Human Embryology and Morphology 19

From Embryology

Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures

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Chapter XIX. Organs of Digestion

Divisions of the Alimentary Tract

It is always advantageous to approach the development of every system of the body by a recapitulation of the various evolutionary stages, so far as these stages are known to us. As regards the evolution of the various parts of the alimentary system, comparative anatomy does not help us greatly, because in even the lowest forms of vertebrates the main parts are already present — the mouth, oesophagus, stomach, liver and intestine. In tracing the development of the earliest digestive cavity (archenteron) of the human embryo (p. 38) we saw that its origin was similar to that of the lower invertebrates and that its first mouth apparently became converted into the blastopore, primitive streak and cloacal membrane. A new mouth is formed by the breaking down of the bucco-pharyngeal membrane (oral membrane. Fig. 273) early in the third week ; we shall see that a new kind of vent or anus is formed at a later stage in the development of the human embryo — namely, at the end of the 2nd month of development. There are other reasons why comparative anatomy does not help us to understand the early stages in the development of the alimentary system. They will be understood by a reference to Fig. 273. In the human embryo a large part of the alimentary cavity has been specialized and precociously developed to form the yolk sac for the nourishment of the embryonic tissues ; the embryonic adaptations mask and obliterate the ancestral stages (see page 35).

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Fig. 273. The Form of the Alimentary Canal in a Human Embryo of the 4th week.

1 For literature on development of alimentary system see A. Oppel, Ergebnisse der Anat. 1905, vol. 15, p. 207 ; 1906, vol. 16, p. 216 ; 6. Grosser, Verhand. Aimt. Gesellsch. 1911, p. 173 ; Keibel and Mall's Manual of Human Embryology, 1912, vol. 2, p. 291.

With the development of the cephalic and caudal evaginations of the embryonic plate the archenteron becomes differentiated into three parts (Fig. 274) — the Mid-gut, which represents the body and chief part of the primitive cavity ; the Fore-gut and Hind-gut. There can be no doubt these represent three functional divisions. The mid-gut is supplied by the superior mesenteric artery and serves for one kind of digestion and absorption ; the hind-gut, supplied by the inferior mesenteric artery, is mainly excretory in nature ; the fore-gut, separated by the outgrowth of the liver from the mid-gut, is supplied mainly by the coeliac axis and serves the preparatory purposes of digestion. The pharynx, respiratory tract, oesophagus, stomach, liver and pancreas represent parts of the fore-gut. The hind-gut gives rise to the colon from the splenic flexure to the anus ; the allantois, bladder and urethra are also separated from its hinder end — the cloaca.

Differentiation of Parts

How rapidly the various parts of the alimentary system are differentiated during the 4th week of development will be seen by comparing Figs. 274 and 275. Fig. 274, which represents the alimentary tract of a human embryo near the beginning of the 4th week, shows the pharynx large, the lung bud beginning to evaginate from the floor of the fore-gut just behind the pharynx and at this date lying directly under the occipital part of the head ; the oesophagus and stomach and first part of the duodenum scarcely marked off from one another, all of them lying on the dorsal wall of the pericardium and lying under the cervical segments of the embryo. The evagination to form the liver indicates the junction of the fore-gut with the mid-gut. The latter division is in wide communication with the yolk sac. The various parts of the hind-gut are already indicated. The condition towards the end of the 4th week is shown in Fig. 275. The oral membrane is gone ; the pharynx is relatively smaller ; the outgrowth of the pulmonary system is now very apparent, the oesophagus and stomach are longer and narrower ; the liver outgrowth has become massive ; the mid-gut is tubular, and the neck of the yolk sac reduced to a duct (vitello-intestinal duct). The parts of the hind-gut have assumed a more definite shape.

Primitive Mesentery and Coelom

It will be remembered that almost as soon as it appears, the mesoderm is cleft into two layers — an outer applied to the ectoderm to form the somatopleure or body wall, and an inner to the entoderm or archenteron to form the visceral wall or splanchnopleure. The cavity formed by the cleavage of the mesoderm is the coelom (Fig. 39). Originally the cavity was designed for the purposes of excretion ; its wall also served as the nidus for the reproductive cells. In vertebrates the coelom came to serve the purposes of a large bursa, in order that the muscular movements of the digestive canal, lungs and heart might proceed without undue friction. Hence the alimentary canal is developed within the cavity of the coelom, being situated within the median partition, which separates the right coelomic space from the left. The median partition suspends the alimentary canal to the dorsal or vertebral wall of the body cavity, and forms the primitive dorsal mesentery ; the part of the median partition which fixes the tract to anterior or ventral wall of the body cavity forms the primitive ventral mesentery which, however, is formed only in connection with the fore-gut and the cloacal segment of the hind-gut, all the rest being destitute of a ventral mesentery from the beginning. Hence the right and left coelomic spaces in the abdomen are thrown into one, and form the peritoneal cavity. The only parts of the alimentary canal which never come to lie within the coelom are the anterior part or pharynx and the most posterior part of the cloaca. The anterior part of the coelomic space forms the cavity of the pericardium, which lies beneath the pharynx (Fig. 274) ; it is separated from the peritoneal space by a transverse partition — the septum transversum,^ already well marked at the beginning of the 4th week. The primitive oesophagus crosses the upper or dorsal border of the septum transversum (Fig. 279). At each side of it is situated a communication between the pericardial and peritoneal spaces — the pleuroperitoneal passages. These two passages are separated not only by the primitive oesophagus, but also by the primitive median mesentery, which encloses the oesophagus (Fig. 279).

1 Broman, Ergebnisse der Anat. 1905, vol. 15, p. 332.

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Fig. 274. The Alimentary System of a Human Embryo 2'5 mm. long, and near the commencement of the 4th week of development. (Professor Peter Thompson.)

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Fig. 275. The Alimentary System of a Human Embryo, although only 3 mm. long, is in the stage of development reached at the end of the 4th week. (After Professor Broman.)

^ P. Thompson, Journ. Anat. and Physiol. 1908, vol. 42, p. 170.


In the 4th week the oesophagus of the human embryo resembles that of a fish ; it is merely a sphincter or constricted part between the pharynx and stomach (Fig. 274). During the 6th and 7th weeks, when the neck is being difierentiated, and the pharynx and head separated from the heart and thorax, the oesophagus undergoes a rapid elongation. The chief cause of the elongation of the oesophagus is to be sought for in the development of the lungs and pleural cavities (Fig. 277), by which the stomach is forced backwards in the body cavity. The oesophagus is of double origin ; the upper or paratracheal part is derived with the trachea from the retropharyngeal segment of the fore-gut ; the lower or retrotracheal part arises from the pregastric segment of the fore-gut. In the 5th week the jDulmonary bud and tracheal groove are being separated from the oesophagus, the lateral septa which effect the separation, beginning behind and spreading forwards (Figs. 276, 277). Children are sometimes born in which the process of separation has taken place in an irregular manner (Fig. 278). The paratracheal part ends blindly, and is surrounded by striated pharyngeal musculature ; the retrotracheal part opens from the trachea, and is covered by non-striated muscle.^ The oesophagus is at first lined by columnar epithelium, but in the 2nd month, as it elongates the epithelium proliferates, forming several irregular layers, which almost occlude the lumen of the tube for a time. In the 5th month glands are formed in the submucous tissue. In the 6th week the oesophagus is only 2 mm. long ; at birth it measures 100 mm. (4 inches). Its commencement is surrounded by a sphincter formed by part of the inferior constrictor of the pharynx ; above this sphincter, in later life, a pouch (retropharyngeal diverticulum) may arise ; such pouches are never congenital in origin.

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Fig. 276. Fore-gut of an Embryo in the 4th week of development. (Broman.)

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Fig. 277. Fore-gut of an Embryo at the end of the 5th week of development. A, hepatic stalk ; B, ventral pancreatic bud. (Broman.)

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Fig. 278. Irregular Separation of the Trachea and Oesophagus. The upper or pharyngeal part of the oesophagus forms a blind sac ; the lower part passes from the trachea to the stomach. The normal trachea — oesophageal septum — is marked * ; the abnormal septum **.

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Fig. 279. The Mesentery of the Fore-gut and its Contents, viewed from the left side (schematic).

At the lower end the oesophagus is also closed by a sphincter. The muscle coats are difierentiated in the 7th week, the circular first, the longitudinal later.

Development of the Liver

Before proceeding to describe the development of the stomach, it is convenient to deal first with the Hver, because the manner in which this viscus arises gives the key to the complicated developmental changes of the abdominal viscera. The human liver in its development repeats broadly the forms met with in ascending the animal scale. In amphioxus the liver is merely a caecal diverticulum of the digestive canal ; in amphibians it is a modified tubular gland — the hepatic cells being arranged in cylinders around the bile ducts. In mammals the tubular arrangement is lost and a lobular form substituted. In every case it is so placed that the blood, laden with the products of absorption from the alimentary tract or from the placenta, must come into intimate relationship with the hepatic tissue before passing into the general circulation of the body.

1 See Keith, Brit. Med. Journ. 1910, vol. 1, p. 301. For development of mucous membrane see F. P. Johnson, Amer. Journ. Anat. 1910, vol. 10, p. 521.

2 0. Charnock Bradley, Journ. Anat. and Physiol. 1909, vol. 43, p. 1 ; F. P. Mall, Amer. Journ. Anat. 1906, vol. 5, p. 227 ; F. T. Lewis, Keibel and Hall's Manual of Human Embryology, 1912, vol. 2, p. 403 ; Prof. P. Thompson, Journ. Anat, 1914, vol. 48, p. 222. See also references to Bamiville (p. 47) and Waterston (p. 18) ; Prof. Frazer, Journ. Anat. 1919, vol. 54, p. 116.

To understand the development of the liver, the condition of parts at the commencement of the 4th week must be studied. At this time, the anterior wall of the yolk sac and that part of the fore-gut which becomes the stomach, lie in the dorsal wall of the septum transversum (Fig. 274), or to be more accurate, in the substance of the dorsal and ventral mesentery which have not yet been differentiated from the septum transversum (Fig. 279). Two other views of the septum transversum are given in Figs. 280, 281, which will assist the reader to understand the early relationship of the liver. When the liver bud grows out, it springs from the junction of the fore-gut and yolk sac (Fig. 279) ; and spreads into the tissue which becomes the ventral mesentery of the fore-gut. The part of the gut from which it arises afterwards becomes the second stage of the duodenum. The hepatic bud is at first a hollow, a fold-like diverticulum of the fore- gut, lined with entoderm ; from the upper or cranial end of the diverticulum arises the outgrowth of liver tissue ; its lower or caudal end becomes the gall bladder and main bile ducts (Fig. 276). The diverticulum is surrounded in the mesogastrium by a mass of mesodermal cells which form the vessels, capsule and connective tissue of the liver. From the hollow hepatic diverticulum arise right and left solid processes of entodermal cells, which invade and form masses round the right and left veins from the yolk sac — the vitelline veins (Figs. 279, 281). Professor Bradley ^ has pointed out that the right and left masses do not correspond to the right and left lobes of the fully formed liver ; the separation between the right and left lobes is formed late, and has no functional significance. A line from the fundus of the gall bladder to the caval impression divides the liver into embryonic and functional right and left halves (Canthe).

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Fig. 280. Dissection of the Septum TransversTim of a Human Embryo early in the 4th week of development. The right half is cut away to expose the yolk sac. (After Low.)

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Fig. 281. Coronal Section of the Septum Transversum of a Human Embryo in the 5th week of development, showing the liver trabeculae invading the terminal parts of the vitelline veins. After Wilhelm His (1831-1904)

The hepatic buds are developed just behind the sinus venosus and between both the vitelline and umbilical veins which are also situated in the ventral mesentery (Figs. 279, 281, 282). The veins are broken up by the ingrowth ; from them starts an invasion of sinus-like capillaries which, with the surrounding mesoderm, penetrates the liver bud and breaks the solid entodermal processes into reticulating cylinders. According to F. T. Lewis the hepatic processes perforate and proliferate within the lumina of the vitelline veins, the venous capillaries thus arising directly from venous spaces. Secondary processes arise from the primary hepatic reticulating cylinders and form smaller and smaller meshes of hepatic cells. The hepatic cells, first grouped in trabeculae, become arranged in lobular units ; round the periphery of the units are the terminal portal venules ; in the centre of each unit is the beginning of a tributary of the hepatic vein ; the portal or placental blood as it passes from the periphery to the centre of each lobule is exposed to the action of the liver cells. Growth takes place by successive division or dichotomy of the lobules, the chief areas of proliferation being always at the surface of the organ or subcapsular. Growth is particularly rapid during the 2nd and 3rd months, the liver reaching its largest relative size at this time. Up to the 10th week, when the foetus is 42 mm. long, the right and left halves have grown symmetrically, but then occurs the retraction of the bowel from the umbihcal cord and the enlargement of the stomach, leading to an atrophy of part of the left lobe. The ducts of the liver, unlike those of any other gland, arise by a secondary process. Undifferentiated tissue lying along the distribution of the portal vein in the liver group themselves in cords, develop lumina, become covered by mesodermal tissue and thus form the intra-hepatic bile ducts.

Veins of the Liver

Within the liver the two vitelline veins become divided so as to form two sets of vessels — afferent or distributing and efferent or collecting veins (Fig. 282). In the 5th week a number of remarkable changes occur : (1) The left umbilical vein, which opens at first in the left duct of Cuvier, establishes a communication with the portal sinus in the transverse fissure of the liver (Figs. 281, 282, 283) ; (2) the right umbilical vein disappears ; (3) a new channel — the ductus venosus — is opened between the portal sinus and the inferior vena cava ; (4) the right vitelline vein, all except its terminal part, becomes obliterated (Fig. 283).

1 Journ. Anat. and Physiol. 1909, vol. 43, p. 1. s

Gall Bladder and Bile Ducts

The hepatic diverticulum, from which the liver buds arise, may be regarded as a direct extension of the wall of the fore-gut. From its hinder part (Fig. 276) are developed lies in the ventral mesentery (gastro-hepatic omentum) — a position which is permanent in some vertebrates and may occur as a rare anomaly in man. In the second month it becomes embedded in the hepatic tissue, its fundus appearing on the diaphragmatic surface ; at a later date it the common bile duct, the gall bladder, and the cystic duct formed at the junction of the gall bladder and common bile duct. The hepatic ducts arise within the stalks of the solid hepatic buds. At first the gall bladder assumes its superficial position. The lumen of the ducts is occluded by an epithelial proliferation until the 3rd month ; bile enters the gall bladder in the 6th month. Originally its veins end in the adjoining hepatic tissue. Occasionally the bud for the gall bladder divideS; giving rise to a bifid or double gall bladder. Eound the termination of the common bile duct a sphincter is developed from the musculature of the duodenum. The manner in which the common bile duct, hepatic artery and portal vein come to occupy the free edge of the ventral mesogastrium will be described in another paragraph.

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Fig. 282. The Liver Mass invading the Vitelline Veins during the 4th week of development. (Professor Mall.)

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Fig. 283. Diagram to show the Transformation in the Veins round the Liver at the end of the 5th week of development. (After Professor Mall.)

^ A. Pensa, Aiiat. Anz. 1912, vol. 41, p. 155.

Separation of the Liver from the Septum Transversum

As the liver develops, the dorsal and ventral mesenteries of the fore-gut, in the substance of which the liver and stomach are formed, become differentiated from the tissues of the septum transversum. The typical arrangement of these membranes, as seen in reptiles, is shown in Fig. 284. In the dorsal mesentery (mesogastrium) lie the inferior vena cava and arteries of the fore-gut ; in the ventral mesentery (gastro-hepatic omentum) are contained the terminal parts of three veins — the umbilical, portal and inferior vena cava, the last vessel reaching the ventral mesentery by passing to the right of the oesophagus. The liver develops within both ventral and dorsal mesenteries, but that part of the mesentery in which it and the inferior vena cava lie — the mesohepar — becomes separated from the part which is occupied by the bile ducts, portal vein and the stomach. Broman found that this separation, which occurs in all higher vertebrates, takes place towards the end of the 4th week in the human embryo, by the development of a recess in the mesentery — the Mesenteric Recess — which commences to the right side of the duodenum, and extends forwards (see Fig. 287). The mesenteric recess ^ (bursa omentalis, Broman) forms the vestibular or hepatic part of the lesser sac of the peritoneum, and extends from the foramen of Winslow to behind the Spigelian lobe of the liver (see Figs. 287, 288 and 286). When the liver and stomach are removed in the course of dissection, the attachment of the mesohepar will be seen to bound the Spigelian part of the lesser sac on the right, while on its left side, the dorsal mesogastrium has been evaginated to form the main body of the lesser sac (Fig. 288). Thus it will be seen that the dorsal and ventral mesenteries of the fore-gut are split into a right lamina — the mesohepar, and a left lamina — the mesogastrium — by the development of a recess which forms the earliest and first part of the lesser sac. The mesenteric recess at first extends forwards in the mesentery of the oesophagus almost to the right lung bud — a condition which is constant in reptiles. When the lungs expand and the diaphragm is being formed during the 7th week, the apical part of the mesenteric recess is cut ofi and left within the thorax — ^to the right of the oesophagus, and just above the diaphragm. To this detached part, Broman has given the name of infra-cardiac bursa (Fig. 286). It usually disappears at the end of foetal life, but a remnant can often be found in adults if careful search is made.

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Fig. 284. The origin of the Peritoneal Ligaments connected with the Liver. Diagram to show the foetal relationship of the ventral mesentery to veins and the stomach, the liver being removed.

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Fig. 285. The Liver viewed from beliind to show its relationship to the Gastrohepatic Omentum, part of the Ventral Mesentery.

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Fig. 286. The Visceral Surface of the Liver of a Foetus, 16 mm., in the 7th week of development. (P. Thompson.)

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Fig. 287. Diagram of the Primitive Attachments of the Visceral Mesenteries to the Posterior Wall of the Abdomen as . seen in a Low Primate (Lemur Coronatus). The condition illustrates the earlier developmental phases of the human foetus.

1 See F. T. Lewis, Anat. Rec. 1916, vol. 10, p. 220.

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Fig. 288. Diagram of the Attachments of the Visceral Mesenteries to the Posterior Abdominal Wall of an Adult. The three chief modifications seen, when compared with Fig. 246, are (1) the extensive adhesion of the mesogastrium, (2) of the mesocolon, and (3) mesentery of small intestine, to the posterior wall of the abdomen.

The Ligaments of the Liver

When the liver separates from the septum transversum towards the end of the 2nd month of development, it is attached to the walls of the abdomen by peritoneal ligaments derived from the dorsal and ventral mesenteries of the fore-gut (Figs. 284, 285). These are the following :

  1. The gastro-hepatic omentum is that part of the ventral mesentery which passes from (1) the oesophagus, (2) lesser curvature or ventral border of stomach, and (3) first stage of duodenum to (1) the diaphragm, (2) the posterior part of the longitudinal fissure of the liver, the ductus venosus lying within its hepatic attachment, and (3) the transverse fissure of the liver (Fig. 285). The portal and umbilical veins lie in the ventral mesentery (Fig. 284) ; the hepatic artery passes by it to the liver. The right or free border of the gastro-hepatic omentum, with the falcifoT-m ligament containing the remnant of the umbilical vein, represents the posterior border of the primitive ventral mesentery (Fig. 284).
  2. The falciform ligament, containing the umbilical vein, also represents part of the ventral mesentery (Fig. 284). At an early stage the umbilical veins reached the sinus venosus by passing through the septum transversum. The terminal parts of both veins become obliterated (Fig. 283) ; the new terminal channel for the left vein is formed in the ventral mesentery.
  3. The coronary, the right and left lateral ligaments, and the attachments to the vena cava and diaphragm. — These ligaments, which are the chief hepatic bonds, are derived from the mesohepar in the later part of the 2nd month, when the liver is being separated from the diaphragm by invading pockets or recesses of peritoneum. It would be extremely convenient to retain the term mesohepar to designate the bonds between the liver and diaphragm in the adult, looking on the right and left lateral ligaments as merely processes of the mesohepar.

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Fig. 289. Diagram of a Mammalian Liver viewed from behind and below.

Morphology o£ the Liver

The liver of orthograde (upright) animals (man, anthropoids) differs widely in form and lobulation from that of mammals generally, but Professor Arthur Thomson has shown that traces of the fissures and lobes of the typical mammalian liver can be seen in the human organ. The liver of a dog or dog-like ape consists of three main lobes — right, middle and left — and two accessory lobes — the caudate and Spigelian (Fig. 289). In man the right and middle lobes have fused, but traces of the fissure which separates them (the right lateral fissure) are frequently to be seen in the liver of the newly born child (Fig. 290). The caudate lobe has been reduced in man to a vestige, but in the third month foetus it is of considerable size (Figs. 286, 290). It projects from the liver at the upper boundary of the foramen of Winslow ; in many animals it rivals the right lobe in size. The caudate fissure separates the caudate from the right lobe, and a trace of this fissure is very frequently to be seen in the human liver (Fig. 286). Irregular lobulation of the liver is not uncommon ; the condition seen in the 6th week, when the gall bladder and umbilical vein occupy a common fissure, may be retained. The quadrate lobe arises in the 7th week (Fig. 286) from the left lobe and grows across the fissure occupied by the umbilical vein to occupy the space between the vein and gall bladder (P. Thompson).

1 1 have dealt with some of the factors which determine the shape of the liver in lectures on enteroptosis ; see Lancet, 1903, March 7th and 14th. For cases of malformation of liver see E. Barclay-Smith, Journ. Anat. and Physiol. 1909, vol. 43, p. 346 ; Prof. P. Thompson, Journ. Anat. 1914, vol. 48, p. 222.

Changes in the Liver after Birth

During foetal life the liver increases rapidly in size in comparison with the other abdominal organs.

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Fig. 290. The Lower Surface of the Liver of a Human Foetus during the Srd month, showing Vestiges of Fissures and Lobes of the typical Mammalian Liver.

At birth it occupies nearly half of the abdominal space, and measures Y^^th of its final volume. The left lobe may still reach, and even overlap, the spleen. Up to the time of birth nucleated red blood corpuscles multiply within it (page 334). After birth two factors come into operation which lead to a diminution in size and change of shape. It is supplied before birth by placental instead of portal blood ; at birth, its blood-forming function ceases ; its rate of growth becomes proportionately less than that of other abdominal organs. The stomach, formerly empty, is now filled, and presses the liver towards the right side, causing a change in shape and partial atrophy of the left lobe. Riedel's lobe is a linguiform prolongation of the right lobe below the 10th right costal cartilage caused by compression. It is never present at birth.

The Stomach

The stomach is developed out of that part of the foregut which lies between the oesophagus and pharynx in front, and the yolk sac, duodenum and liver bud behind. In the ith week (Fig. 274) it lies in the neck, with the cervical §omite§ dorsal to it, the pericardium ventral to it, while on each side is the coelomic passage which leads from the pericardial to the peritoneal spaces (Fig. 279). At this time heart, lungs and stomach lie near the exit of the vagal fibres from the central nervous system. During the 6th and 7th weeks, as we have already seen, the growth of the lung buds leads to an elongation of the oesophagus and a backward migration of the stomach which, from being a cervical structure comes to lie level with the lower thoracic segments (Figs. 276, 277). At first its dorsal and ventral mesenteries are undifferentiated from the septum transversum. In the 5th week the gastric part of the fore-gut shows a dorsal bulging — the greater curvature (Fig. 277). As the liver and gut are developed, the stomach separates itself from the septum transversum and conies to be suspended from the dorsal wall of the coelom by the dorsal mesogastrium (Fig. 284). The gastro-hepatic omentum is part of the ventral mesogastrium. The oesophageal end of the stomach lies between the spinal fibres of the diaphragm which develop in its mesentery ; the outgrowth of the liver bud fixes its pyloric end in the ventral mesogastrium. Three changes quickly ensue during the 5th and 6th weeks, the one being partly dependent on the other :

  1. The dorsal border of the stomach, to which the dorsal mesogastrium is attached, grows more rapidly than the ventral border to which the ventral mesogastrium is attached. The greater and lesser curvatures are thus produced.
  2. The fundus of the stomach is produced as an outgrowth from the dorsal border, its origin being similar to that of the caecum from the small intestine (Fig. 291, A).
  3. While the ventral mesogastrium attached to the lesser curvature undergoes a relatively slow growth, the dorsal mesogastrium is affected by a very rapid expansion. Because of the discrepancy in the growth of these two membranes, the greater curvature of the stomach becomes freely movable, while the lesser curvature remains relatively fixed.

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Fig. 291, A.— Stomach of a Human Foetus about the end of the Srd month, showing the outgrowth of the Fundus of the Stomach. (Wood Jones.) B. Section across the Fundus (the line of section is indicated in A), showing the differentiation of the four coats of the Stomach. (Wood Jones.)

The three factors just enumerated lead to a rotation of the stomach, the greater curvature moving to the left, while the surfaces, formerly right and left, carrying the corresponding vagus nerves, become posterior and anterior. The rotation is already evident at the end of the first month of development (Broman). All of these changes are adaptations to allow the stomach to expand when filled and contract when emptied. As the stomach fills, it is the greater curvature which expands ; the lesser curvature remains relatively fixed. By the commencement of the 4th month the stomach is demarcated into a wide, vertical, cardiac part, and a narrower horizontal or pyloric part. The pyloric sphincter becomes difierentiated towards the end of the 2nd month, and it is then possible to see a distinction between pylorus and duodenum.

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Fig. 292. The Relationship of the Spleen, Pancreas and Liver to the Mesogastrium in the Embryo.

Differentiation of the Coats of the Stomach

(Fig. 291, B). — A section of the wall of the stomach at the end of the 3rd month of foetal life shows (1) an entodermal lining everywhere showing depressions or pits — the primary gastric pits — from which gastric glands will be produced during the 4th month, (2) an extremely thick submucous layer, (3) a circular muscle coat, with nerve fibres and ganglion cells applied to its outer surface ; while the circular coat appears during the 7th week, the outer longitudinal coat is not differentiated until the 4th month, and (4) peritoneal coat. From the primary gastric pits solid processes grow within the submucous coat, thus forming the epithelial bases of the gastric glands. Even as late as the 5th month of foetal life the mucous membrane in the pyloric region has a villous appearance owing to upgrowths between the mouths of the primary gastric pits. True villi, however, commence at the distal border of the pylorus.

^ See reference under Johnson, p. 271.

The Spleen

The spleen is formed in the dorsal mesogastrium above the cardiac end of the stomach (Fig. 292) and grows out of the left surface of the mesogastrium (Fig. 293). It appears at the beginning of the 6th week by a localized growth of the mesoderm in the mesogastrium. The thickening becomes vascularized. The coelomic mesothelium, which covers this thickening on the left aspect of the dorsal mesogastrium, rapidly proliferates, the deeper cells invading the vascular basis of the spleen. The tail of the pancreas (Fig. 292) reaches its point of origin. The splenic artery is one of the vessels of the mesogastrium (Fig. 293) ; its branches end in the developing tissues of the spleen and greater curvature of the stomach. The splenic blood spaces are formed during the earlier part of the 3rd month by a dilatation of the capillaries, and perhaps also from veins which, in the developing spleen, are lined by columnar cells. The trabecular and muscular tissues, and the capsule, are derived from the mesoderm of the dorsal mesogastrium. Small masses of splenic tissue (accessory spleens) are occasionally formed in the dorsal mesogastrium near the hilum of the spleen. In lower mammals the splenic formation spreads backwards until it forms a colic lobe lying in the dorsal mesentery of the hind-gut.^ In the 3rd month the surface of the spleen is nodular and deeply incised ; about the middle of foetal life the fissure begins to disappear ; only on the anterior or gastric border do they persist. The spleen differs from a lymph gland in that its spaces are formed by dilatations of blood vessels in place of lymph vessels. Lymphoid nodules appear in the spleen about the 6th month. The development of the spleen in the mesogastrium and the termination of its blood in the portal circulation suggest that the spleen is concerned in some way with digestion.

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Fig. 293. A Diagrammatic Transverse Section of the Mesogastrium viewed from behind.

^ W. Colin Mackenzie, Journ. Anat. 1917, vol 51, p. 1.

The gastro-splenic omentum is that part of the dorsal mesogastrium which unites the spleen to the stomach (Figs. 292 and 293). It becomes elongated and stretched as the stomach rotates, and as its greater curvature is developed. The spleen comes to lie against the posterior (right) surface of the cardiac end of the stomach. The dorsal part of the mesogastrium between the roof of the coelom and the spleen becomes the lieno-renal ligament. The rotation of the stomach also leads to the spleen being thrust towards the left side ; the dorsal or renal surface of the spleen becomes applied to the peritoneum covering the anterior surface of the left kidney and supra-renal body (Fig. 293). The part of the mesogastrium between the spleen and oesophagus adheres to the diaphragm and forms the lieno-phrenic ligament. The manner in which the dorsal mesogastrium becomes applied and adherent to the posterior wall of the abdomen during the 2nd and 3rd months will be described in connection with the secondary attachments of the peritoneum and mesenteries.

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Fig. 294. The Pancreatic and Hepatic Processes of a 4th week Human Embryo. (After Kollmann.)

The Pancreas

The Pancreas appears during the 4th week as two processes from that part of the gut which afterwards becomes the second stage of the duodenum (Fig. 294). The pancreatic buds develop within the ventral as well as within the dorsal mesentery for, at their points of origin from the duodenum, these two mesenteries are continuous (Fig. 292). Of the two buds, one is a minor process which springs from the ventral aspect of the duodenum in common with the hepatic diverticulum. This ventral bud forms only the lower part of the head of the pancreas (Fig. 295). The greater part is formed from a process which springs from the dorsal border of the duodenum, nearer the stomach than the ventral process, and grows into the dorsal mesogastrium above the stomach until it readies the spleen (Figs. 294, 295, 296). A developmental rotation in the wall of the duodenum, brings the bile duct and ventral pancreatic bud in contact with the right or dorsal aspect of the dorsal pancreatic outgrowth. In many animals there are two ventral pancreatic buds, one of which sends a process within the gastrohepatic omentum, round the bile duct, almost to the transverse fissure of the liver. A representative of this omental lobe is occasionally present in man (Fig. 295). The ducts of both processes may persist, the duct of the dorsal bud (duct of Santorini) opening half an inch above the opening of the bile duct ; the duct of the ventral bud (Wirsung's) terminates with the common bile duct (Fig. 295). The terminal part of the duct of Santorini commonly becomes obliterated and the secretion of the dorsal pancreatic outgrowth finds a new exit through an anastomosis between its duct system and that of the ventral bud, which is effected in the 3rd month. Even if the duct of Santorini persist, the secretion from the dorsal bud reaches the duodenum mostly through the duct of the ventral bud — the duct of Wirsung. Occasionally the duct of Wirsung does not join the common bile duct, but opens separately in the duodenum.

^ For development of pancreas : F. W. Thyng, Amer. Journ. Anat. 1907-8, vol. 7, p. 489 ; F. T. Lewis, Keibel and Mall's Manual of Embryology, 1912, vol. 2, p. 429 ; Margaret Tribe, Phil. Trans. 1918, vol. 20S (B), p. 307 ; Geo. W. Comer, Amer. Journ. Anat. 1914, vol. 16, p. 207.

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Fig. 295. Diagram of the Pancreas showing (1) its Primary Relationship to the Dorsal and Ventral Mesenteries ; (2) the parts formed from the Ventral and Dorsal Outgrowths ; (3) the Formation of the Duct of Wlrsung (Duct of W.) by a union between the Ducts of Dorsal and Ventral Buds.

The developing pancreatic process is at first hollow, like the primary liver process, but the secondary outgrowths are solid and cylindrical. They divide and re-divide, acquire lumina, and form an acino-tubular gland. About the end of the 3rd month some of the acini, particularly in the tail of the pancreas, already distinguished by the staining reaction of their cells, become partially or entirely separated from the duct-system and form the islands of Langerhans.^ Rennie, from a study of these in fishes, concludes they are permanent bodies, while the investigations of Dale led him to regard them as temporary in nature, representing resting acini.

IK. A. Heiberg, Ergebnisse der Anat, 1909, vol. 19, p. 948.

The semi-isolated acini, of which there arc several hundreds, are found in all parts of the pancreas, and represent for us the first stage in the separation of an ordinary duct gland into two elements — one connected with an external secretion, the other with a highly important internal secretion. We see from the example of the pancreas how ductless glands like the thyroid and pituitary may have arisen from duct glands by atrophy of the excretory part. The capsule and connective tissue of the pancreas are derived from the mesoderm of the dorsal mesentery.

Relationship of the Pancreas to the Peritoneum and Vessels

1. In the Embryo

The pancreas develops between the layers of the dorsal mesogastrium, just where this structure is being expanded to form the wall of the omental sac. From the first it is completely surrounded by peritoneum, and it lies with its tail directed forwards against the spleen and its head on the dorsal bend of the duodenal loop (Fig. 296). It comes to lie parallel to the great curvature (dorsal border) of the stomach. In Fig. 296 a schematic drawing is given of the essential relationship of the pancreas to the dorsal mesogastrium in lower vertebrate animals ; it also represents the condition seen in a human embryo in the 5th week of development, when the dorsal mesentery is swollen with young tissue (Fig. 281) and attached along the mid-dorsal line. The coeliac axis (Fig. 296) is the artery of the mesogastrium and of the structures which it contains. It supplies the fore-gut and its derivatives, between the septum transversum in front and yolk sac behind. The coronary artery passes direct to the cardiac end of the stomach ; the splenic is a short vessel ending on the cardiac dilatation of the stomach and supplying the spleen ; the hepatic passes on the right side of the pancreas to the duodenum and pyloric end of the stomach, and ends in the liver by passing through the ventral mesentery. As the stomach migrates backwards during the 6th and 7th weeks, the origin of the coeliac axis moves also.

2. In the Adult

The development of the great omentum and the rotation of the stomach to the left, lead to the pancreas being pressed against the left side of the posterior wall of the abdomen. That part of the dorsal mesogastrium which lies between the stomach and pancreas becomes elongated enormously, during the 3rd and 4th months, to form the great omentum, and hence the two anterior layers of the great omentum are attached to the great curvature of the stomach and to the gastrosplenic omentum (Fig. 296). The two posterior layers of the omentum end on the lower (formerly ventral) border of the pancreas. The great omentum is well developed in all mammals, its origin being probably related to that of the diaphragm. Its exact function is unknown, but it is connected with the absorption, and perhaps also with the secretion, of peritoneal fluids ; it is a great phagocytic mechanism. The duodenal loop, with the head of the pancreas in its concavity, is also pressed against the posterior abdominal wall. During all the changes which take place in the position of the pancreas and spleen, owing to the rotation of the stomach and intestine, one structure remains fixed, and that is the coeliac axis. The part of the mesogastriimi in which the spleen and tail of the pancreas are situated becomes greatly drawn out. Both structures, instead of being situated near the middle line dorsal to the stomach, come to occupy a situation in front of the left kidney, the pancreas thus coming to lie across, instead of along, the abdominal cavity. The mesogastrium is ballooned out towards the left side to form the lesser sac of the peritoneum, and as the splenic artery lies in the mesogastrium it also is drawn towards the left, circumventing the lesser sac of the peritoneum (Fig. 297). Up to the 6th week of embryonic life the pancreas lies between the layers of the dorsal mesogastrium and the extension from these layers which forms the mesentery of the duodenal loop (Figs. 295, 296) ; thus right and left surfaces are covered by peritoneum. The left surface, which becomes anterior, retains its covering, but during the 6th week the right aspect of the pancreas and duodenal loop become applied to the posterior abdominal wall in front of the aorta, crura of the diaphragm and left kidney (Fig. 297). The peritoneal covering on the right aspect gradually disappears, and thus in the adult the pancreas comes to appear as if it lay behind and outside the cavity of the peritoneum. The complete application and fixation of the pancreas and duodenum to the posterior abdominal wall only occur in animals adapted to the upright posture (see Figs. 287, 288, 297).

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Fig. 296. Schematic representation of the Dorsal Mesogastrium and its contents.

The part of the dorsal mesogastrium between the pancreas and aorta (Fig. 297) is also applied to the posterior abdominal wall, and forms the posterior lining of the lesser sac.

The Lesser Sac is composed of two parts, a vestibular or hepatic part formed from the recessus mesentericus (Figs. 287, 288) and an omental or gastric part formed by the evagination of the dorsal mesogastrium.

1 For fuller details see P. T. Crymble, Journ. Anat. 1913, vol. 47, p. 207.

These two parts communicate at an isthmus or constriction caused by the coronary and hepatic arteries (Fig. 297). The hepatic recess or pocket separates the Spigelian lobe of the liver from the right crus, and permits the liver to glide during the respiratory movements of the diaphragm (Figs. 286, 288). The gastric part isolates the stomach, allows it to contract, expand and move during digestion and respiration. In the anterior wall of the lesser sac are situated (Fig. 297) : (1) the gastro-hepatic omentum or ventral mesentery, which is at first vertical and median ; (2) the stomach ; (3) the gastro-splenic omentum, a part of the dorsal mesentery ; (4) the two anterior layers of the great omentum, also parts of the dorsal mesentery. In its posterior wall are situated : (1) the lieno-renal ligament (dorsal mesentery) ; (2) the dorsal mesentery of pancreas ; (3) two posterior layers of the great omentum.

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Fig. 297. Diagram to show the Formation of the Lesser Sac of the Peritoneum from the Dorsal Mesogastrium. The arrow lies in the isthmus between the vestibular and omental parts.

Process of Peritoneal Fixation

We have seen that certain developmental processes, such as the obliteration of the embryonic clefts of the lip and of the palate, or the union of the medullary folds to enclose the neural tube, are akin to the processes which lead to the union of the lips of a wound made by a surgeon's knife. In the peritoneal cavity we are to see examples of another process with which surgeons are familiar — the formation of adhesions which follow inflammatory disturbances of the peritoneum. The passages which lead from the pericardium to the pleura, from the pleura to the peritoneum and from the peritoneal cavity to the tunica vaginalis of the testes, are closed by the formation of developmental adhesions. The peritoneal adhesions with which surgeons are familiar follow inflammation, but the developmental process — the process of zygosis — which leads to the adhesion of the mesentery of the duodenum and part of the mesogastrium to the dorsal wall of the abdomen in the latter part of the 2nd month of embryonic life, are not preceded by inflammatory changes, but are the result of growth impulses arising under an unknown stimulus. The process of zygosis is active not only in foetal life but is also to be seen at work at, and even after, birth. The applied peritoneal surfaces become adherent by the proliferation and union of lining cells of the opposed layers of peritoneum. The adhesions as they form, contract and thus draw the various parts of the alimentary canal to their final position, much in the same way as the testes come to be lodged in the scrotum. We are here dealing with growth manifestations utilized for a mechanical purpose. The secondary adhesion of the mesenteries of the abdominal viscera are apparently related to posture ; the degree of adhesion is much more extensive in man than any other animal, with the exception of the great anthropoid apes. Man and the anthropoids are distinguished from all other animal forms by the upright posture of their bodies. The peritoneal adhesions which occur from the middle of the 2nd month onwards must be regarded as adaptations to the upright posture. The suspensory ligament of the spleen, the right and left costo-colic ligaments, the peritoneal bands passing from gall bladder to the colon or omentum are of the same nature, and are formed by secondary adhesions of the peritoneum in the later months of foetal life.^

1 See Keith, Lancet, 1914, vol. 2, p. 362.

The Mid-gut, Yolk Sac and Meckel's Diverticulum

The yolk sac reaches its maximum size in the earlier part of the ith week, when its neck, filling the embryonic umbilicus, extends from the septum transversum in front to the allantois behind (Fig. 274). In the 5th week (Fig. 275) the mid-gut has become a V-shaped tube ; the yolk sac, now lying in the umbilical cord, just beginning to be differentiated, is joined to the apex of the mid-gut by a stalk or neck. The condition reached in the 6th week is shown diagrammatically in Fig. 298. The following points are to be noted: (1) The production of the mid-gut as a U-shaped loop. (2) The formation within the umbilical cord of a long neck to the yolk sac — the vitellointestinal duct ; Meckel's diverticulum is formed by a persistence of the intra-abdominal part of the canal. Normally the duct becomes occluded, and shrivels up during the 6th week ; this is the case in all mammals, but in birds the yolk sac is large at the time of hatching, and part of it always persists as an intestinal diverticulum. (3) The yolk sac, by the constriction of the umbilical orifice and formation of the cord, comes to lie on the placenta where a remnant of it may be found at birth near the implantation of the cord (Fig. 298).

Vessels of the Yolk Sac

Although at first the yolk sac receives a series of branches from the aorta, by the time of its separation from the mid-gut the number has been reduced to one — ^the superior mesenteric, which becomes the artery of the U-shaped loop (Fig. 298). Its vein, however, the left vitelline, has no connection with the superior mesenteric vein but, when the U-shaped loop is formed continues its original course and ends in the portal vein at the lower border of the pylorus (Fig. 301). When the vitello-intestinal duct atrophies in the 6th week, the same fate overtakes the vessels of the yolk sac, but they, too, may persist as cords.

^ For many details connected with the formation of these adhesions see papers by Dr. Douglas G. Reid, Journ. of Anat. and Physiol, vols. 1911-1915.

The Umbilical Coelom and Intestinal Loop

At first the coelom extends into the proximal segment of the umbilical cord and it is within this umbilical recess of the peritoneal cavity that the U-shaped loop — the mid-gut — undergoes its earlier developmental changes. The structural features of the loop are shown in Fig. 298 ; it is made up of a proximal or jejunal limb and of a distal or caecal limb, for already in the 6th week, when the embryo is little more than 5 mm. in length, the caecal diverticulum is apparent. In Fig. 299 a dissection of the intestinal loop is shown, from an embryo in the 6th week of development. Already the process of rotation has commenced — the jejunal limb coming to lie to the right and dorsal to the caecal limb. The mesoduodenum is becoming adherent to the dorsal wall (Fig. 299) while, as Professor Frazer has shown, certain " traction bands " are forming within the common mesentery and thus guiding and regulating the movement and fixation of the loop. The condition in the 9th week is shown in Fig. 300 ; within the umbilical coelom coils of small intestine have been produced from the jejunal and ileal parts of the loop ; also a jejunal coil within the abdomen from the proximal limb. The duodeno-jejunal flexure has become closely bound to the dorsal wall by traction bands — part of which become muscular and form the Muscle of Treitz. Then, suddenly, in the 10th week, when the foetus is about 42 mm. long, the loop is retracted within the abdomen and the umbilical recess becomes closed. We must regard the withdrawal as due to the development of " contraction " or " retraction " bands in the mesentery. During the weeks spent by the intestinal coils in the umbilical recess, the lung buds are expanding and the pleural cavities and diaphragm are being formed, and the safe-guarding of these processes may be the reason for an extra-abdominal development of the intestinal loop.^

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Fig. 298. Schematic representation of the Alimentary Canal, and of its Mesenteries and Arteries during the Cth week of development.

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Fig. 299. The Intestinal Loop, seen from the right side, in an embryo in the Cth week of development. (Prof. Frazer.)

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Fig. 300. The Intestinal Loop, with the Umbilical Coelom, of a foetus in the 9th week, seen from the left side. (Prof. Bardeen.)

Persistence of Certain Embryonic Structures

Many of the structural features seen in the human embryo at the stage of development reached during the fifth or sixth weeks may persist.

(1) The most common structure to remain is the intestinal end of the neck of the yolk sac — Meckel's diverticulum. It occurs in 2 per cent, of subjects, and commonly forms a finger-like sac on the free border of the ileum from two to four feet above the ileo-caecal orifice. Hence we know that this part of the ileum forms the apex of the U-shaped loop of intestine. The point on the ileum at which the canal of the yolk sac was attached is frequently the seat of a narrowing, which may be more or less marked. This forms a favourable site at which intussusception of the bowel occurs. The diverticulum varies in length and shape ; its blind end is frequently bulbous and the site of secondary diverticula. Occasionally pancreatic masses are developed at its extremity. It is lined by a glandular epithelium similar to that of the ileum. Frequently a fold of the mesentery descends to it (Fig. 302). In the mesenteric fold there is usually to be found a vestige of the artery of the yolk sac (Fig. 298). The attached base of the mesenteric fold may atrophy, while the free margin forms a cord, under which a loop of bowel may become strangulated (Fig. 302).

(2) The vitello-intestinal duct may remain patent, and, when the cord is cut at birth, form a fistulous opening at the umbilicus, by which the contents of the ileum escape. Or part may become grafted on the umbilicus and give rise to a " weeping navel " (Stiles).

^For further details see articles by Frazer and Robbins, Journ. Anal. 1916, vol. 50, p. 75; C. R. Bardeen, Amer. Journ. Anat. 1914, vol. 16, p. 427.

2 For an account of the structure of the yolk-sac see papers by Dr. H. E. Jordan, Anat. Anzeiger, 1907, vol. 31, p. 291 ; 1910, vol. 37, p. 56. For an account of Meckel's diverticulum and of malformations of the bowel see Keith, Brit. Med. Journ. 1910, vol. 1, p. 301 ; Ivar Broman, Ergebnisse Anat. Entw. 1913, vol. 21, p. 99.

(3) The artery of the yolk sac, the terminal part of the superior mesenteric, may persist as a fibrous band which stretches from the mesentery at the situation of a Meckel's diverticulum to the umbilicus. Over it the gut may become strangulated. The young of all carnivora are born with thread-like remains of both artery and vein, stretching from the umbilicus to the mesentery (Fig. 301). A remnant of the vein is rarely seen in the human subject. The vitello-intestinal duct may also be reduced to a fibrous structure, over which a loop of intestine may fall and thus become strangulated.

(4) The U-shaped loop, instead of retreating within the abdomen at the beginning of the third month, may remain within the umbilical recess. This gives rise to a congenital umbilical hernia. Such herniae occur in all degrees ; they may contain a piece of intestine, or almost the whole of the abdominal contents. In such cases the somatopleure, or belly wall, which forms the covering of the hernia, is commonly thin and transparent.

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Fig. 301. Fibrous Remnants of the Artery (a) and Vein (6) of the Yolk Sac in a Kitten.

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Fig. 302. Meckel's Diverticulum provided with a Mesentery. The arrow marks the site at which an aperture may be formed in the mesenteric fold.

Congenital Diverticula

During the third month numerous outgrowths of intestinal epithelium are formed, which perforate the muscular coat. They usually disappear, but may give rise to diverticula, a common site being the ileo-caecal junction where a diverticulum may develop into a large cyst. Frequently masses of pancreatic tissue are attached to intestinal diverticula (Lewis and Thyng).

Congenital Occlusion of the Duodenum

The part of the duodenum just above the opening of the bile ducts may be partially or completely closed — a rare occurrence (Fig. 303). After the liver and pancreatic buds grow out, this part of the duodenum becomes occluded by the proliferation of the epithelium lining the gut (Tandler). We have seen that a rotatory movement occurs at this site (p. 273). The proliferation of the intestinal epithelium — in the second month — is not confined to the duodenum ; hence congenital occlusions may occur at any part of the intestine.

1 For literature on congenital diverticula see F. T. Lewis and F. W. Thyng, Amer. Journ. Anat. 1907-8, vol. 7, p. 505.

2 For congenital occlusions see H. Forssner, Anat. Hefle, 1907, vol. 34, p. 1 ; C. P, G. Wakeley, Journ. Anat. 1917, vol. 51, p. 65; R. J. Gladstone, Journ. Anat. 1914, vol. 48, p. 47.

Duodeno-jejunal Loop and Junction

The junction between the duodenal and U-shaped loops becomes the most fixed point in the whole intestinal tract (Fig. 298). Within its dorsal mesentery a band of nonstriated fibres is developed which binds the junction to the right crus of the diaphragm. The suspensory band ^ is generally known as the muscle of Treitz. The functional meaning of the duodeno-jejunal loop and its muscular band is unknown, but they are found in all the higher vertebrates (see p. 290).

Villi of the Intestine

As early as the 7th week circular muscle fibres appear in the coat of the duodenum and by the 10th week the process has spread downwards to the ileo-caecal junction. The longitudinal coat appears in the 12th week and a little later meconium is being propelled towards the great intestine. A germinal zone is formed between the circular and longitudinal coats, in which Auerbach's plexus become developed. Villi begin to form at the end of the second month while the glands of Lieberklihn appear in the 3rd month, both structures being developed in the proximal part first and spreading downwards. The villi arise by subdivision of the ridges (Berry). Lymphoid follicles make an appearance in the 4th month and Peyer's patches begin to form in the 7th month, and are apparent to the naked eye in the 1st month after birth. The valvulae eonniventes arise as folds of the mucous membrane in the 8th month, thus increasing the surface for absorption. They are formed first in the duodenum ; their development gradually ceases at the upper part of the ileum.

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Fig. 303. Congenital Occlusion of the Duodenum.

1 A. Low, Journ. Anat. and Physiol. 1908, vol. 42, p. 93 ; P. T. Crymble, Brit. Med. Journ. 1910, vol. 2, p. 1156.

2W. A. Hilton, Amer. Journ. Anat. 1901-2, vol. 1, p. 459 (Dev. of Villi and Valvulae Conniventes).

Derivates of the Hind-Gut

At the beginning of the 2nd month the hind-gut is almost equal in length to the mid-gut, but its calibre is less. Indeed, it is not until the 5th month that the hind-gut is marked off from the mid-gut by its greater diameter. By the end of the 2nd month, as we have just seen, the anterior (jejunal) limb of the intestinal loop has grown very rapidly, and become thrown into a number of distinct loops. At birth the small intestine is six times the length of the large bowel.

The Rectum is formed out of the posterior end of the hind-gut. The manner in which the rectum is separated from the cloaca, the anal canal formed, and the permanent anus produced, will be described in connection with the perineum and urogenital passages, for their history is closely associated with the development of these structures (see p. 381).

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Fig. 304. The Mesentery of the Hind-gut. The position assumed by the colon after the rotation of the gut has taken place.

The Descending Iliac and Pelvic Segments of the Colon are also formed out of the hind-gut. The artery of the hind-gut is the inferior mesenteric (Fig. 304). Hence it supplies the rectum, sigmoid and descending colon. In the 6th week the hind-gut is suspended from the front of the aorta and spine by the dorsal mesentery of the hind-gut (Figs. 298, 299). This becomes transformed into the meso-rectum, meso-sigmoid and descending meso-colon. The angle between the hind-gut and U-shaped loop becomes the splenic flexure (Figs. 299, 304). At the commencement of the third month, when the intestine takes up its permanent position within the abdomen, the U-shaped loop has become twisted round on the axis of the superior mesenteric artery (Fig. 304), so that the part of the hind-gut which forms the splenic flexure is turned forwards and to the left until it touches the spleen (Fig. 310). It carries its artery, the left colic, with it. At this time the anterior limb of the U-loop elongates much more rapidly than the posterior limb, and is produced into coils of small intestine — the jejunum and ileum — which press the descending meso-colon against the kidney and the parietal peritoneum covering the left kidney (Fig. 305). The left surface and layer of the meso-colon adheres by the process of zygosis to the pre-renal layer of the peritoneum, both layers subsequently being absorbed. Thus the descending meso-colon, originally situated in the middle line, comes to be attached in the left lumbar region.

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Fig. 305. Diagram to show how the descending Meso-colon becomes applied to the Parietal Peritoneum of the left Lumbar Region.

The Intersigmoid Fossa

The sigmoid flexure, which is made up of the pelvic colon and part of the iliac segment, after the rotation of the gut, forms a loop, with its convexity directed towards the liver. The mesosigmoid is originally attached in the middle line, but the pressure of the developing loop of small bowel presses it against the posterior abdominal wall and left iliac fossa. It may become completely adherent like the descending meso-colon, or only partially. When the sigmoid is lifted up a recess or fossa may be apparent beneath the meso-sigmoid, to the outer side of the left common iliac artery, which is due to a failure of adhesion between the meso-sigmoid and parietal peritoneum. It occurs opposite the convexity of the sigmoid loop (Fig. 288). At birth the mesosigmoid is relatively extensive ; the sigmoid loop lies with its convexity towards the right side of the abdomen, and well above the pelvis. During adolescence the sigmoid grows more slowly than the rest of the colon. It sinks within the pelvis, and forms the greater part of the pelvic colon.

Morphology of the Ileo-coUc Part of the Bowel

In all vertebrates, from fishes upwards, the junction of the small with the great intestine is demarcated by the ileo-colic sphincter, developed from the circular coat of the bowel.^ As a rare abnormality the caecum may be absent in man, the only external indication of the ileo-colic junction being the presence of the ileo-colic sphincter. This is the normal condition in the frog, and in several mammals such as the racoon. The sphincter marks the junction of two difierent functional segments of the alimentary tract. Villi, which are originally developed in the great bowel, disappear in the later months of foetal life. The proximal part of the colon from which the caecum is developed forms the caecal colon (Fig. 306) ; it is frequently demarcated from the ascending colon by a thickening of the circular muscular coat — the caeco-colic sphincter (Fig. 306), c) — which can commonly be recognized in the bowel of man. The caecum is developed as a diverticulum of the caecal colon. In all vertebrates its submucous coat is rich in lymphocytes, which in mammals collect in the form of solitary follicles more or less closely crowded together. R. J. Berry found that in the primates there is a tendency for the lymphoid tissue to be aggregated in the apex of the caecum. In man, in anthropoids, and a few other forms, the lymphoid tissue becomes richly developed in the distal part of the caecum, which has a narrow lumen, strong muscular coat, and is of great functional activity during digestion. This highly specialized part of the caecum is the appendix ; it is well developed in man, and is certainly not a vestigial structure. The lymphoid tissue undergoes a great reduction in size and growth when the period of adolescence is past. Thus there are five structures to be observed in the ileo-colic region of a typical mammal (Fig. 306) : (1) an ileo-colic sphincter, (2) a caeco-colic sphincter, (3) a caecal segment of the colon, (4) a caecum, the distal part of which may be specialized to form (5) an appendix. Further, a study of the comparative anatomy of this region shows that the caecum is largest in vegetablefeeding animals, and that there is a correlationship between the development of the stomach and caecum. In the horse, for instance, the caecum and caecal colon are comphcated, the stomach simple ; in the ruminants the stomach is complex, the caecum comparatively simple. In animals which live on a flesh diet the caecum is small.

^For literature on shape and development of caecum and appendix see R. J. A. Berry and L. A. H. Lack, Journ. Anat. and Physiol. 1906, vol. 40, p. 247 (Nature of Appendix) ; F. G. Parsons, Journ. Anat. and Physiol. 1908, vol. 42, p. 30 (Age Changes in Shape of Caecum) ; R. J. A. Berry, " Intercolon," Med. Journ. Australia, 1907, June 20 (Nature of Appendix) ; G. S. Huntingdon, The Anatomy of the Human Peritoneum and Abdominal Cavity, 1903 ; H. A. Kelly and E. Hurdon, The Vermiform Appendix and its Diseases, 1905.

" Keith, " Anatomical Evidence as to the Nature of the Caecum and Appendix," Proc. Anat. Soc. Nov, 1903. See also Prof. T. B. Johnston, Journ. Anat. 1920, vol. 54, p. 67.

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Fig. 306. Diagram to show the parts of a typical Mammalian Caecum. Five parts are shown in the figure : (1) the termination of the ileum ; (2) the caecal colon in which the ileum ends ; (3) the caecum which opens from the caecal colon ; (4) the apex of the caecum ; (5) the commencement of the ascending colon. At three points the circular muscular fibres are thickened to form sphincters : (a) ileo-colic junction ; (b) at the junction of caecum and caecal colon (in man a and b are combined in the ileo-caecal orifice and its retinacula) ; (c) in the first part of the ascending colon.

Development of the Colon and Caecum

Early in the 6th week of development an elevation appears on the free border of the posterior limb of the U-sliaped loop (Figs. 298, 299). The elevation contains a diverticulum of the caecal colon, which forms the caecum and appendix. It continues to grow outwards and forwards in close contact with the free border of the ileum. At first the colic part of the intestinal loop and the caecal process are not of larger calibre than the small intestine, but in the fifth month the colon and caecum undergo an enlargement, but the terminal or apical part of the caecum retains its foetal dimensions, and forms the appendix. As in the small bowel the circular coat appears long before the longitudinal, but whereas the muscle appears first at the proximal end of the small bowel and spreads distally, the muscle of the colon appears first at the rectal end — where the sacral visceral nerves enter — and spreads towards the ileo-caecal junction. The longitudinal coat appears in the 3rd month along the mesenteric border — representing the mesenteric taenia ; the remaining two in the 4th month. The evaginations or haustra are distinct in the 7th month of foetal life.^

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Fig. 307. Diagram of the Apex of the Caecum at the time of Birth and the Diverticula which may be produced later in the Fundus of the Caecum.

As the superior mesenteric (vitelline) artery descends in the intestinal loop, it gives off three branches to the posterior limb — the middle colic, right colic and ileo-colic arteries (Figs. 304, 308). The mesentery of the U-shaped loop may be divided into two parts, the fate of the two parts being different :

1. The mesentery of the anterior limb in front of the superior mesenteric artery — forms the pre-arterial part. This gives rise to the greater part of the mesentery of the small bowel.

2. The mesentery of the posterior limb, behind the artery — is the postarterial part. It forms the mesentery of the ascending and transverse colon, and also the lower part of the mesentery of the small bowel.

1 See Th. Thaysen, Anat. Hefte, 1916, vol. 54, p. 321 ; P. E. Lineback, Anat. Rec. 1919, vol. 16, p. 155 ; E. J. Carey, Anat, Rec, 1920, vol. 18, p. 224,

When the rotation of the intestinal loop takes place (p. 289) the splenic flexure of the colon comes against the spleen, while the transverse mesocolon, containing the middle colic artery, is brought into apposition with that part of the mesogastrium which forms the great omentum (Figs. 300, 310). These two layers adhere ; thus the transverse colon is formed by the fusion of a part of the dorsal mesogastrium with the mesentery of the posterior limb of the U-shaped loop (Fig. 288). The rotation places that part of the loop mesentery which forms the mesentery of the caecum and ascending colon against the duodenum, and at the same time the duodenal loop becomes fixed in its permanent position in front of the right kidney and inferior vena cava. The caecum thus comes to be situated in the majority of foetuses in front of the right kidney, near the gallbladder, and there it remains until about the time of birth, when the ascending colon elongates and the caecum thus moves towards the right iliac fossa. An iliac position of the caecum is a feature which occurs only in animals adapted to the upright posture. Thus the attachment of the ascending meso-colon is effected by secondary adhesions which are formed during the migration of the colon and caecum. The appendix, during the migration, may be caught behind the colon, thus assuming a retro-colic position ; it is then lodged and fixed in the ascending meso-colon. The peritoneal adhesions, which are formed in the 4th and 5th months of foetal life, between the transverse meso-colon and great omentum, and especially the adhesions which the ascending colon forms just before and after birth, as the caecum assumes its position in the iliac fossa, are subject to a great range of variations, and many peritoneal folds and recesses may be formed. The object of all of them is to give a fixation of the viscera to the abdominal wall — a fixation which occurs only in orthograde primates.^

The Appendix

At first, and until the fifth month, the caecal diverticulum is of the same calibre throughout, but from that month onwards the appendix remains small while the caecum grows, keeping pace in diameter with the colon. At birth the appendix is still the tapered apex of the caecal diverticulum (Fig. 307), but during cliildhood, an outer, or an inner sacculation, or botli together, arise in the fundus of the caecum and thrust the appendix backwards and to the left into an asymmetrical position.^ Villi are formed in the mucous coat in the early part of the 4th month ; Lieberkiihn's glands appear a little later. Lymphoid follicles make their appearance in the 5th month. The villi disappear in the 8th month.

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Fig. 308. The Appendix and Peritoneal Folds at the end of the 2nd montli of Foetal Life. The intestinal loop is ^iewed on its left, later its dorsal, aspect.

^ Dr. Douglas Reid has described the various forms of foetal adhesions in Journal of Anatomy and Physiology, vols. 1911-1915.

Although a distinctly marked appendix is only seen in man, the anthropoids, lemur, opossum and certain rodents, still a corresponding lymphoid structure is present generally in mammals. The appendix is a lymphoid diverticulum of the caecal apex (R. J. Berry). It must be regarded as a lymphoid structure, and although it can be dispensed with, is not therefore to be regarded as vestigial in nature any more than is the tonsil. In 30 % of adults both muscular and mucous coats have undergone a partial degeneration under modern conditions of diet, and the appendix does tend to become a useless structure.

Heo-caecal Valves

At the ileo-colic junction, the development of villi ends. In the higher primates the junction is invaginated within the caecum, in the form of two lips or valves. The invagination becomes apparent in the human foetus of the 3rd month. Within these folds are (1) the ileo-colic sphincter ; (2) muscular bands developed in the retinacula from the circular musculature of the caecum and representing the midcaecal sphincter of the typical caecum (Fig. 306). The retinacular musculature assists in the emptying and filling of the caecum. To a very slight extent the ileo-colic lips can serve as mechanical valves in the living subject ; they assume a valvular form only when dead and dried.

Ileo-caecal Fossae

When the caecal diverticulum grows out from the hinder limb of the U-shaped loop it carries with it three folds (see Fig. 309) :

  1. The ileo-colic fold, a process from the right side of the mesentery containing the anterior caecal artery ; in a small proportion of cases this fold forms the mesentery of the appendix; ^
  2. The bloodless or ileo-caecal fold, a process from the coat of the ileum;
  3. The mesentery of the appendix, a process from the left side of the mesentery, containing the artery to the appendix (Fig. 308).

These three folds give rise to three fossae (Fig. 309) :

  1. The ileo-colic, between the termination of ileum and ileo-colic fold;
  2. The ileo-caecal, between the bloodless fold and mesentery of the appendix ;
  3. The retro-caecal, between the mesentery of the appendix and commencement of the ascending meso-colon.

The caecum and appendix are made up of bilateral halves ; there are right (anterior caecal fold) and left (mesentery of appendix) mesenteries. In birds the appendix is divided ; it is occasionally double in malformed human infants. ^ There is no reason to suppose, however, that the appendix was ever a double structure in the stem from which man has descended.

The duodeno-jejunal fossa is formed to the left of the duodeno-jejunal flexure after the transverse colon and caecum have rotated to the right hypocliondrium and when the transverse meso-colon has fused with the omental layers of the lesser sac (Fig. 310). The fossa is occupied by a bend of intestine at the duodeno-jejunal junction and serves as a bursa for this knuckle of gut. Its origin is connected with (1) the traction bands developed at this junction (see p. 290), the passage of the inferior mesenteric vein in or near its left border. It lies in the axis at which the mesenteric rotation takes place (Fig. 310), and when the plastic nature of the peritoneal tissue is remembered, it is easy to realize how this and other recesses may be formed near the termination of the duodenum.

1 See F. G. Parsons, Journ. Anat. and Physiol. 1908, vol. 42, p. 30.

2 See Dr. Geo. M. Smith, Anat. Record, 1911, vol. 5, p. 549 ; A. Forster, Anat. Hefte, 1918, vol. 56, p. 5.

' Dr. F. Wood Jones, Journ. Anat. and Physiol. 1912, vol. 46, p. 193.

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Fig. 309. Peritoneal Fossae in the Ileo-caecal Region.

-To show the Rotation of the Intestinal Loop and Formation of the Duodeno-jejunal Fossa.

The mesentery of the small gut is formed out of the primitive mesentery of the U-shaped intestinal loop, chiefly from that part of it (the pre-arterial) which lies between the superior mesenteric artery and the anterior limb of the loop (Fig. 304). After the rotation, the aspect of the mesentery, which was directed towards the right, becomes left and anterior (compare Figs. 298, 310). During the rotation of the gut the superior mesenteric artery comes to lie in front of the third stage of the duodenum. At first the mesentery is attached in front of the spine only at the origin of the superior mesenteric artery (see Figs. 287, 288). Its oblique attachment to the posterior abdominal wall, from the duodenum to the right iliac fossa, is effected by secondary adhesions which are formed after the rotation of the gut and during the 4th and 5th months, and this extensive attachment is found only in animals adapted to the upright posture. The last part of the mesentery to become adherent to the posterior wall of the abdomen is the angular area between the ileum and ascending colon. Not unfrequently this part remains free, and it is then possible for a volvulus to form by a rotation of the ileo-colic loop.

By the rotation of the U-shaped loop, the small intestine becomes confined in a bursa or peritoneal compartment formed by the mesentery of the large bowel (Fig. 310).

Abnormal Fixation of the Mesentery

The rotation of the bowel is subject to three forms of disturbance, giving rise to three varieties in the fixation of the mesentery, which are of importance to medical men. (1) The bowel may undergo its normal rotation, but the process of adhesion may fail ; the bowel is thus suspended by a free fan-shaped mesentery. During life it may become twisted round its stalk, formed by the superior mesenteric artery, and thus give rise to obstruction of the bowel (complete volvulus). (2) It may not undergo a rotation ; the caecum then lies on the left side of the abdomen, and the colon — ascending and descending — are situated behind and to the left of the small bowel. (3) The rotation may occur in a direction opposite to the normal — the duodenum and mesentery coming to lie in front of the transverse colon in place of being situated behind it. Several cases of this nature have been recorded of late by surgeons and anatomists.


At birth, the great intestine and the ileum are distended by meconium, a black, semi-fluid substance secreted by the liver and mucous membrane of the bowel. Dr. A. Low found that the meconium reaches the ileo-colic junction in the 4th month, the rectum in the 5th. The meconium passes quickly along the jejunum. At birth the lower part of the ileum and whole of the great intestine are distended with it. By the 3rd or 4th day after birth all the meconium has been passed, a fact which may be utilized to prove that a child had lived for a certain time after birth.

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Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures