically obliterated in that portion of its course which lies in the region occupied by the corpus callosum, its fissure in this region becoming known as the callosal fissure, while the corresponding portions of the dentate gyrus become reduced to narrow and insignificant bands of nerve tissue which rest upon the upper surface of the corpus callosum and are known as the lateral longitudinal stria.
The Olfactory Lobes. - At the time when the cerebral hemispheres
Fig. 244. - Median Longitudinal Section or the Brain of an Embryo of Four Months.
c, Calcarine fissure; ca, anterior commissure; cc, corpus callosum; cf. chorioidal fissure; dg, dentate gyrus; fm, interventricular foramen; h, hippocampal fissure; po, parieto-o c c i p i t a 1 fissure. - (Mihalkovicz.)
THE OLFACTORY LOBES
begin to enlarge - that it to say, at about the fourth week - a slight furrow, which appears on the ventral surface of each anteriorly, marks off an area which, continuing to enlarge with the hemispheres, gradually becomes constricted off from them to form a distinct lobelike structure, the olfactory lobe (Fig. 233, VI 3). In most of the lower mammalia these lobes reach a very considerable size, and consequently have been regarded as constituting an additional division of the brain, known as the rhinencephalon, but in man they remain smaller, and although they are at first hollow, containing prolongations from the lateral ventricles, the cavities later on disappear and the lobes become solid. Each lobe becomes differentiated into two portions, its terminal portion becoming converted into the club-shaped structure, the olfactory bulb and stalk, while its proximal portion gives rise to the olfactory tracts, the trigone, and the anterior perforated substance.
Histogenesis of the Cerebral Cortex. - A satisfactory study of the histogenesis of the cortex has not yet been made. In embryos of three months a marginal velum is present and probably gives rise to the stratum zonale of the adult brain; beneath this is a cellular layer, perhaps representing the mantle layer; beneath this, again, a layer of nerve-fibers is beginning to appear, representing the white substance of the pallium; and, finally, lining the ventricle is an ependymal layer. In embryos of the fifth month, toward the innermost part of the second layer, cells are beginning to differentiate into the large pyramidal cells, but almost nothing is known as to the
Fig. 245. - Median Longitudinal Section of the Brain oe an Embryo of the Fifth Month.
ac, Anterior commissure; cc, corpus callosum; dg, dentate gyrus;/, fornix; i, infundibulum; mc, intermediate mass; si, septum pellucidum; vi, velum interpositum. - (Mihal kovicz.)
408 THE SPINAL NERVES origin of the other layers recognizable in the adult cortex, nor is it known whether any migration, similar to what occurs in the cerebellar cortex, takes place. The fibers of the white substance do not begin to acquire their myelin sheaths until toward the end of the ninth month, and the process is not completed until some time after birth (Flechsig), while the fibers of the cortex continue to undergo myelination until comparatively late in life (Kaes).
The Development of the Spinal Nerves. - It has already been seen that there is a fundamental difference in the mode of development of the two roots of which the typical spinal nerves are composed, the ventral root being formed by axis-cylinders which arise from neuroblasts situated within the substance of the spinal cord, while the dorsal roots arise from the cells of the neural crests, their axiscylinders growing into the substance of the cord while their dendrites become prolonged peripherally to form the sensory fibers of the nerves. Throughout the thoracic, lumbar and sacral regions of the cord the fibers which grow out from the anterior horn cells converge to form a single nerve-root in each segment, but in the cervical region fibers which arise from the more laterally situated neuroblasts make their exit from the cord independently of the more ventral neuroblasts and form the roots of the spinal accessory nerve (see p. 416). In the cervical region there are accordingly three sets of nerve-roots, the dorsal, lateral, and ventral sets.
In a typical spinal nerve, such as one of the thoracic series, the dorsal roots as they grow peripherally pass ventrally as well as outward, so that they quickly come into contact with the ventral roots with whose fibers they mingle, and the mixed nerve so formed soon after divides into two trunks, a dorsal one, which is distributed to the dorsal musculature and integument, and a larger ventral one. The ventral division as it continues its outward growth soon reaches the dorsal angle of the pleuro-peritoneal cavity, where it divides, one branch passing into the tissue of the body- wall while the other passes into the splanchnic mesoderm. The former branch, continuing its onward course in the body- wall, again divides, one branch becoming the lateral cutaneous nerve, while the other continues inward to
THE CRANIAL NERVES 409 terminate in the median ventral portion of the body as the anterior cutaneous nerve. The splanchnic branch forms a ramus communicans to the sympathetic system and will be considered more fully later on.
The conditions just described are those which obtain throughout the greater part of the thoracic region. Elsewhere the fibers of the ventral divisions of the nerves as they grow outward tend to separate from one another and to become associated with the fibers of adjacent nerves, giving rise to plexuses. In the regions where the limbs occur the formation of the plexuses is also associated with a shifting of the parts to which the nerves are supplied, a factor in plexus formation which is, however, much more evident from comparative anatomical than from embryological studies.
The Development of the Cranial Nerves. - During the last thirty years the cranial nerves have received a great deal of attention in connection with the idea that an accurate knowledge of their development would afford a clue to a most vexed problem of vertebrate morphology, the metamerism of the head. That the metamerism which was so pronounced in the trunk should extend into the head was a natural supposition, strengthened by the discovery of head-cavities in the lower vertebrates and by the indications of metamerism seen in the branchial arches, and the problem which presented itself was the correlation of the various structures belonging to each metamere and the determination of the modifications which they had undergone during the evolution of the head.
In the trunk region a nerve forms a conspicuous element of each metamere and is composed, according to what is known as Bell's law, of a ventral or efferent and a dorsal or afferent root. Until comparatively recently the study of the cranial nerves has been dominated by the idea that it was possible to extend the application of Bell's law to them and to recognize in the cranial region a number of nerve pairs serially homologous with the spinal nerves, some of them, however, having lost their afferent roots, while in others a dislocation, as it were, of the two roots had occurred.
The results obtained from investigation along this line have not,
4IO . THE CRANIAL NERVES however, proved entirely satisfactory, and facts have been elucidated which seem to show that it is not possible to extend Bell's law, in its usual form at least, to the cranial nerves. It has been found that it is not sufficient to recognize simply afferent and efferent roots, but these must be analyzed into further components, and when this is done it is found that in the series of cranial nerves certain components occur which are not represented in the nerves of the spinal series.
Before proceeding to a description of these components it will be well to call attention to a matter already alluded to in a previous chapter (p, 84) in connection with the segmentation of the mesoderm of the head. It has been pointed out that while there exist "head-cavities" which are serially homologous with the mesodermal somites of the trunk, there has been imposed upon this primary cranial metamerism a secondary metamerism represented by the branchiomeres associated with the branchial arches, and, it may be added, this secondary metamerism has become the more prominent of the two, the primary one, as it developed, gradually slipping into the background until, in the higher vertebrates, it has become to a very considerable extent rudimentary. In accordance with this double metamerism it is necessary to recognize two sets of cranial muscles, one derived from the cranial myotomes and represented by the muscles of the eyeball, and one derived from the branchiomeric mesoderm, and it is necessary also to recognize for these two sets of muscles two sets of motor nerves, so that, with the dorsal or sensory nerve-roots, there are altogether three sets of nerve-roots in the cranial region instead of only two, as in the spinal region.
These three sets of roots are readily recognizable both in the embryonic and in the adult brain, especially if attention be directed to the cell groups or nuclei with which they are associated (Fig. 246). Thus there can be recognized: (1) a series of nuclei from which nerve-fibers arise, situated in the floor of the fourth ventricle and aquaeduct close to the median line and termed the ventral motor nuclei; (2) a second series of nuclei of origin, situated more laterally
THE CRANIAL NERVES
and in the substance of the formatio reticularis, and known as the lateral motor nuclei; and (3) a series of nuclei in which afferent nervefibers terminate, situated still more laterally in the floor of the ventricle and forming the dorsal or sensory nuclei. None of the twelve cranial nerves usually recognized in the text-books contains fibers associated with all three of these nuclei; the fibers from the lateral motor nuclei almost invariably unite with sensory fibers to form a
Fig. 246. - Transverse Section through the Medulla Oblongata of an Embryo of 10 mm., showing the Nuclei of Origin of the Vagus (X) and Hypoglossal (XII) Nerves. - (His.) mixed nerve, but those from all the ventral motor nuclei form independent roots, while the olfactory and auditory nerves alone, of all the sensory roots (omitting for the present the optic nerve), do not contain fibers from either of the series of motor nuclei. The relations of the various cranial nerves to the nuclei may be seen from the following table, in which the + sign indicates the presence and the - sign the absence of fibers from the nuclear series under which it stands':
THE CRANIAL NERVES
Vagus. 1 Spinal Accessory. J
Two nerves - namely, the second and twelfth - have been omitted from the above table. Of these, the second or optic nerve undoubtedly belongs to ah entirely different category from the other peripheral nerves, and will be considered in the following chapter in connection with the sense-organ with which it is associated (see especially p. 460). The twelfth or hypoglossal nerve, on the other hand, really belongs to the spinal series and has only secondarily been taken up into the cranial region in the higher vertebrates. It has already been seen (p. 170) that the bodies of four vertebrae are included in the basioccipital bone, and that three of the nerves corresponding to these vertebrae are represented in the adult by the hypoglossal and the fourth by the first cervical or suboccipital nerve. The dorsal roots of the hypoglossal nerves seem to have almost disappeared, although a ganglion has been observed in embryos of 7 and 10 mm. in the posterior part of the hypoglossal region (His), and probably represents the dorsal root of the most posterior portion of the hypoglossal nerve. This ganglion disappears, as a rule, in later stages, and it is interesting to note that the ganglion of the suboccipital nerve is also occasionally wanting in the adult condition. The hypoglossal roots are to be regarded, then, as equivalent to the ventral roots of the cervical spinal nerves, and the nuclei from which they arise lie in series with the cranial ventral motor roots, a
THE CRANIAL NERVES 413 fact which indicates the equivalency of these latter with the fibers which arise from the neuroblasts of the anterior horns of the spinal cord.
The equivalents of the lateral motor roots may more conveniently be considered later on, but it may be pointed out here that these are the fibers which are distributed to the muscles of the branchiomeres. In the case of the sensory nerves a further analysis is necessary before their equivalents in the spinal series can be determined. For this the studies which have been made in recent years of the components entering into the cranial nerves of the amphibia (Strong) and fishes (Herrick) must supply a basis, since as yet a direct analysis of the mammalian nerves has not been made. In the forms named it has been found that three different components enter into the formation of the dorsal roots of the cranial nerves: (i) fibers belonging to a general cutaneous or somatic sensory system, distributed to the skin without being connected with any special sense-organs; (2) fibers belonging to what is termed the communis or viscerosensory system, distributed to the walls of the mouth and pharyngeal region and to special organs found in the skin of the same character as those occurring in the mouth; and (3) fibers belonging to a special set of cutaneous sense-organs largely developed in the fishes and known as the organs of the lateral line.
The fibers of the somatic sensory system converge to a group of cells, situated in the lateral part of the floor of the fourth ventricle and forming what is termed the trigeminal lobe, and also extend posteriorly in the substance of the medulla (Fig. 247), forming what has been termed the spinal root of the trigeminus and terminating in a column of cells which represents the forward continuation of the posterior horn of the cord. In the fishes and amphibia fibers belonging to this system are to be found in the fifth, seventh, and tenth nerves, but in the mammalia their distribution has apparently become more limited, being confined almost exclusively to the trigeminus, of whose sensory divisions they form a very considerable part. Since the cells around which the fibers of the spinal root of the trigeminus terminate are the forward continuations of the posterior
THE CRANIAL NERVES
horn of the cord, it seems probable that the fibers of this system are the cranial representatives of the posterior roots of the spinal nerves, which, it may be noted, are also somatic in their distribution. The fibers of the viscero-sensory system are found in the lower forms principally in the ninth and tenth nerves (see Fig. 247), although groups of them are also incorporated in the seventh and fifth. They converge to a mass of cells, known as the lobus vagi, and like the first set are also continued down the medulla to form
Fig. 247. - Diagrams showing the Sensory Components of the Cranial Nerves of a Fish (Menidia) . The somatic sensory system is unshaded, the viscero-sensory is cross-hatched, and the lateral line system is black, asc.v, Spinal root of trigeminus; brx, branchial branches of vagus; Ix, lobus vagi; ol, olfactory bulb; op, optic nerve; rc.x, cutaneous branch of the vagus; rix, intestinal branch of vagus; rl, lateral line nerve; rl.acc, accessory lateral line nerve; ros, superficial ophthalmic; rp, ramus palatinus of the facial; thy, hyomandibular branch of the facial; t.inf, infraorbital nerve. - (Herrick.) a tract known as the fasciculus solitarius or: fasciculus communis. In the mammalia the system is represented by the sensory fibers of the glosso-pharyngeo-vagus set of nerves, of which it represents practically the entire mass; by the sensory fibers of the facial arising from the geniculate ganglion and included in the chorda tympani and probably also the great superficial petrosal; and also, probably, by
THE CRANIAL NERVES 415 the lingual branch of the trigeminus. Furthermore, since the mucous membrane of the palate is supplied by branches from the trigeminus which pass by way of the spheno-palatine (Meckel's) ganglion, and the same region is supplied in lower forms by a palatine branch from the facial, it seems probable that the palatine nerves of the mammalia are also to be assigned to this system.* If this be the case, a very evident clue is afforded to the homologies of the system in the spinal nerves, for since the spheno-palatine ganglion is to be regarded as part of the sympathetic system, the sensory fibers which pass from the viscera to the spinal cord by way of the sympathetic system (p. 420) present relations practically identical with those of the palatine nerves.
Finally, with regard to the system of the lateral line, there seems but little doubt that it has no representation whatsoever in the spinal nerves. It is associated with a peculiar system of cutaneous senseorgans found only in aquatic or marine animals, and also with the auditory and possibly the olfactory organs, the former of which are certainly and the latter possibly primarily parts of the lateral line system of organs. The organs are principally confined to the head, although they also extend upon the trunk, where they are followed by a branch from the vagus nerve, the entire system being accordingly supplied by cranial nerves. In the fishes, in which the development of the organs is at a maximum, fibers belonging to the system are found in all the branchiomeric nerves and all converge to a portion of the medulla known as the tuberculum acusticum. In the Mammalia, with the disappearance of the lateral line organs there has been a disappearance of the associated nerves, and the only certain representative of the system which persists is the auditory nerve.
The table given on page 412 may now be expanded as follows, though it must be recognized that such an analysis of the mammalian nerves is merely a deduction from what has been observed in lower
- The fact that the palatine branches are associated with the trigeminus in the Mammalia and with the facial in the Amphibia is readily explained by the fact that in the latter the Gasserian and geniculate ganglia are not always separated, so that it is possible for fibers originating from the compound ganglion to pass into either
THE CRANIAL NERVES
forms, and may require some modifications when the components have been subjected to actual observation:
An additional word is necessary concerning the spinal accessory nerve, for it presents certain interesting relations which possibly furnish a clue to the spinal equivalents of the lateral motor roots. In the first place, the neuroblasts which give rise to those fibers of the nerve which come from the spinal cord are situated in the dorsal part of the ventral zones. As the nuclei of origin are traced anteriorly they will be found to change their position somewhat as the medulla is reached and eventually come to lie in the reticular formation, the most anterior of them being practically continuous with the motor nucleus of the vagus. Indeed, it seems that the spinal accessory nerve is properly to be regarded as an extension of the vagus downward into the cervical region (Furbringer, Streeter), a process which reaches its greatest development in the mammalia and seems to-stand in relation to the development of those portions of the trapezius and sterno-mastoid muscles which are supplied by the spinal accessory nerve.
It is believed that the white rami communicantes which pass from the spinal cord to the thoracic and upper lumbar sympathetic
THE CRANIAL NERVES 417 ganglia arise from cells situated in the dorso-lateral portions of the ventral horns, and it is noteworthy that white rami are wanting in the region in which the spinal accessory nerve occurs. Since this nerve represents a cranial lateral motor root the temptation is great to regard the cranial lateral motor roots as equivalent to the white rami of the cord, and this temptation is intensified when it is recalled that there are both embryological and topographical reasons for regarding the branchiomeric muscles, to which the cranial lateral motor nerves are supplied, as equivalent to the visceral muscles of the trunk. But in view of the fact that a sympathetic neurone is always interposed between a white ramus fiber and the visceral musculature (see Fig. 249), while the lateral motor fibers connect directly with the branchiomeric musculature, it seems advisable to await further studies before yielding to the temptation.
As regards the actual development of the cranial nerves, they follow the general law which obtains for the spinal nerves, the motor fibers being outgrowths from neuroblasts situated in the walls of the neural tube, while the sensory nerves are outgrowths from the cells of ganglia situated without the tube. In the lower vertebrates a series of thickenings, known as the suprabranchial placodes, are developed from the ectoderm along a line corresponding with the level of the auditory invagination, while on a line corresponding with the upper extremities of the branchial clefts another series occurs which has been termed that of the epibranchial placodes, and with both of these sets of placodes the cranial nerves are in connection. In the human embryo epibranchial placodes have been found in connection with the fifth, seventh, ninth and tenth nerves, to whose ganglia they contribute cells. The suprabranchial placodes, which in the lower vertebrates are associated with the lateral line nerves, are unrepresented in man, unless, as has been maintained, the sense-organs of the internal ear are their representatives.
From what has been said above it is clear that the usual arrangement of the cranial nerves in twelve pairs does not represent their true relationships with one another. The various pairs are serially homologous neither 27
418 THE SYMPATHETIC SYSTEM with one another nor with the typical spinal nerves, nor can they be regarded as representing twelve cranial segments. Indeed, it would seem that comparatively little information with regard to the number of myotomic segments which have fused together to form the head is to be derived from the cranial nerves, for while there are only four of these nerves which are associated with structures equivalent to the mesodermic somites of the trunk, a much greater number of head cavities or mesodermic somites has been observed in the cranial region of the embryos of the lower vertebrates, Dohrn, for instance, having found nineteen and Killian eighteen in the cranial region of Torpedo. Furthermore, it is not possible to say at present whether the branchiomeres and their associated nerves correspond with one or several of the cranial mesodermic somites, or whether, indeed, any correspondence whatever exists.
In early stages of development a series of constrictions have been observed in the cranial portion of the neural tube and have been regarded as indicating a primitive segmentation of that structure. The neuromeres, as the intervals between successive constrictions have been termed, seem to correspond with the cranial nerves as usually recognized and hence cannot be regarded as primitive segmental structures. They are more probably secondary and due to the arrangement of the neuroblasts corresponding to the various nerves.
The Development of the Sympathetic Nervous System. - From the embryological standpoint the distinction which has been generally recognized between the sympathetic and central nervous systems does not exist, the former having been found to be an outgrowth from the latter. This mode of origin has been observed with especial clearness in the embryos of some of the lower vertebrates, in which masses of cells have been seen to separate from the posterior root ganglia to form the ganglia of the ganglionated cord (Fig. 248). In the mammalia, including man, the relations of the two sets of ganglia to one another is by no means so apparent, since the sympathetic cells, instead of being separated from the posterior root ganglion en masse, migrate from it singly or in groups, and are therefore less readily distinguishable from the surrounding mesodermal tissues.
To understand the development of the sympathetic system it must be remembered that it consists typically of three sets of ganglia. One of these is constituted by the ganglia of the ganglionated cord (Fig. 249, GC), the second is represented by the ganglia of the
THE SYMPATHETIC SYSTEM
Fig. 248. - Transverse Section through an Embryo Shark (Scyllium) of ii mm., SHOWING THE ORIGIN OF A SYMPATHETIC GANGLION.
Ch, Notochord; E, ectoderm; G, posterior root ganglion; Gs, sympathetic ganglion; .1/, spinal cord. - (Onodi.)
THE SYMPATHETIC SYSTEM
prevertebral plexuses (PVG), such as the cardiac, solar, hypogastric, and pelvic, while the third or peripheral set (PG) is formed by the cells which occur throughout the tissues of probably most of the visceral organs, either in small groups or scattered through plexuses such as the Auerbach and Meissner plexuses of the intestine. Each cell in these various ganglia stands in direct contact with the axiscylinder of a cell situated in the central nervous system, probably in the lateral horn of the spinal cord or the corresponding region of the brain, so that each cell forms the terminal link of a chain whose first link is a neurone belonging to the central system (Huber) . Through
Fig. 249. - Diagram showing the Arrangement of the Neurones of the Sympathetic System. The fibers from the posterior root ganglia are represented by the broken black lines; those from the anterior horn cells by the solid black; the white rami by red; and the sympathetic neurones by blue. DR, Dorsal ramus of spinal nerve; GC, ganglionated cord; GR, gray ramus communicans; PG, peripheral ganglion; PVG, prsevertebral ganglion; VR, ventral ramus of spinal nerve; WR, white ramus communicans. - (Adapted from Huber.) out the thoracic and upper lumbar regions of the body the central system neurones form distinct cords known as the white rami communicantes (Fig. 249, WR), which pass from the spinal nerves to the adjacent ganglia of the ganglionated cord, some of them terminating around the cells of these ganglia, others passing on to the cells of the prsevertebral ganglia, and others to those of the peripheral plexuses. In the cervical, lower lumbar and sacral regions white rami are wanting, the central neurones in the first-named region probably making their way to the sympathetic cells by way of the upper
THE SYMPATHETIC SYSTEM
thoracic nerves, while in the lower regions they may pass down the ganglionated cord from higher regions or may join the prevertebral and peripheral ganglia directly without passing through the proximal ganglia. In addition to these white rami, what are known as gray rami also extend between the proximal ganglia and the spinal nerves; these are composed of fibers, arising from sympathetic cells,
Fig. 250. - Transverse Section through the Spinal Cord of an Embryo of 7 mm.
c, Notochord; g, posterior root ganglion; m, spinal cord; s, sympathetic cell migrating from the posterior root ganglion; wr, white ramus.- - (His.) which join the spinal nerves in order to pass with them to their ultimate distribution.
The brief description here given applies especially to the sympathetic system of the neck and trunk. Representatives of the system are also found in the head, in the form of a series of ganglia connected with the trigeminal and facial nerves and known as the ciliary, spheno-palatine, otic, and submaxillary ganglia; and, as will
422 THE SYMPATHETIC SYSTEM be seen later, there are probably some sympathetic cells which owe their origin to the root ganglia of the vagus and glossopharyngeal nerves. There is nothing, however, in the head region corresponding to the longitudinal bundles of fibers which unite the various proximal ganglia of the trunk to form the ganglionated cord.
The first distinct indications of the sympathetic system are to be seen in a human embryo of about 7 mm. As the spinal nerves reach the level of the dorsal edge of the body-cavity, they branch, one of the branches continuing ventrally in the body-wall, while the other (Fig 250, wr) passes mesially toward the aorta, some of its fibers reaching that structure, while others bend so as to assume a longitudinal direction. These mesial branches represent the white rami communicantes, but as yet no ganglion cells can be seen in their course. The cells of the posterior root ganglia have already, for the most part, assumed their bipolar form, but among them there may still be found a number of cells in the neuroblast condition, and these (Fig. 250, s), wandering out from the ganglia, give rise to a column of cells standing in relation to the white rami. At first there is no indication of a segmental arrangement of the cells of the column (Fig. 251), but at about the seventh week such an arrangement makes its appearance in the cervical region, and later, extends posteriorly, until the column assumes the form of the ganglionated cord.
This origin of the ganglionated cord from cells migrating out from the posterior root ganglia has been described by various authors, but recently the origin of the cells has been carried a step further back, to the mantle layer of the central nervous system (Kuntz). Indifferent cells and neuroblasts are said to wander out from the walls of the medullary canal by way of both the posterior and anterior nerve roots and it is claimed that these are the cells that give rise to the ganglionated cord in the manner just described.
Before, however, the segmentation of the ganglionated cord becomes marked, thickenings appear at certain regions of the cell column, and from these, bundles of fibers may be seen extending ventrally toward the viscera. The thickenings represent certain of
THE SYMPATHETIC SYSTEM
the prevertebral ganglia, and later cells wander out from them and take a position in front of the aorta. In an embryo of 10.2 mm. two ganglionic masses (Fig. 251, pc) occur in the vicinity of the origin
Fig. 251. - Reconstruction of the Sympathetic System of an Embryo of 10.2 mm. am, Vitelline artery; ao, aorta; au, umbilical artery; bg, ganglionic mass representing the pelvic plexus; d, intestine; oe, oesophagus; pc, ganglia of the cceliac plexus; ph, pharynx; rv, right vagus nerve; sp, splanchnic nerves; sy, ganglionated cord; t, trachea; *, peripheral sympathetic ganglia in the walls of the stomach. - (His, Jr.)
of the vitelline artery (am), one lying above and the other below that vessel; these masses represent the ganglia of the cceliac
424 LITERATURE plexus and have separated somewhat from the ganglionated cord, the fiber bundles which unite the upper mass with the cord representing the greater and lesser splanchnic nerves (sp), while that connected with the lower mass represents the connection of the cord with the superior mesenteric ganglion. Lower down, in the neighborhood of the umbilical arteries, is another enlargement of the cord (bg), which probably represents the inferior mesenteric and hypogastric ganglia which have not yet separated from the cell column.
With the peripheral ganglia the conditions are slightly different, in that they are formed very largely, if not exclusively, from cells that migrate from the walls of the hind-brain by way of the vagus nerves (Fig. 251). In this way the ganglia of the myenteric, pulmonary and cardiac plexuses are formed, though in the case of the last named it is probable that contributions are also received from the ganglionated cord.
The elongated courses of the cardiac sympathetic and splanchnic nerves in the adult receive an explanation from the recession of the heart arid diaphragm (see pp. 239 and 322), the latter process forcing downward the coeliac plexus, which originally occupied a position opposite the region of the ganglionated cord from which the splanchnic nerves arise.
As regards the cephalic sympathetic ganglia, the observations of Remak on the chick and Kolliker on the rabbit show that the ciliary, sphenopalatine, and otic ganglia arise by the separation of cells from the semilunar (Gasserian) ganglion, and from their adult relations it may be supposed that the cells of the submaxillary and sublingual ganglia have similarly arisen from the geniculate ganglion of the facial nerve. Evidence has also been obtained from human embryos that sympathetic cells are derived from the ganglia of the vagus and glossopharyngeal nerves, but, instead of forming distinct ganglia in the adult, these, in all probability, associate themselves with the first cervical ganglia of the ganglionated cord.
C. R. Bardeen: "The Growth and Histogenesis of the Cerebrospinal Nerves in Mammals," Amer. Journ. AnaL, 11, 1903.
LITERATURE 425 S. R. Cajal: "Nouvelles Observations sur revolution des neuroblasts avec quelques remarques sur l'hypothese neurogenetique de Hensen-Held," Anal. Anzeiger, xxxii, 1908. A. F. Dixon: "On the Development of the Branches of the Fifth Cranial Nerve in Man," Sclent. Trans. Roy. Dublin Soc, Ser. 1, VI, 1896. C. R. Essick: "The Development of the Nuclei pontis and the Nucleus Arcuatus in Man," Amer. Journ. Anat., xiii, 1912. E. Giglio-Tos: "Sugli organi branchiali e laterali di senso nell' uomo nei primordi del suo sviluppo," Monit. Zool. Ital., xill, 1902. E. Giglio-Tos: "SulP origine embrionale del nervo trigemino nell' uomo," Anat.
Anzeiger, xxi, 1902. E. Giglio-Tos: "Sui primordi dello sviluppo del nervo acustico-faciale nell' uomo," Anat. Anzeiger, xxi, 1902. K. Goldstein: "Die erste Entwicklung der grossen Hirncommissuren und die 'Verwachsung' von Thalamus und Striatum" Archiv jiir Anat. und Physiol., Anat. Abth., 1903. G. Groenberg: "Die Ontogenese einer niederen Saugergehirns nach Untersuchungen an Erinaceus europaeus," Zoolog. Jahrb. Abth. f. Anat. und Ontogen., xv, 1901. I. Hardesty: "On the Development and Nature of the Neuroglia," Amer. Journ Anat., in, 1904. R. G. Harrison: "Further Experiments on the Development of Peripheral Nerves,' Amer. Journ. of Anat., v, 1906. W. His: "Zur Geschichte des menschlichen Ruckenmarkes und der Nervenwurzeln,' Abhandl. der konigl. Sachsischen Gesellsch., Math.-Physik. Classe, xiii, 1886. W. His: "Zur Geschichte des Gehirns sowie der centralen und peripherischen Nerven bahnen beim menschlichen Embryo," Abhandl. der konigl. Sachsischen Gesellsch., Math.-Physik. Classe, xiv, 1888. W. His: "Die Formentwickelung des menschlichen Vorderhirns vom Ende des ersten bis zum Beginn des dritten Monats," Abhandl. der konigl. Sachsischen Gesellsch., Math.-Physik. Classe, xv. 1889. W. His: "Histogenese und Zusammenhang der Nervenelemente," Archiv fur Anat.
und Physiol., Anat. Abth., Supplement, 1890. W. His: "Die Entwickelung des menschlichen Gehirns wahrend der ersten Monate," Leipzig, 1904. W. His, Jr.: "Die Entwickelung des Herznervensystem bei Wirbelthieren," Abhandl.
der konigl. Sachsischen Gesellsch., Math.-Physik. Classe, xvni, 1893. W. His, Jr.: "Ueber die Entwickelung des Bauchsympathicus beim Huhnchen und Menschen," Archiv filr Anat. und Physiol., Anat. Abth., Supplement, 1S97. C. J. Herrick: " The Cranial and First Spinal Nerves of Menidia: A Contribution upon the Nerve Components of the Bony Fishes," Journ. of Comp. Neurol., ix, 1899. C. J. Herrick: "The Cranial Nerves and Cutaneous Sense-organs of the North American Siluroid Fishes," Journ. of Comp. Neurol., xi, 1901. G. C. Huber: "Four Lectures on the Sympathetic Nervous System," Journ. of Comp.
Neurol., vn, 1897. A. Kuntz: "A Contribution to the Histogenesis of the Sympathetic System," Anat.
Record, in, 1909.
426 LITERATURE A. Kuntz: "The role of the Vagi in the Development of the Sympathetic Nervous System," Anat. Anzeiger, xxxv, 1909. A. Kuntz: "The Development of the Sympathetic Nervous System in Mammals,' Journ. Compar. Neurol., xx, 1910. M. von Lenhossek: "Die Entwickelung der Ganglienanlagen bei dem menschlichen Embryo," Archiv filr Anat. und Physiol., Anat. Abth., 1891.
F. Marchand: "Ueber die Entwickelung des Balkens im menschlichen Gehirn," Archiv filr mikrosk. Anat., xxxvn, 1891. V. VON Mihalkovicz: " Entwickelungsgeschichte des Gehirns," Leipzig, 1877. A. D. Onodi: "Ueber die Entwickelung des sympathischen Nervensystems," Archiv filr mikrosk. Anat., xxvn, 1886.
G. Retzius: "Das Menschenhirn," Stockholm, 1896.
A. Schaper: "Die friihesten Differenzirungsvorgange im Central-nerven-system,' Archiv filr Entwicklungsmechanik, v, 1897. G. L. Streeter: " The Development of the Cranial and Spinal Nerves in the Occipita Region of the Human Embryo," Amer. Journ. Anat., iv, 1904. O. S. Strong: "The Cranial Nerves of Amphibia," Journal of Morphol., x, 1895. R. Wlassak: "Die Herkunft des Myelins," Archiv filr Entwicklungsmechanik, VI 1898. E. Zuckerkandl: "Zur Entwicklung des Balkens," Arbeiten aus neurol. Inst. Wien.
THE DEVELOPMENT OF THE ORGANS OF SPECIAL SENSE.
Like the cells of the central nervous system, the sensory cells are all of ectodermal origin, and in lower animals, such as the earthworm, for instance, they retain their original position in the ectodermal epithelium throughout life. In the vertebrates, however, the majority of the sensory cells relinquish their superficial position and sink more or less deeply into the subjacent tissues, being represented by the posterior root ganglion cells and by the sensory cells of the special sense-organs, and it is only in the olfactory organ that the original condition is retained. Those cells which have withdrawn from the surface receive stimuli only through overlying cells, and in certain cases these transmitting cells are not specially differentiated, the terminal branches of the sensory dendrites e ding among ordinary epithelial cells or in such structures as the Pacinian bodies or the end-bulbs of Krause situated beneath undifferentiated epithelium. In other cases, however, certain specially modified superficial cells serve to transmit the stimuli to the peripheral sensory neurones, forming such structures as the hair-cells of the auditory epithelium or the gustatory cells of the taste-buds.
Thus three degrees of differentiation of the special sensory cells may be recognized and a classification of the sense-organs may be made upon this basis. One organ, however, the eye, cannot be brought into such a classification, since its sensory cells present certain developmental peculiarities which distinguish them from those of all other sense-organs. Embryologically the retina is a portion of the central nervous system and not a peripheral organ, and hence it will be convenient to arrange the other sense-organs 427
428 THE OLFACTORY ORGANS according to the classification indicated and to discuss the" history of the eye at the close of the chapter.
The Development of the Olfactory Organ. - The general development of the nasal fossa, the epithelium of which contains the olfactory sense cells, has already been described (pp. 99 and 283), as has also the development of the olfactory lobes of the brain (p. 406), and there remains for consideration here merely the formation of the olfactory nerve and the development of the rudimentary organ of Jacobson.
The Olfactory Nerve. - Very diverse results have been obtained by various observers of the development of the olfactory nerve, it having been held at different times that it was formed by the outgrowth of fibers from the olfactory lobes (Marshall), from fibers which arise partly from the olfactory lobes and partly from the olfactory epithelium (Beard), from the cells of an olfactory ganglion originally derived from the olfactory epithelium but later separating from it (His), and, finally, that it was composed of the prolongations of certain cells situated and, for the most part at least, remaining permanently in the olfactory epithelium (Disse). The most recent observations on the structure of the olfactory epithelium and nerve indicate a greater amount of probability in the last result than in the others, and the description which follows will be based upon the observations of His, modified in conformity with the results obtained by Disse from chick embryos.
In human embryos of the fourth week the cells lining the upper part of the olfactory pits show a distinction into ordinary epithelial and sensory cells, the latter, when fully formed, being elongated cells prolonged peripherally into a short but narrow process which reaches the surface of the epithelium and proximally gives rise to an axis-cylinder process which extends up toward and penetrates the tip of the olfactory lobe to come into contact with the dendrites of the first central neurones of the olfactory tract (Fig. 252). These cells constitute a neuro-epithelium and in later stages of development retain their epithelial position for the most part, a few of them, however, withdrawing into the subjacent mesenchyme and becoming
THE OLFACTORY ORGANS
bipolar, their peripheral prolongations ending freely among the cells of the olfactory epithelium. These bipolar cells resemble closely in form and relations the cells of the embryonic posterior root ganglia, and thus form an interesting transition between these and the neuroepithelial cells.
The Organ of Jacohson. - In embryos of three or four months a
Fig. 252. - Diagram Illustrating the Relations of the Fibers of the Olfactory Nerve.
Ep, Epithelium of the olfactory pit; C, cribiform plate of the'ethmoid, G, glomerulus of the olfactory bulb; M, mitral cell. - (Van Gekuchten.) j small pouch-like invagination of the epithelium covering the lower anterior portion of the median septum of the nose can readily be seen. This becomes converted into a slender pouch, 3 to 5 mm. long, ending blindly at its posterior extremity and opening at its other end
430 THE ORGANS OF TASTE into the nasal cavity. Its lining epithelium resembles that of the respiratory portion of the nasal cavity, and there is developed in the connective tissue beneath its floor a slender plate of cartilage, distinct from that forming the septum of the nose.
This organ, which may apparently undergo degeneration in the adult, and in some cases completely disappears, appears to be the representative of what is known as Jacobson's organ, a structure which reaches a much more extensive degree of development in many of the lower mammals, and in these contains in its epithelium sensory cells whose axis-cylinder processes pass with those of the olfactor} sense cells to the olfactory bulbs. In man, however, it seems to be a rudimentary organ, and no satisfactory explanation of its function has as yet been advanced.
The olfactory neuro-epithelium, considered from a comparative standpoint, seems to have been derived from the system of lateral line organs so highly developed in the lower vertebrates (Kupffer). In higher forms the system, which is cutaneous in character, has disappeared except in two regions where it has become highly specialized. In one of these regions it has given rise to the olfactory sense cells and in the other to the similar cells of the auditory apparatus.
The Organs of Touch and Taste. - Little is yet known concerning the development of the various forms of tactile organs, which belong to the second class of sensory organs described above.
The Organs of Taste. - The remaining organs of special sense belong to the third class, and of these the organs of taste present in many respects the simplest condition. They are developed principally in connection with the vallate and foliate papillae of the tongue, and of the former one of the earliest observed stages has been found in embryos of 9 cm. in the form of two ridges of epidermis, lying toward the back part of the tongue and inclined to one another in such a manner as to form a V with the apex directed backward. From these ridges solid downgrowths of epidermis into the subjacent tissue occur, each downgrowth having the form of a hollow truncated cone with its basal edge continuous with the
THE INTERNAL EAR 43 1 superficial epidermis (Fig. 253, A). In later stages lateral outgrowths develop from the deeper edges of the cone, and about the same time clefts appear in the substance of the original downgrowths (Fig. 253, B) and, uniting together, finally open to the surface, forming a trench surrounding a papilla (Fig. 253, C). The lateral outgrowths, which are at first solid, also undergo an axial degeneration and become converted into the glands of Ebner (b), which open into the trench near its floor. The various papillae which occur in the adult do not develop simultaneously, but their number increases with the age of the fetus, and there is, moreover, considerable variation in the time of their development.
The taste-buds are formed by a differentiation of the epithelium which covers the papillae, and this differentiation appears to stand
B C Fig. 253. - Diagrams Representing the Development of a Vallate Papilla. a, Valley surrounding the papilla; b, von Ebner's gland. - (Graberg.) in intimate relation with the penetration of fibers of the glossopharyngeal nerve into the papillae. The buds form at various places upon the papillae, and at one period are especially abundant upon their free surfaces, but in the later weeks of intrauterine life these surface buds undergo degeneration and only those upon the sides of the trench persist, as a rule.
The foliate papillae do not seem to be developed until some time after the circumvallate, being entirely wanting in embryos of four and a half and five months, although plainly recognizable at the seventh month.
The Development of the Ear. - It is customary to describe the mammalian ear as consisting of three parts, known as the inner, middle, and outer ears, and this division is, to a certain extent at
THE INTERNAL EAR
least, confirmed by the embryonic development. The inner ear, which is the sensory portion proper, is an ectodermal structure, which secondarily becomes deeply seated in the mesodermal tissue of the head, while the middle and outer ears, which provide the apparatus necessary for the conduction of the sound-waves to the inner ear, are modified portions of the anterior branchial arches. It will be convenient, accordingly, in the description of the ear, to accept the usually recognized divisions and to consider first of all the development of the inner ear, or, as it is better termed, the otocyst. The Development of the Otocyst. - In an embryo of 2.4 mm. a pair of pits occur upon the surface of the body about opposite the middle portion of the hind-brain (Fig. 254, A). The ectoderm lining the pits is somewhat thicker than is the neighboring ectoderm
a - B Fig. 254. - Transverse Section Passing through the Otocyst (ot) of Embryos of (A) 2.4 mm. and (B) 4 mm. - (His.) of the surface of the body, and, from analogy with what occurs in other vertebrates, it seems probable that the pits are formed by the invagination of localized thickenings of the ectoderm. The mouth of each pit gradually becomes smaller, until finally the invagination is converted into a closed sac (Fig. 254, B), which separates from the surface ectoderm and becomes enclosed within the subjacent mesoderm. This sac is the otocyst, and in the stage just described, found in embryos of 4 mm., it has an oval or more or less spherical form. Soon, however, in embryos of 6.9 mm., a prolongation arises from its dorsal portion and the sac assumes the form shown in Fig. 255, A; this prolongation, which is held by some authors to be the remains of the stalk which originally connected the otocyst sac
THE INTERNAL EAR
with the surface ectoderm, represents the ductus endolymphaticus , and, increasing in length, it soon becomes a strong club-shaped process, projecting considerably beyond the remaining portions of the otocyst (Fig. 255, B). In embryos of about 10.2 mm. the sac begins to show certain other irregularities of shape (Fig. 255, B, sc). Thus, about opposite the point of origin of the ductus endolymphaticus three folds make their appearance, representing the semi
Fig. 255. - Reconstruction of the Otocysts of Embryo of (A) 6.9 mm. and (B) 10.2 MM.
de, Endolymphatic duct; gc, ganglion cochleare; gg, ganglion geniculatum; gv, ganglion vestibulare; sc, lateral semicircular duct. - (His, Jr.) circular ducts, and as they increase in size the opposite walls of the central portion of each fold come together, fuse, and finally become absorbed, leaving the free edge of the fold as a crescentic canal, at one end of which an enlargement appears to form the ampulla. The transformation of the folds into canals takes place somewhat earlier in the cases of the two vertical than in that of the horizontal duct, as 28
THE INTERNAL EAR
may be seen from Fig. 256, which represents the condition occurring in an embryo of 13.5 mm.
A short distance below the level at which the canals communicate with the remaining portion of the otocyst a constriction appears, indicating a separation of the otocyst into a more dorsal portion and a more ventral one. Later, the latter begins to be prolonged into a flattened canal which, as it elongates, becomes coiled upon itself and also becomes separated by a constriction from the remaining portion of the otocyst (Fig. 257). This canal is the ductus cochlearis (scala media of the cochlea), and the remaining portion of the otocyst subsequently becomes divided by a constriction into the utriculus, with which the semicircular ducts are connected, and the sacculus. The constriction which separates the cochlear duct from the sacculus becomes the ductus reuniens, while that between the utriculus and sacculus is converted into a narrow canal with which the ductus endolymphaticus connects, and hence it is that, in the adult, the connection between these two portions of H the otocyst seems to be formed by the ductus dividing proximally into two limbs, one of which is connected with the utricle and the other with the saccule.
When first observed in the human embryo the auditory ganglion is closely associated with the geniculate ganglion of the seventh nerve (Fig. 255, B), the two, usually spoken of as the acustico-facialis ganglion, forming a mass of cells lying in close contact with the
Fig. 256. - Reconstruction of the Otocyst of an Embryo of I3.5 MM.
co, Cochlea; de, endolymphatic duct;.sc, semicircular duct. - (His Jr.)
THE INTERNAL EAR
anterior wall of the otocyst. The origin of the ganglionic mass has not yet been traced in the mammalia, but it has been observed that in cow embryos the geniculate ganglion is connected with the ectoderm at the dorsal end of the first branchial cleft (Froriep), and it may perhaps be regarded as one of the epibranchial placodes (see p. 417), and in the lower vertebrates a union of the ganglion with a suprabranchial placode has been observed (Kupffer), this union
Fig. 257. - Reconstruction of the Otocyst of an Embryo of 20 mm., front view. cc, Common limb of superior and posterior semicircular ducts; eg, cochlear ganglion; co, cochlea; de, endolymphatic duct; s, sacculus; sdl, sdp, and sds x lateral, posterior and superior semicircular ducts; u, utriculus; vg, vestibular ganglion. - (Streeter.) indicating the origin of the auditory ganglion from one or more of the ganglia of the lateral line system.
At an early stage in the human embryo the auditory ganglion shows indications of a division into two portions, a more dorsal one, which represents the future ganglion vestibular e, and a ventral one, the ganglion cochleare. The ganglion cells become bipolar, in which condition they remain throughout life, never reaching the T-shaped condition found in most of the other peripheral cerebro-spinal ganglia. One of the prolongations of each cell is directed centrally to
THE INTERNAL EAR
form a fiber of the auditory nerve, while the other penetrates the wall of the otocyst to enter into relations with certain specially modified cells which differentiate from its lining epithelium.
â– In the earliest stages the ectodermal lining of the otocyst is formed of similar columnar cells, but later over the greater part of the surface the cells flatten down, only a few, aggregated together to
Fig. 258. - The Right Internal Ear of an Embryo of Six Months. ca, ce, and cp, Superior, lateral, and posterior semicircular ducts; cr, crista acustica; de, endolymphatic duct; Is, spiral ligament; mb, basilar membrane; ms and tnu, macula acustica sacculi and utriculi; rb, basilar branches of the cochlear nerve. - (Retzius.) form patches, retaining the high columnar form and developing hairlike processes upon their free surfaces. These are the sensory cells of the ear. In the human ear there are in all six patches of these sensory cells, an elongated patch (crista ampullaris) in the ampulla of each semicircular canal (Fig. 258, cr), a round patch (macula acus
THE INTERNAL EAR 437 tica, mii) in the utriculus and another (ms) in the sacculus, and, finally, an elongated patch which extends the entire length of the scala media of the cochlea and forms the sensory cells of the spiral organ of Corti. The cells of this last patch are connected with the fibers from the cochlear ganglion, while those of the vestibular ganglion pass to the cristas and maculae.
In connection with the spiral organ certain adjacent cells also retain their columnar form and undergo various modifications,
Fig. 259. - Section of the Cochlear Duct of a Rabbit Embryo of 55 mm.
a, Mesenchyme; b to e, epithelium of cochlear duct; M.t, membrana tectoria; V.s.p, vein; 1 to 7, spiral organ of Corti. - (Baginsky.) giving rise to a rather complicated structure whose development has been traced in the rabbit. Along the whole length of the cochlear duct the cells resting upon that half of the basilar membrane which is nearest the axis of the cochlea, and may be termed the inner half, retain their columnar shape, forming two ridges projecting slightly into the cavity of the scala (Fig. 259). The cells of the inner ridge, much the larger of the two, give rise to the membrana tectoria,
438 THE INTERNAL EAR either as a cuticular secretion or by the artificial adhesion of long hair-like processes which project from their free surfaces (Ayers). The cells of the outer ridge are arranged in six longitudinal rows (Fig. 259, 1-6); those of the innermost row (1) develop hairs upon their free surfaces and form the inner hair cells, those of the next two rows (2 and 3) gradually become transformed on their adjacent surfaces into chitinous p /- /^ substance and form the rods of Corti, while the three outer rows
- (4 to 6) develop into the outer e -^~~ __ hair cells. It is in connection
with the hair cells that the peripheral prolongations of the cells of the cochlear ganglion terâ„¢JS= d^t" 1 â„¢ mi Â«ate, and since these hair cells Rabbit Embryo of Twenty-four Days, are arranged in rows extending c, Periotic cartilage; ep, fibrous mem- .1 pnt j rp ] er ,a\h of the cochlear brane beneath the epithelium of the canal; me ermre lengin 01 me COCIliear p, perichondrium; s, spongy tissue. - (Von duct, the ganglion also IS drawn Kolliker.) . 1 .. .
out into a spiral following the coils of the cochlea, and hence is sometimes termed the spiral ganglion.
While the various changes described above have been taking place in the otocyst, the mesoderm surrounding it has also been undergoing development. At first this tissue is quite uniform in character, but later the cells immediately surrounding the otocyst condense to give rise to a fibrous layer (Fig. 260, ep), while more peripherally they become more loosely arranged and form a somewhat gelatinous layer (s) , and still more peripherally a second fibrous layer is differentiated and the remainder of the tissue assumes a character which indicates an approaching conversion into cartilage. The further history of these various layers is as follows: The inner fibrous layer gives rise to the connective-tissue wall which supports the ectodermal lining of the various portions of the otocyst; the gelatinous layer undergoes a degeneration to form a lymph-like
THE INTERNAL EAR
fluid known as the perilymph, the space occupied by the fluid being the perilymphatic space; the outer fibrous layer becomes perichondrium and later periosteum; and the precartilage undergoes chondrification and later ossifies to form the petrous portion of the temporal bone.
The gelatinous layer completely surrounds most of the otocyst structures, which thus come to lie free in the perilymphatic space, but in the cochlear region the conditions are somewhat different. In this region the gelatinous layer is interrupted along two lines,
Fig. 261. - Diagrammatic Transverse Section through a Coil of the Cochlea showing the relation of the scal^e. c, Organ of Corti; co, ganglion cochleare; Is, lamina spiralis; SAT, cochlear duct; ST, scala tympani; SV, scala vestibuli. - (From Gerlach.) an outer broad one where the connective-tissue wall of the cochlear duct is directly continuous with the perichondrium layer, and an inner narrow one, along which a similar fusion takes place with the perichondrium of a shelf-like process of the cartilage, which later ossifies to form the lamina spiralis. Consequently throughout the cochlear region the perilymphatic space is divided into two compartments which communicate at the apex of the cochlea, while below one, known as the scala vestibuli, communicates with the space
440 THE MIDDLE EAR surrounding the saccule and utricle, and the other, the scala tympani, abuts upon a membrane which separates it from the cavity of the middle ear and represents a portion of the outer wall of the petrous bone where chondrification and ossification have failed to occur. This membrane closes what appears in the dried skull to be an opening in the inner wall of the middle ear, known as the fenestra cochlea (rotunda) ; another similar opening, also closed by membrane in the fresh skull, occurs in the bony wall opposite the utricular portion of the otocyst and is known as the fenestra vestibuli (ovalis) .
The Development of the Middle Ear. - The middle ear develops from the upper part of the pharyngeal groove which represents the endodermal portion of the first branchial cleft. This becomes prolonged dorsally and at its dorsal end enlarges to form the tympanic cavity, while the narrower portion intervening between this and the pharyngeal cavity represents the tuba auditiva (Eustachian tube).
To correctly understand the development of the tympanic cavity it is necessary to recall the structures which form its boundaries. Anteriorly to the upper end of the first branchial pouch there is the upper end of the first arch, and behind it the corresponding part of the second arch, the two fusing together dorsal to the tympanic cavity and forming its roof. Internally the cavity is bounded by the outer wall of the cartilaginous investment of the otocyst, while externally it is separated from the upper part of the ectodermal groove of the first branchial cleft by the thin membrane which forms the floor of the groove.
It has been seen in an earlier chapter that the axial mesoderm of each branchial arch gives rise to skeletal structures and muscles. The axial cartilage of the ventral portion of the first arch is what is known as Meckel's cartilage, but in that portion of the arch which forms the roof and anterior wall of the tympanic cavity, the cartilage becomes constricted to form two masses which later ossify to form the malleus and incus (Fig. 262, m and i), while the muscular tissue of this dorsal portion of the arch gives rise to the tensor tympani. Similarly, in the case of the second arch there is to be found, dorsal to
THE MIDDLE EAR 44 1 the extremity of the cartilage which forms the styloid process of the adult, a narrow plate of cartilage which forms an investment for the facial nerve (Fig. 262, VII), and dorsal to this a ring of cartilage (st) which surrounds a small stapedial artery and represents the stapes.
It has been found that in the rabbit the mass of cells from which the stapes is formed is at its first appearance quite independent of the second branchial arch (Fuchs), and it has been held to be a
Fig. 262. - Semi-diagrammatic Viewof the Auditory Ossicles of an Embryo of Six Weeks. i, Incus; j, jugular vein; m, malleus; mc, Meckel's cartilage; oc, capsule of otocyst; R, cartilage of the second branchial arch; st, stapes; VII, facial nerve. - (Siebenmann.) derivative of the mesenchyme from which the periotic capsule is formed. In later stages, however, it becomes connected with the cartilage of the second branchial arch, as shown in Fig. 262, and it is a question whether this connection, which is transitory, does not really indicate the phylogenetic origin of the ossicle from the second arch cartilage, its appearance as an independent structure being a secondary ontogenetic phenomenon. However that may be, the stapedial artery disappears in later stages and the stapedius muscle, derived from the musculature of the second branchial arch and therefore supplied by the facial nerve, becomes attached to the ossicle.
The three ossicles at first lie embedded in the mesenchyme forming the roof of the primitive tympanic cavity, as does also the chorda tympani, a branch of the seventh nerve, as it passes into the substance of the first arch on the way to its destination. The mesenchyme in which these various structures are embedded is rather voluminous (Fig. 264), and after the end of the seventh month becomes converted into a peculiar spongy tissue, which, toward the end of fetal life, gradually degenerates, the tympanic cavity at the same time expanding and wrapping itself around the ossicles and the muscles attached to them (Fig. 263). The bones and their muscles, consequently, while appearing in the adult to traverse the tympanic cavity, are really completely enclosed within a layer of epithelium continuous with that lining the wall of the cavity, while the handle of the malleus and the chorda tympani lie between the epithelium of the outer wall of the cavity and the fibrous mesoderm which forms the tympanic membrane. The extension of the tympanic cavity does not, however, cease with its replacement of the degenerated spongy mesenchyme, but toward the end of fetal life it begins to invade the substance of the temporal bone by a process similar to that which produces the ethmoidal cells and the other osseous sinuses in connection with the nasal cavities (see p. 175). This process continues for some years after birth and results in the formation in the mastoid portion of the bone of the so-called mastoid cells, which communicate with the tympanic cavity and have an epithelial lining continuous with that of the cavity.
Fig. 263. - Diagrams Illustrating the Mode of Extension of the Tympanic Cavity Around the Auditory Ossicles.
M, Malleus; m, spongy mesenchyme; p, surface of the periotic capsule; T, tympanic cavity. The broken line represents the epithelial lining of the tympanic cavity.
The lower portion of the diverticulum from the first pharyngeal groove which gives rise to the tympanic cavity becomes converted into the Eustachian tube. During development the lumen of the tube disappears for a time, probably owing to a proliferation of its lining epithelium, but it is re-established before birth.
In the account of the development of the ear-bones given above it is held that the malleus and incus are derivatives of the first branchial (mandibular) arch and the stapes probably of the second. This view represents the general consensus of recent workers on the difficult question of the origin of these bones, but it should be mentioned that nearly all possible modes of origin have been at one time or other suggested. The malleus has very generally been accepted as coming from the first arch, and the same is true of the incus, although some earlier authors have assigned it to the second arch. But with regard to the stapes the opinions have been very varied. It has been held to be derived from the first arch, from the second arch, from neither one nor the other, but from the cartilaginous investment of the otocyst, or, finally, it has been held to have a compound origin, its arch being a product of the second arch while its basal plate was a part of the otocyst investment.
The Development of the Tympanic Membrane and of the Outer Ear. - Just as the tympanic cavity is formed from the endodermal groove of the first branchial cleft, so the outer ear owes its origin to the ectodermal groove of the same cleft and to the neighboring arches. The dorsal and most ventral portions of the groove flatten out and disappear, but the median portion deepens to form, at about the end of the second month, a funnel-shaped cavity which corresponds to the outer portion of the external auditory meatus. From the inner end of this a solid ingrowth of ectoderm takes place, and this, enlarging at its inner end to form a disk-like mass, comes into relation with the gelatinous mesoderm which surrounds the malleus and chorda tympani. At about the seventh month a split occurs in the disk-like mass (Fig. 264), separating it into an outer and an inner layer, the latter of which becomes the outer epithelium of the tympanic membrane. Later, the split extends outward in the substance of the ectodermal ingrowth and eventually unites with the funnel-shaped cavity to complete the external meatus.
Fig. 264. - Horizontal Section Passing through the Dorsal Wall of the External Auditory Meatus in an Embryo of 4.5 cm.
c, Cochlea; de, endolymphatic duct; i, incus; Is, transverse sinus; m, malleus; me, meatus auditorius externus; me' , cavity of the meatus; s, sacculus; sc, lateral semicircular canal; sc', posterior semicircular canal; st, stapes; t, tympanic cavity; u, utriculus; 7, facial nerve. - (Siebenmann.)
The tympanic membrane is formed in considerable part from the substance of the first branchial arch, the area in which it occurs not being primarily part of the wall of the tympanic cavity, but being brought into it secondarily by the expansion of the cavity. The membrane itself is mesodermal in origin and is lined on its outer surface by an ectodermal and on the inner by an endodermal epithelium.
The auricle (pinna) owes its origin to the portions of the first and second arches which bound the entrance of the external meatus. Upon the posterior edge of the first arch there appear about the end of the fourth week two transverse furrows which mark off three tubercles (Fig. 258, A, 1-3) and on the anterior edge of the second arch a corresponding number of tubercles (4-6) is formed, while, in addition, a longitudinal furrow, running down the middle of the arch, marks ofT a ridge (c) lying posterior to the tubercles. From these six tubercles and the ridge are developed the various parts of the auricle, as may be seen from Fig. 265 which represents the transformation as described by His. According to this, the most ventral tubercle of the first arch (i) gives rise to the tragus, and the middle one (5) of the second arch furnishes the antitragus. The middle and dorsal tubercles of the first arch (2 and 3) unite with the ridge (c) to produce the helix, while from the dorsal tubercle of the second arch (4) is produced the anthelix and from the ventral one (6) the lobule. More recent observations, however, seem to indicate that the lobule is an accessory structure unrelated to the tubercles and that the sixth tubercle gives rise to the antitragus, while the fifth is either included in the anthelix or else disappears. It is noteworthy that up to about the third month of development the upper and posterior portion of the helix is bent forward so as to conceal the anthelix (Fig. 265, D); it is at just about a corresponding stage that the pointed form of the ear seen in the lower mammals makes its appearance, and it is evident that, were it not for the forward bending, the human ear would also be assuming at this stage a more or less pointed form. Indeed, there is usually to be found upon the incurved edge of the helix, some distance below the upper border of the auricle, a more or less distinct tubercle, known as Darwin's tubercle, which seems to represent the point of the typical mammalian ear, and is, accordingly, the morphological apex of the pinna.
Fig. 265. - Stages in the Development of the Auricle.
A, Embryo of n mm.; B, of 13.6 mm.; C, of 15 mm.; D, at the beginning of the third month; E, fetus of 8.5 cm.; F, fetus at term. - (His.)
There seems to be little room for doubt that the otocyst belongs primarily to the system of lateral line sense-organs, but a discussion of this interesting question would necessitate a consideration of details concerning the development of the lower vertebrates which would be foreign to the general plan of this book. It may be recalled, however, that the analysis of the components of the cranial nerves described on page 415 refers the auditory nerve to the lateral line system.
The Development of the Eye. - The first indications of the development of the eye are to be found in a pair of hollow outgrowths from the side of the first primary brain vesicle, at a level which corresponds to the junction of the dorsal and ventral zones. Each evagination is directed at first upward and backward, and, enlarging at its extremity, it soon shows a differentiation into a terminal bulb and a stalk connecting the bulb with the brain (Fig. 232). At an early stage the bulb comes into apposition with the ectoderm of the side of the head, and this, over the area of contact, becomes thickened and then depressed to form the beginning of the future lens (Fig. 266).
Fig. 266. - Early Stages in the Development of the Lens in a Rabbit Embryo. The nucleated layer to the left is the ectoderm and the thicker lens epithelium, beneath which is the outer wall of the optic evagination; above and below between the two is mesenchyme. - (Rabl.)
As the result of the depression of the lens ectoderm, the outer wall of the optic bulb becomes pushed inward toward the inner wall, and this invagination continuing until the two walls come into contact, the bulb is transformed into a double-walled cup, the optic cup, in the mouth of which lies the lens (Fig. 268). The cup is not perfect, however, since the invagination affects not only the optic bulb, but also extends medially on the posterior surface of the stalk, forming upon this a longitudinal groove and producing a defect of the ventral wall of the cup, known as the chorioidal fissure (Fig. 267). The groove and fissure become occupied by mesodermal tissue, and in this, at about the fifth week, a blood-vessel develops which traverses
Fig. 267. - Reconstruction of the Brain of an Embryo of Four Weeks, showing the Chorioid Fissure. - (His.) the cavity of the cup to reach the lens and is known as the arteria hyaloidea.
In the meantime further changes have been taking place in the lens. The ectodermal depression which represents it gradually deepens to form a cup, the lips of which approximate and finally meet, so that the cup is converted into a vesicle which finally separates completely from the ectoderm (Fig. 268), much in the same way as the otocyst does. As the lens vesicle is constricted off, the surrounding mesodermal tissue grows in to form a layer between it and the overlying ectoderm, and a split appearing in the layer divides it into an outer thicker portion, which represents the cornea, and an inner thinner portion, which covers the outer surface of the lens and becomes highly vascular. The cavity between these two portions represents the anterior chamber of the eye. The cavity of the optic cup has also become filled by a peculiar tissue which represents the vitreous humor, while the mesodermal tissue surrounding the cup condenses to form a strong investment for it, which is externally continuous with the cornea, and at about the sixth week shows a differentiation into an inner vascular layer, the chorioid coat, and an outer denser one, which becomes the sclerotic coat.
Fig. 268. - Horizontal Section through the Eye of an Embryo Pig of 7 mm. Br, Diencephalon; Ec, ectoderm; I, lens; P, pigment, and R, retinal layers of the retina.
The various processes resulting in the formation of the eye, which have thus been rapidly sketched, may now be considered in greater detail.
The Development of the Lens. - When the lens vesicle is complete, it forms a more or less spherical sac lying beneath the superficial ectoderm and containing in its cavity a few cells, either scattered or in groups (Fig. 268). These cells, which have wandered into the cavity of the vesicle from its walls, take no part in the further development of the lens, but early undergo complete degeneration, and the first change which is concerned with the actual formation of the lens is an increase in the height of the cells forming its inner wall and a thinning out of its outer wall (Fig. 269, A). These changes continuing, the outer half of the vesicle becomes converted into a single layer of somewhat flat cells which persist in the adult condition to form the anterior epithelium of the lens, while the cells of the posterior wall form a marked projection into the cavity of the vesicle and eventually completely obliterate it, coming into contact with the inner surface of the anterior epithelium (Fig. 269, B).
These posterior elongated cells form, then, the principal mass of the lens, and constitute what are known as the lens fibers. At first those situated at the center of the posterior wall are the longest, the more peripheral ones gradually diminishing in length until at the equator of the lens they become continuous with and pass into the anterior epithelium. As the lens increases in size, however, the most centrally situated cells fail to elongate as rapidly as the more peripheral ones and are pushed in toward the center of the lens, the more peripheral fibers meeting below them along a line passing across the inner surface of the lens. The disparity of growth continuing, a similar sutural line appears on the outer surface beneath the anterior epithelium, and the fibers become arranged in concentric layers around a central core composed of the shorter fibers. In the human eye the line of suture of the peripheral fibers becomes bent so as to consist of two limbs which meet at an angle, and from the angle a new sutural line develops during embryonic life, so that the suture assumes the form of a three-rayed star. In later life the stars become more complicated, being either six-rayed or more usually nine-rayed in the adult condition '(Fig. 270).
Fig. 269. - Sections through the Lens (4) of Human Embryo of Thirty Thirty-one Days and (B) of Pig Embryo of 36 Mu. - (Rabl.)
As early as the second month of development the lens vesicle becomes completely invested by the mesodermal tissue in which blood-vessels are developed in considerable numbers, whence the
Fig. 270. - Posterior (Inner) Surface of the Lens from an Adult showing the Sutural Lines. - (Rabl.)
investment is termed the tunica vasculosa lends (Fig. 278, tv). The arteries of the tunic are in connection principally with the hyaloid artery of the vitreous humor (Fig. 276), and consist of numerous fine branches which envelop the lens and terminate in loops almost at the center of its outer surface. This tunic undergoes degeneration after the seventh month of development, by which time the lens has completed its period of most active growth, and, as a rule, completely disappears before birth. Occasionally, however, it may persist to a greater or less extent, the persistence of the portion covering the outer surface of the lens, known as the membrana papillaris, causing the malformation known as congenital atresia of the pupil.
In addition to the vascular tunic, the lens is surrounded by a non-cellular membrane termed the capsule. The origin of 'this structure is still in doubt, some observers maintaining that it is a product of the investing mesoderm, while others hold it to be a product of the lens epithelium.
It is interesting from the standpoint of developmental mechanics to note that W. H. Lewis and Spemann have shown that in the Amphibia contact of the optic vesicle with the ectoderm is necessary for the formation of the lens, and, furthermore, if the vesicle be transplanted to other regions of the body of a larva, a lens will be developed from the ectoderm with which it is then in contact, even in the abdominal region, The Development of the Optic Cup.- - When the invagination of the outer wall of the optic bulb is completed, the margins of the resulting cup are opposite the sides of the lens vesicle (Fig. 268), but with the enlargement of the lens and cup the margins of the latter gradually come to lie in front of - that is to say, upon the outer surface of - the lens, forming the boundary of the opening known as the pupil. The lens, consequently, is brought to lie within the mouth of the optic cup, and that portion of the latter which covers the lens takes part in the formation of the iris and the adjacent ciliary body, while its posterior portion gives rise to the retina.
The chorioidal fissure normally disappears during the sixth or seventh week of development by a fusion of its lips, and not until this is accomplished does the term cup truly describe the form assumed by the optic bulb after the invagination of its outer wall. In certain cases the lips of the fissure fail to unite perfectly, producing the defect of the eye known as coloboma; this may vary in its extent, sometimes affecting both the iris and the retina and forming what is termed coloboma iridis, and at others being confined to the retinal portion of the cup, in which case it is termed coloboma chorioidae.
Up to a certain stage the differentiation of the two layers which form the optic cup proceeds along similar lines, in both the ciliary and retinal regions. The layer which represents the original internal portion of the bulb does not thicken as the cup increases in size, and becomes also the seat of a deposition of dark pigment, whence it may be termed the pigment layer of the cup; while the other layer - â– that formed by the invagination of the outer portion of the bulb, and which may be termed the retinal layer - remains much thicker (Fig. 268) and in its proximal portions even increases in thickness. Later, however, the development of the ciliary and retinal portions of the retinal layers differs, and it will be convenient to consider first the history of the ciliary portion.
The Development of the Iris and Ciliary Body
The first change noticeable in the ciliary portion of the retinal layer is its thinning out, a process which continues until the layer consists, like the pigment layer, of but a single layer of cells (Fig. 271), the transition of which to the thicker retinal portion of the layer is somewhat abrupt and corresponds to what is termed the ora serrata in adult anatomy. In embryos of 10.2 cm. the retinal layer throughout its entire extent is readily distinguishable from the pigment layer by the absence in it of all pigmentation, but in older forms this distinction gradually diminishes in the iris region, the retinal layer there acquiring pigment and forming the uvea.
When the anterior chamber of the eye is formed by the splitting of the mesoderm which has grown in between the superficial ectoderm and the outer surface of the lens, the peripheral portions of its posterior (inner) wall are in relation with the ciliary portion of the optic cup and give rise to the stroma of the ciliary body and of the iris (Fig. 271), this latter being continuous with the tunica vasculosa lentis so long as that structure persists (Fig. 278). In embryos of about 14.5 cm. the ciliary portion of the cup becomes thrown into radiating folds (Fig. 271), as if by a too rapid growth, and into the folds lamellae of mesoderm project from the stroma. These folds occur not only throughout the region of the ciliary body, but also extend into the iris region, where, however, they are but temporary structures, disappearing entirely by the end of the fifth month. The folds in the region of the corpus ciliare persist and produce the ciliary processes of the adult eye.
Embedded in the substance of the iris stroma in the adult are non-striped muscle-fibers, which constitute the sphincter and dilatator iridis. It has long been supposed that these fibers were differentiated from the stroma of the iris, but recent observations have shown that they arise from the cells of the pigment layer of the optic cup, the sphincter appearing near the pupillary border (Fig. 271, Sph) while the dilatator is more peripheral.
Fig. 271. - Radial Section through the Iris of an Embryo of 19 cm. AE, Pigment layer; CC, ciliary folds; IE, retinal layer; I.Str, iris stroma; Pm, pupillary membrane; Rs, marginal sinus; Sph, sphincter iridis. - (Szili.)
The Development of the Retina
Throughout the retinal region of the cup the pigment layer, undergoing the same changes as in the ciliary region, forms the pigment layer of the retina (Fig. 272, p). The retinal layer increases in thickness and early becomes differentiated into two strata (Fig. 268), a thicker one lying next the pigment layer and containing numerous nuclei, and a thinner one containing no nuclei. The thinner layer, from its position and structure, suggests an homology with the marginal velum of the central nervous system, and probably becomes converted into the nerve-fiber layer of the adult retina, the axis-cylinder processes of the ganglion cells passing into it on their way to the optic nerve. The thicker layer similarly suggests a comparison with the mantle layer of the cord and brain, and in embryos of 38 mm. it becomes differentiated into two secondary layers (Fig. 272), that nearest the pigment layer if) consisting of smaller and more deeply staining nuclei, probably representing the rod and cone and bipolar cells of the adult retina, while the inner layer, that nearest the marginal velum, has larger nuclei and is presumably composed of the ganglion cells.
Fig. 272. - Portion of a Transverse Section of the Retina of a New-born Rabbit. ch, Chorioid coat; g, ganglion-cell layer; r, outer layer of nuclei; p, pigment layer. - (Falchi.)
Little is as yet known concerning the further differentiation of the nervous elements of the human retina, but the history of some of them has been traced in the cat, in which, as in other mammals, the histogenetic processes take place at a relatively later period than in man. Of the histogenesis of the inner layer the information is
Fig. 273. - Diagram showing the Development of the Retinal Elements.
a, Cone cell in the unipolar, and b, in the bipolar stage; c, rod cells in the unipolar, and d, in the bipolar stage; e, bipolar cells; /and i, amacrine cells; g, horizontal cells; h, ganglion cells; k, Muller's fiber; I, external limiting membrane. - (Kallius, after Cajal.) rather scant, but it may be stated that the ganglion cells are the earliest of all the elements of the retina to become recognizable. The rod and cone cells, when first distinguishable, are unipolar cells (Fig. 273, a and c), their single processes extending outward from the cell-bodies to the external limiting membrane which bounds the outer surface of the retinal layer. Even at an early stage the cone cells (a) are distinguishable from the rod cells (c) by their more decided reaction to silver salts, and at first both kinds of cells are scattered throughout the thickness of the layer from which they arise. Later, a fine process grows out from the inner end of each cell, which thus assumes a bipolar form (Fig. 2 73 , b and d) , and, later still, the cells gradually migrate toward the external limiting membrane, beneath which they form a definite layer in the adult. In the meantime there appears opposite the outer end of each cell a rounded eminence projecting from the outer surface of the external limiting membrane into the pigment layer. The eminences over the cone cells are larger than those over the rod cells, and later, as both increase in length, they become recognizable by their shape as the rods and cones.
The bipolar cells are not easily distinguishable in the early stages of their differentiation from the other cells with which thy are mingled, but it is believed that they are represented by cells which are bipolar when the rod and cone cells are still in a unipolar condition (Fig. 273, e). If this identification be correct, then it is noteworthy that at first their outer processes extend as far as the external limiting membrane and must later shorten or fail to elongate until their outer ends lie in what is termed the outer granular layer of the retina, where they stand in relation to the inner ends of the rod and cone cell processes. Of the development of the amacrine (/", i) and horizontal cells (g) of the retina little is known. From their position in new-born kittens it seems probable that the former are derived from cells of the same layer as the ganglion cells, while the horizontal cells may belong to the outer layer.
In addition to the various nerve-elements mentioned above, the retina also contains neuroglial elements known as Muller's fibers (Fig. 273, k), which traverse the entire thickness of the retina. The development of these cells has not yet been thoroughly traced, but they resemble closely the ependymal cells observable in early stages of the spinal cord.
The Development of the Optic Nerve
The observations on the development of the retina have shown very clearly that the great majority of the fibers of the optic nerve are axis-cylinders of the ganglion cells of the retina and grow from these cells along the optic stalk toward the brain. Their embryonic history has been traced most thoroughly in rat embryos (Robinson), and what follows is based upon what has been observed in that animal.
The optic stalk, being an outgrowth from the brain, is at first a hollow structure, its cavity communicating with that of the third ventricle at one end and with that of the optic bulb at the other. When the chorioid fissure is developed, it extends, as has already been described, for some distance along the posterior surface of the stalk and has lying in it a portion of the hyaloid artery. Later, when the lips of the fissure fuse, the artery becomes enclosed within the stalk to form the arteria centralis retina of the adult (Fig. 276). By the formation of the fissure the original cavity of the distal portion of the stalk becomes obliterated, and at the same time the ventral and posterior walls of the stalk are brought into continuity with the retinal layer of the optic cup, and so opportunity is given for the passage of the axis-cylinders of the ganglion cells along those walls (Fig. 274). At an early stage a section of the proximal portion of the optic stalk (Fig. 275, A) shows the central cavity surrounded by a number of nuclei representing the mantle layer, and surrounding these a non-nucleated layer, resembling the marginal velum and continuous distally with the similar layer of the retina. When the ganglion cells of the latter begin to send out their axis-cylinder processes, these pass into the retinal marginal velum and converge in this layer toward the bottom of the chorioidal fissure, so reaching the ventral wall of the optic stalk, in the velum of which they may be distinguished in rat embryos of 4 mm., and still more clearly in those of 9 mm. (Fig. 275, A). Later, as the fibers become more numerous, they gradually invade the lateral and finally the dorsal
Fig. 2 74. - Diagrammatic Longitudinal Section of the Optic Cup and Stalk passing through the chorioid fisSURE.
Ah, Hyaloid artery; L, lens; On, fibers of the optic nerve; Os, optic stalk; PI, pigment layer, and R, retinal layer of the retina.
THE OPTIC NERVE
walls of the stalk, and, at the same time the mantle cells of the stalk become more scattered and assume the form of connective-tissue (neuroglia) cells, while the original cavity of the stalk is gradually obliterated (Fig. 275, B). Finally, the stalk becomes a solid mass of nerve-fibers, among which the altered mantle cells are scattered.
Fig. 275. - Transverse Sections through the Proximal Part of the Optic Stalk of Rat Embryos of (A) 9 mm. and (5) 11 mm. - (Robinson.)
From what has been stated above it will be seen that the sensory cells of the eye belong to a somewhat different category from those of the other sense-organs. Embryologically they are a specialized portion of the mantle layer of the medullary canal, whereas in the other organs they are peripheral structures either representing or being associated with representatives of posterior root ganglion cells. Viewed from this standpoint, and taking into consideration the fact that the sensory portion of the retina is formed from the invaginated part of the optic bulb, some light is thrown upon the inverted arrangement of the retinal elements, the rods and cones being directed away from the source of light. The normal relations of the mantle layer and marginal velum are retained in the retina, and the latter serving as a conducting layer for the axis-cylinders of the mantle layer (ganglion) cells, the layer of nerve-fibers becomes interposed between the source of light and the sensory cells. Furthermore, it may be pointed out that if the differentiation of the retina be imagined to take place before the closure of the medullary canal - a condition which is indicated in some of the lower vertebrates - there would be then no inversion of the elements, this peculiarity being due to the conversion of the medullary plate into a tube, and more especially to the fact that the retina develops from the outer wall of the optic cup. In certain reptiles in which an eye is developed in connection with the epiphysial outgrowths of the diencephalon, the retinal portion of this pineal eye is formed from the inner layer of the bulb, and in this case there is no inversion of the elements.
A justification of the exclusion of the optic nerve from the category which includes the other cranial nerves has now been presented. For if the retina be regarded as a portion of the central nervous system, it is clear that the nerve is not a nerve at all in the strict sense of that word, but is a tract, confined throughout its entire extent within the central nervous system and comparable to such groups of fibers as the direct cerebellar or fillet tracts of that system.
The Development of the Vitreous Humor. - It has already been pointed out (p. 448) that a blood-vessel, the hyaloid artery, accompanied by some mesodermal tissue makes its way into the cavity
Fig. 276. - Reconstruction of a Portion of the Eye of an Embryo of 13.8 mm. ah, Hyaloid artery; ch, chorioid coat; /, lens; r, retina. - (His.) of the optic cup through the chorioid fissure. On the closure of the fissure the artery becomes enclosed within the optic stalk and appears to penetrate the retina, upon the surface of which its branches ramify. In the embryo the artery does not, however, terminate in these branches as it does in the adult, but is continued on through the cavity of the optic cup (Fig. 276) to reach the lens, around which it sends branches to form the tunica vasculosa lentis.
According to some authors, the formation of the vitreous humor is closely associated with the development of this artery, the humor being merely a transudate from it, while others have maintained that it is a derivative of the mesoderm which accompanies the vessel, and is therefore to be regarded as a peculiar gelatinous form of connective tissue. More recently, however, renewed observations by several authors have resulted in the deposition of the mesoderm from the chief role in the formation of the vitreous and the substitution in it of the retina. At an early stage of development delicate protoplasmic processes may be seen projecting from the surface of the retinal layer into the cavity of the optic cup, these processes probably arising from those cells which will later form the Muller's
Fig. 277. - Transverse Section through the Ciliary Region of a Chick Embryo of Sixteen Days. ac, Anterior chamber of the eye; cj, conjunctiva; co, cornea; i, iris; I, lens; mc, ciliary muscle; rl, retinal layer of optic cup; sf, spaces of Fontana; si, suspensory ligament of the lens; v, vitreous humor. - (Angelucci.) (neuroglia) fibers of the retina. As development proceeds they increase in length, forming a dense and very fine fibrillar reticulum traversing the space between the lens and the retina and constituting the primary vitreous humor. The formation of the fibers is especially active in the ciliary portion of the retina and it is probable that it is from some of the fibers developing in this region that the suspensory ligament of the lens (zonula Zinnii) (Fig. 277, si) is formed spaces which occur between the fibers of the ligament enlarging to produce a cavity traversed by scattered fibers and known as the canal of Petit.
A participation of similar protoplasmic prolongations from the cells of the lens in the formation of the vitreous humor has been maintained (von Lenhossek) and as strenuously denied. But it is generally admitted that at the time when the hyaloid artery penetrates the vitreous to form the tunica vasculosa lentis it carries with it certain mesodermal elements, whose fate is at present uncertain. It has been held that they take part in the formation of the definitive vitreous, which, according to this view, is of mixed origin, being partly ectodermal and partly mesodermal (Van P6e), and, on the contrary, it has been maintained that they eventually undergo complete degeneration, the vitreous being of purely ectodermal origin (von Kolliker).
The degeneration of the mesodermal elements which the latter view supposes is associated with the degeneration of the hyaloid artery. This begins in human embryos in the third month and is completed during the ninth month, the only trace after birth of the existence of the vessel being a more fluid consistency of the axis of the vitreous humor, this more fluid portion representing the space originally occupied by the artery and forming what is termed the hyaloid canal (canal ofCloquet).
The Development of the Outer Coat of the Eye, of the Cornea, and of the Anterior Chamber. - Soon after the formation of the optic bulb a condensation of the mesoderm cells around it occurs, forming a capsule. Over the medial portions of the optic cup the further differentiation of this capsule is comparatively simple, resulting in the formation of two layers, an inner vascular and an outer denser and fibrous, the former becoming the chorioid coat of the adult eye and the latter the sclera.
More laterally, however, the processes are more complicated. After the lens has separated from the surface ectoderm a thin layer of mesoderm grows in between the two structures and later gives place to a layer of homogeneous substance in which a few cells,
The Anterior Chamber Of The Eye
more numerous laterally than at the center, are embedded. Still later cells from the adjacent mesenchyme grow into the layer, which increases considerably in thickness, and blood-vessels also grow into that portion of it which is in contact with the outer surface of the lens. At this stage the interval between the surface ectoderm and the lens is occupied by a solid mass of mesodermal tissue (Fig. 278, co and tv), but as development proceeds, small spaces (ac) filled with fluid begin to appear toward the inner portion of the mass, and these, increasing in number and size, eventually fuse together to form a single cavity which divides the mass into an inner and an outer portion. The cavity is the anterior chamber of the eye, and it has served to separate the cornea (co) from the tunica vasculosa lentis (tv) , and, extending laterally in all directions, it also separates from the cornea the mesenchyme which rests upon the marginal portion of the optic cup and constitutes the stroma of the iris. Cells arrange themselves on the corneal surface of the cavity to form a continuous endothelial layer, and the mesenchyme which forms the peripheral boundary of the cavity assumes a fibrous character and forms the ligamentum pectinatum iridis, among the fibers of which cavities, known as the spaces of Fontana (Fig. 277, sf), appear. Beyond the margins of the cavity the corneal tissue is directly continuous with the sclerotic, beneath the margin of which is a distinctly thickened portion of mesenchyme resting upon the ciliary processes and forming the stroma of the ciliary body, as well as giving rise to the muscle tissue which constitutes the ciliary muscle (Figs. 277 and 278, mc).
Fig. 278. - Transverse Section through the Ciliary Region of a Pig Embryo or 23 MM. ac, Anterior chamber of the eye; co, cornea; ec, ectoderm; I, lens; mc, ciliary muscle; p, pigment layer of the optic cup; r, retinal layer; tv, tunica vasculosa lentis. - (Angelucci.)
The ectoderm which covers the outer surface of the eye does not proceed beyond the stage when it consists of several layers of cells, and never develops a stratum corneum. In the corneal region it rests directly upon the corneal tissue, which is thickened slightly upon its outer surface to form the anterior elastic lamina; more peripherally, however, a quantity of loose mesodermal tissue lies between the ectoderm and the outer surface of the sclerotic, and, together with the ectoderm, forms the conjunctiva (Fig. 277, cj).
The Development of the Accessory Apparatus of the Eye. - The eyelids make their appearance at an early stage as two folds of skin, one a short distance above and the other below the cornea. The center of the folds is at first occupied by indifferent mesodermal tissue, which later becomes modified to form the connective tissue of the lids and the tarsal cartilage, the muscle tissue probably secondarily growing into the lids as a result of the spreading of the platysma over the face, the orbicularis oculi apparently being a derivative of that sheet of muscle tissue.
At about the beginning of the third month the lids have become sufficiently large to meet one another, whereupon the thickened epithelium which has formed upon their edges unites and the lids fuse together, in which condition they remain until shortly before birth. During the stage of fusion the eyelashes (Fig. 279, h) develop at the edges of the lids, having the same developmental history as ordinary hairs, and from the fused epithelium of each lid there grow upward or downward, as the case may be, into the mesodermic tissue, solid rods of ectoderm, certain of which early give off numerous short lateral processes and become recognizable as the tarsal (Meibomian) glands (m), while others retain the simple cylindrical form and represent the glands of Moll. When the eyelids separate, these solid ingrowths become hollow by a breaking down of their
Fig. 279. - Section through the Margins of the Fused Eyelids in an Embryo^ of Six Months. i "1
h, Eyelash; //, lower lid; m, tarsal gland; mu, muscle bundle; ul, upper lid. Seidl.)
central cells, just as in the sebaceous and sudoriparous glands of the skin, the tarsal glands being really modifications of the former glands, while the glands of Moll are probably to be regarded as specialized sudoriparous glands.
A third fold of skin, in addition to^the two which produce the eyelids, is also developed in connection with the eye, forming the plica semilunaris. This is a rudimentary third eyelid, representing the nictitating membrane which is fairly well developed in many of the lower mammals and especially well in birds.
THE LACHRYMAL GLAND 467 The lachrymal gland is developed at about the third month as a number of branching outgrowths of the ectoderm into the adjacent mesoderm along the outer part of the line where the epithelium of the conjunctiva becomes continuous with that covering the inner surface of the upper eyelid. As in the other epidermal glands, the outgrowths and their branches are at first solid, later becoming hollow by the degeneration of their axial cells.
The naso-lachrymal duct is developed in connection with the groove which, at an early stage in the development (Fig. 62), extends
Fig. 280. - Diagram showing the Insertions of the Lachrymal Ducts in EMBRYOS OF 40 MM. AND 170 MM.. THE CaRUNCULA LaCRIMALIS BEING FORMED IN the Latter.
The eyelids are really fused at these stages but have been represented as separate â€¢ for the sake of clearness. - (Ask.) from the inner corner of the eye to the olfactory pit and is bounded posteriorly by the maxillary process of the first visceral arch. The epithelium lying in the floor of this groove thickens toward the begining of the sixth week to form a solid cord, which sinks into the subjacent mesoderm. From its upper end two outgrowths arise which become connected with the ectoderm of the edges of the upper and lower lids, respectively, and represent the lachrymal ducts, and, finally, the solid cord and its outgrowths acquire a lumen and a connection with the mucous membrane of the inferior meatus of the nasal cavity.
The inferior duct connects with the border of the eyelid some distance lateral to the inner angle of the eye, and between its opening and the angle a number of tarsal glands develop. The superior duct, on the other hand, opens at first close to the inner angle and later moves laterally until its opening is opposite that of the inferior duct. During this change the portion of the lower lid between the opening of the inferior duct and the angle is drawn somewhat upward, and, with its glands, forms a small reddish nodule, resting upon the plica semilunaris and known as the caruncula lacrimalis (Fig. 280).
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Zoologie, lxiii and lxv, 1898; lxviii, 1899. A. Robinson: "On the Formation and Structure of the Optic Nerve and Its Relation to the Optic Stalk," Journal of Anat. and Physiol., xxx, 1896. G. Speciale-Cirincione: "Ueber die Entwicklung der Tranendriise beim Menschen " Arch.filr Ophthalmol., LXIX, 1908. J. P. Schaefeer: "The Genesis and Development of the Nasolacrimal Passages in Man," Amer. Journ. Anat., xm, 1912. G. L. Streeter: "On the Development of the Membranous Labyrinth and the Acoustic and Facial Nerves in the Human Embryo," Amer. Journ. of Anat.
vi, 1907. N. van der Stricht: "L'histogenese des parties constituantes du neuroepithelium acoustique, des taches et des cretes acoustiques et de l'organe de Corti " Arch.
de Biol., xxiii, 1908. A. Szili: "Zur Anatomie und Entwickelungsgeschichte der hinteren Irisschichten mit besonderer Beriicksichtigung des Musculus sphincter iridis des Menschen " Anat. Anzeiger, xx, 1901. A. Szili: "Ueber das Entstehen eines fibrillares Stutzgewebes im Embryo und dessen Verhaltnis zur Glaskorperfrage," Anat. Hejte, xxxv, 190S. F. Tuckerman: "On the Development of the Taste Organs in Man," Journal of Anat.
and Physiol., xxiv, 1889. R. Versari: "Ueber die Entwicklung der Blutgefasse des menschlichen Au^es " Anat. Anzeiger, xxxv, 1909.
Chapter XVII. Post-Natal Development
In the preceding pages attention has been directed principally to the changes which take place in the various organs during the period before birth, for, with a few exceptions, notably that of the liver, the general form and histological peculiarities of the various organs are acquired before that epoch. Development does not, however, cease with birth, and a few statements regarding the changes which take place in the interval between birth and maturity will not be out of place in a work of this kind.
The conditions which obtain during embryonic life are so different from those to which the body must later adapt itself, that arrangements, such as those connected with the placental circulation, which are of fundamental importance during the life in utero, become of little or no use, while the relative importance of others is greatly diminished, and these changes react more or less profoundly on all parts of the body. Hence, although the post-natal development consists chiefly in the growth of the structures formed during earlier stages, yet the growth is not equally rapid in all parts, and indeed in some organs there may even be a relative decrease in size. That this is true can be seen from the annexed figure (Fig. 281), which represents the body of a child and that of an adult man drawn as of the same height. The greater relative size of the head and upper part of the body in the child is very marked, and the central point of the height of the child is situated at about the level of the umbilicus, while in the man it is at the symphysis pubis.
That there is a distinct change in the geometric form of the body during growth is also well shown by the following consideration. (Thoma). Taking the average height of a new-born male as 500 mm., and that of a man of thirty years of age as 1686 mm., the height of the body will have increased from birth to adolescence - v _â– = 3.37 times. The child will weigh 3.1 kilos and the man 5:: 66.1 kilos, and if the specific gravity of the body with the included gases be taken in the one case as 0.90 and in the other as 0.93, then the volume of the child's body will be 3.44 liters and that of the man's 71.08 liters, and the increase in volume will be - - =20.66.
Fig. 281. - Child ast> }vL\x Drawn as of th* " Growth of the Brain, " Contemporary Science Series Sons.)
If the increase in volume had taken place without any alteration in the geometric form of the body, it should be equal to the cube of the increase in height; this, however, is 3-37 s =38.27, a number wellnigh twice as large as the actual increase.
But in addition to these changes, which are largely dependent upon differences in the supply of nutrition, there are others associated with alterations in the general metabolism of the body. Up to adult life the constructive metabolism or anabolism is in excess of the destructive metabolism or katabolism, but the amount of the excess is much greater during the earlier periods of development and gradually diminishes as the adult condition is approached. That this is true during intrauterine life is shown by the following figures, compiled by Donaldson:
Age in Weeks
Weight in Grams
Age in Weeks
Weight in Grams
o . 0006
From this table it may be seen that the embryo of eight weeks is six thousand six hundred and sixty-seven times as heavy as the ovum from which it started, and if the increase of growth for each of the succeeding periods of four weeks be represented as percentages, it will be seen that the rate of increase undergoes a rapid diminution after the sixteenth week, and from that on diminishes gradually but less rapidly, the figures being as follows :
Periods of Weeks
Periods of Weeks
8-12 12-16 16-20 20-24
400 500 137 123
24-28 28-32 32-36 36-40
92 39 32 45
That the same is true in a general way of the growth after birth may be seen from the following table, representing the average weight of the body in English males at different years from birth up to twenty-three (Roberts), and also the percentage rate of increase.
Number of Cases
Weight in Kilograms
Certain interesting peculiarities in post-natal growth become apparent from an examination of this table. For while there is a general diminution in the rate of growth, yet there are marked irregularities, the most noticeable being (i) a rather marked diminution during the eleventh and twelfth years, followed by (2) a rapid
- From a comparison with other similar tables there is little doubt but that the weight given above for the second year is too high to be accepted as a good average
1 Z 3 * 5 6 f a 9 ID ft 12 13 14 1$ 16 17 19
2 3 4-5 6 7 8 9 10 11 12 13 J& I
5 17 18
Age Lbs/4 " 12 " 10 â€¢â– 8 " 6 " * ' Z
Fig. 282. - Curves Showing the Annual Increase in Weight in (I) Boys and (II) Girls.
The faint line represents the curve from British statistics, the dotted line that from American (Bowditch), and the heavy line the average of the two. Before the sixth year the data are unreliable. - (Stephenson.) acceleration which reaches its maximum at about the sixteenth year and then very rapidly diminishes. These irregularities may be more Consequently the percentage increase for the second year is too high and that for the third year too low.
It may be mentioned that the weights in the original table are expressed in pounds avoirdupois and have been here converted into kilograms, and further the figures representing the percentage increase have been added.
clearly seen from the charts on page 474, which represent the curves obtained by plotting the annual increase of weight in boys (Chart I) and girls (Chart II). The diminution and acceleration of growth referred to above are clearly observable and it is interesting to note that they occur at earlier periods in girls than in boys, the diminution occurring in girls at the eighth and ninth years and the acceleration reaching its maximum at the thirteenth year.
Considering, now, merely the general diminution in the rate of growth which occurs from birth to adult life, it becomes interesting to note to what extent the organs which are more immediately associated with the metabolic activities of the body undergo a relative reduction in weight. The most important of these organs is undoubtedly the liver, but with it there must also be considered the thyreoid and thymus glands, and probably the suprarenal bodies. In all these organs there is a marked diminution in size as compared with the weight of the body, as will be seen from the following table (H. Vierordt), which also includes data regarding other organs in which a marked relative diminution, not in all cases readily explainable, occurs.
ABSOLUTE WEIGHT IN GRAMS.
New-born and Adult.
â€ž . Spmal Brain _; . Cord
PERCENTAGE WEIGHT OF ENTIRE BODY New-born and Adult.
4-57 2 -57
12 .29 2 .16
Recent observations by Hammar render necessary some modification of the figures given for the thymus in the above table. He finds the average weight of the gland at birth to be 13.26 grms., and that the weight increases up to puberty, averaging 37.52 grms. between the ages of 11 and 15. After that period it gradually diminishes, falling to 16.27 g rm sbetween 36 and 45, and to 6.0 grms. between 66 and 75. Expressed in percentage of the body weight this gives a value in the new-born of 0.42 and in an individual of 50 years of 0.02, a difference much more striking than that shown in Vierordt's table.
It must be mentioned, however, that the gland is subject to much individual variation, being largely influenced by nutritive conditions.
The remaining organs, not included in the tables given above, when compared with the weight of the body, either show an increase or remain practically the same.
ABSOLUTE WEIGHT IN GRAMS. New-born and Adult.
Skin and Subcutaneous Tissues
Stomach and Musculature , T Intestines
776.5 65 28,732.0 1,364
PERCENTAGE OF BODY-WEIGHT. New-born and Adult.
Skin and Subcutaneous Tissues
Stomach and Intestines
2 5-05 43-40
2 . 1 2 .06
0. 11 015
From this table it will be seen that the greatest increment of weight is that furnished by the muscles, the percentage weight of which is one and three-fourths times as great in the adult as in the child. The difference does not, however, depend upon the differentiation of additional muscles; there are just as many muscles in the new-born child as in the adult, and the increase is due merely to an enlargement of organs already present. The percentage weight of the digestive tract, pancreas, and lungs remains practically the same, while in the case of the skeleton there is an appreciable increase, and in that of the skin and subcutaneous tissue a slight
Fig. 283. - Longitudinal Section through the Sacrum of a New-born Female Child. - (Fehling.)
diminution. The latter is readily understood when it is remembered that the area of the skin, granting that the geometric form of the body remains the same, would increase as the square of the length, while the mass of the body would increase as the cube, and hence in comparing weights the skin might be expected to show a diminution even greater than that shown in the table.
The increase in the weight of the skeleton is due to a certain extent to growth, but chiefly to a completion of the ossification of the cartilage largely present at birth. A comparison of the weights of this system of organs does not, therefore, give evidence of the many changes of form which may be perceived in it during the period under consideration, and attention may be drawn to some of the more important of these changes.
In the spinal column one of the most noticeable peculiarities observable in the new-born child is the absence of the curves so characteristic of the adult. These curves are due partly to the weight of the body, transmitted through the spinal column to the hipjoint in the erect position, and partly to the action of the muscles, and it is not until the erect position is habitually assumed and the musculature gains in development that the curvatures become pronounced. Even the curve of the sacrum, so marked in the adult, is but slight in the new-born child, as may be seen from Fig. 283, in which the ventral surfaces of the first and second sacral vertebrae look more ventrally than posteriorly, so that there is no distinct promontory.
But, in addition to the appearance of the curvatures, other changes also occur after birth, the entire column becoming much more slender and the proportions of the lumbar and sacral vertebrae becoming quite different, as may be seen from the following table (Aeby) :
Lengths Of The Vertebral Regions Expressed As Percentages Of The Entire Column
25.6 23-3 20.3 19.7 22 .1
47-5 46.7 45-6 47.2 46.6
Male 2 years
Male 5 years
Male 1 1 years Male adult
The cervical region diminishes in length, while the lumbar gains, the thoracic remaining approximately the same. It may be noticed, furthermore, that the difference between the two variable regions is greater during youth than in the adult, a condition possibly associated with the general more rapid development of the lower portion of the body made necessary by its imperfect development during fetal life. The difference is due to changes in the vertebrae, the intervertebral disks retaining approximately the same relative thickness throughout the period under consideration.
The form of the thorax also alters, for whereas in the adult it is barrel-shaped, narrower at both top and bottom than in the middle, in the new-born child it is rather conical, the base of the cone being below. The difference depends upon slight differences in the form and articulations of the ribs, these being more horizontal in the child and the opening of the thorax directed more directly upward than in the adult.
As regards the skull, the processes of growth are very complicated. Cranium and brain react on one another, and hence, in harmony with the relatively enormous size of the brain at birth, the cranial cavity has a relatively greater volume in the child than in the adult. The fact that the entire roof and a considerable part of the sides of the skull are formed of membrane bones which, at birth, are not in sutural contact with one another throughout, gives opportunity for considerable modifications, and, furthermore, the base of the skull at the early stage still contains a considerable amount of unossified cartilage. Without entering into minute details, it may be stated that the principal general changes which the skull undergoes in its post-natal development are (i) a relative elongation of its anterior portion and (2) an increase in the relative height of the maxillae.
If a line be drawn between the central points of the occipital condyles, it will divide the base of the skull into two portions, which in the child's skull are equal in length. The portion of the skull in front of a similar line in the adult skull is very much greater than that which lies behind, the proportion between the two parts being 5:3, against 3:3 in the child (Froriep). There has, therefore, been a decidedly more rapid growth of the anterior portion of the skull, a growth which is asssociated with a corresponding increase in the dorso-ventral dimensions of the maxillae. These bones, indeed, play a very important part in determining the proportions of the skull at different periods. They are so intimately associated with the cranial portions of the skull that their increase necessitates a corresponding increase in the anterior part of the cranium, and their increase in this direction stands in relation to the development of the teeth, the eight teeth which are developed in each maxilla (including the premaxilla) in the adult requiring a longer bone than do the five teeth of the primary dentition, these again requiring a greater length when completely developed than they do in their immature condition in the new-born child.
Fig. 284. - Skull of a New-born Child and of an Adult Man, Drawn as of Approximately the Same Size. - (Henke.)
But far more striking than the difference just described is that in the relative height of the cranial and facial regions (Fig. 284). It has been estimated that the volumes of the two portions have a ratio of 8: 1 in the new-born child, 4: 1 at five years of age, and 2:1 in the adult skull (Froriep) , and these differences are due principally to changes in the vertical dimensions of the maxillae. As with the increase in length, the increase now under consideration is, to a certain extent at least, associated with the development of the teeth, hese structures calling into existence the alveolar processes which ,re practically wanting in the child at birth. But a more important actor is the development of the maxillary sinuses, the practically olid bodies of the maxillae becoming transformed into hollow shells, rhese cavities, together with the sinuses of the sphenoid and frontal >ones, which are also post-natal developments, seem to stand in elation to the increase in length of the anterior portion of the skull, erving to diminish the weight of the portion of the skull in front Â»f the occipital condyles and so relieving the muscles of the neck of a onsiderable strain to which they would otherwise be subjected.
These changes in the proportions of the skull have, of course, nuch to do with the changes in the general proportions of the face. 3ut the changes which take place in the mandible are also imporant in this connection, and are similar to those of the maxillae in leing associated with the development of the teeth. In the new10m child the horizontal ramus is proportionately shorter than in he adult, while the vertical ramus is very short and joins the Lorizontal one at an obtuse angle. The development of the teeth if the primary dentition, and later of the three molars, necessitates ,n elongation of the horizontal ramus equivalent to that occurring n the maxillae, and, at the same time, the separation of the alveolar â€¢orders of the two bones requires an elongation of the vertical ramus f the condyle is to preserve its contact with the mandibular fossa, ,nd this, again, demands a diminution of the angle at which the ami join if the teeth of the two jaws are to be in proper apposition.
In the bones of the appendicular skeleton secondary epiphysial enters play an important part in the ossification, and in few are hese centers developed prior to birth, while the union of the epiphyes to the main portions of the bones takes place only toward maurity. The dates at which the various primary and secondary enters appear, and the time at which they unite, may be seen from he following table:
POST-NATAL DEVELOPMENT UPPER EXTREMITY.
Appearance of Secondary
(At sternal end) 17th year.
8th week. <.
2 acromial 15th year.
2 on vertical border 16th year.
> 20th year.
1 st year.
15 th year.
Head 1st year.
Great tuberosity 3d year.
> 20th year.
Lesser tuberosity 5th year.
â– jth week.
Inner condyle 5th year.
1 8th year.
Capitellum 3d year.
Trochlea 10th year.
[â€¢ 17 th year.
Outer condyle 14th year.
Olecranon 10th year.
Distal epiphysis 4th year.
1 8th year.
Proximal epiphysis 5th year.
17 th year.
Distal epiphysis 2d year.
Triquetrum . . .
12 th year.
Metacarpals . . .
17 th-! 8th years.
The dates in italics are before birth.
POST-NATAL DEVELOPMENT LOWER EXTREMITY.
Appearance of Secondary Fusion of
Crest 15th year.
Anterior inferior spine 15 th year.
â– 22d year.
Tuberosity 15th year.
Crest 1 Sth year.
Cartilage appears at 4th month, ossification in 3d year.
Head 1st year.
20 th year.
â– jth week.
Great trochanter 4th year. Lesser trochanter 13 th- 14th year. Condyle gth month.
19 th year. 1 Sth year. 2 1 st year.
Head end of gth month. Distal end 2d year.
2ist-2 5thyear. 1 Sth year.
Upper epiphysis 5th year. Lower epiphysis 2d year.
21st year. 20th year.
1 6 th year.
A few days after birth.
1 st year.
The dates in italics are before birth.
So far as the actual changes in the form of the appendicular bones are concerned, these are most marked in the case of the lower limb. The ossa innominata alter somewhat in their proportions after birth, a fact which may conveniently be demonstrated by considering the changes which occur in the proportions of the pelvic diameters, although it must be remembered that these diameters are greatly influenced by the development of the sacral curve. Taking the conjugate diameter of the pelvic brim as a unit for comparison, the antero-posterior (dorso-ventral) and transverse diameters of the child and adult have the proportions shown in the table on the opposite page (Fehling).
It will be seen from this that the general form of the pelvis in the new-born child is that of a cone, gradually diminishing in diameter from the brim to the outlet, a condition very different from what obtains in the adult. Furthermore, it is interesting to note
(Conjugata vera . Transverse >, f Antero-posterior rt 1 U y Transverse -^ ( Antero-posterior O Transverse
1.294 1. 18 1. 14 1 .07 1 -153
that sexual differences in the form of the pelvis are clearly distinguishable at birth; indeed, according to Fehling's .observations, they become noticeable during the fourth month of intrauterine development.
The upper epiphysis of the femur is entirely unossified at birth and consists of a cartilaginous mass, much broader than the rather slender shaft and possessing a deep notch upon its upper surface (Fig. 285). This notch marks off the great trochanter from the head of the bone, and at this stage of development there is no neck, the head being practically sessile. As development proceeds the inner upper portion of the shaft grows more rapidly than the outer portion, carrying the head away from the great trochanter and forming the neck of the bone. The acetabulum is shallower at birth than in the adult and cannot contain more than half the head of the femur; consequently the articular portion of the head is much less extensive than in the adult.
It is a well-known fact that the new-born child habitually holds the feet with the soles directed toward one another, a position only reached in the adult with some difficulty, and associated with this supination or inversion there is a pronounced extension of the foot (i. e., flexion upon the leg as usually understood; see p. 102), it being difficult to flex the child's foot beyond a line at right angles with the axis of the leg. These conditions are due apparently to the extensor and tibialis muscles being relatively shorter and the opposing muscles relatively longer than in the adult, and with the elongation or shortening, as the case may be, of the muscles on the assumption
Fig. 2S5. - Longitudinal Sections of the Head of the Femur of (.4) New-born Child and (B) a Later Stage of Development.
ep, Epiphysial center for the head; h, head; /, trochanter. - (Henke.) of the erect position, the bones in the neighborhood of the anklejoint come into new relations to one another, the result being a modification of the form of the articular surfaces, especially of the talus (astragalus). In the child the articular cartilage of the trochlear surface of this bone is continued onward to a considerable extent upon the neck of the bone, which comes into contact with the tibia in the extreme extension possible in the child. In the adult, however, such extreme extension being impossible, the cartilage upon the neck gradually disappears. The supination in the child brings the talus
in close contact with the inner surface of the calcaneus and with the sustentaculum tali; with the alteration of position a growth of these portions of the calcaneus occurs, the sustentaculum becoming higher and broader, and so becoming an obstacle in the way of supination in the adult. At the same time a greater extent of the outer surface of the talus comes into contact with the lateral malleolus, with the result that the articular surface is considerably increased on that portion of the bone. Marked changes in the form of the talo-navicular articulation also occur, but their consideration would lead somewhat further than seems desirable.
C. Aeby: "Die Altersverschiedenheiten der menschlichen Wirbelsaule." Archiv fur Anal, und Physiol., Anat. Abth., 1879. W. Camerer: " Utersuchungen iiber Massenwachsthum und Langen wachsthum der Kinder," Jahrbuchfiir Kinderheilkunde, xxxvi, 1893. H. H. Donaldson: "The Growth of the Brain," London, 1895. H. Fehling: "Die Form des Beckens beim Fotus und Neugeborenen und ihre Bezie hung zu der beim Erwachsenen," Archiv fur Gynakol., x, 1876. H. Friedenthal: " Das Wachsthum des Korpergewichtes des Menschen und anderer Saugethiere in verschiedenen Lebensaltern," Zeit. allgem. Physiol., ix, 1909. J. A. Hammar: "Ueber Gewicht, Involution und Persistenz der Thymus im Post fotalleben des Menschen," Archiv fur Anat. und Phys., Anat. Abth., Supplement, 1906. W. Henke: " Anatomie des Kindersalters," Handbuch der Kinder krankheiten (Cerhardt) , Tubingen, 1881. C. Hennig: "Das kindliche Becken," Archiv fur Anat. und Physiol., Anat. Abth., 1880. C. Huter: "Anatomische Studien an den Extremitatengelenken Neugeborener und Erwachsener," Archiv fur patholog. Anat. und Physiol., xxv, 1862. W. Stephenson: "On the Relation of Weight to Height and the Rate of Growth in Man," TheLancet, 11, 1888. R. Thoma: " Untersuchungen iiber die Grosse und das Gewicht der anatomischen Bestandtheile des menschlichen Korpers," Leipzig, 1882. H. Vierordt: "Anatomische, Physiologische und Physikalische Daten und Tabellen," Jena, 1893. H. Welcker: "Untersuchungen iiber Wachsthum und Bau des menschlichen Schadels," Leipzig, 1862.
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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