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 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.)
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 from Fig 195, which represents the anterior surface of the septum, together with the related parts, in a rabbit embryo of nine days.
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.)
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
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 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 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.
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.)
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 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).
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.
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 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 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.)
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 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 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 subserous areolar tissue, the duodenum assuming the retroperitoneal position which characterizes it in the adult.
Fig. 198. - Diagram showing the arrangement of the Mesentery and Visceral Branches of the Abdominal Aorta in an Em
Fig. 199. -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. - (Hertwig.)
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 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 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.
Fig. 200 - Diagrams Illustrating the Manner in Which the Fixation of the Descending Colon (C) takes Place.
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 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.
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.)
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.
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'arrierecavite 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.
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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