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the abdomen. Already, at the fifth or sixth week, the adult form of the stomach is clearly indicated.
This rotation of the stomach around its long axis, which is accompanied by a rotation of the lower end of the oesophagus, explains the asymmetrical position of the two vagi. In the adult the left nerve is found on the front of the stomach, which was originally the left side of the organ; similarly, the right nerve lies on the back, which was originally the right side.
Intestines. At first there is no separation into large and small intestines; the primitive canal simply forms a slender tube, with a convexity towards the umbilical orifice, through which the vitelline duct passes to the yolk sac. Later, the tube increases in length, and in embryos of 11 or 12 mm. and about five weeks old an outgrowth of the canal appears, which represents the future cæcum, and indicates the separation into large and small intestines. Growing longer, the intestine forms a large loop with the vitelline duct springing from its apex (Fig. 973), and the superior mesenteric artery running down between the layers of its mesentery. At the same time the two extremities of the coil approach one another, and form a narrow neck to the loop, as shown in Fig. 973. There now takes place a change which entirely modifies the position of the parts this is a rotation of the whole loop, with its mesentery, around the superior mesenteric artery as an axis (Fig. 973). The result of this rotation is that the original right side of the loop of gut and mesentery becomes the left side; and the beginning of the large intestine is carried across the duodenum (Fig. 973), thus explaining the passage of the transverse colon in front of the second part of the duodenum in the adult. At the same time the cæcum comes to lie near the middle of the abdomen below the liver, a position in which it is found during the third month. Subsequently, it passes farther to the right; and finally, descending, comes to occupy its adult position. The small intestine continues to grow in length, and, as a result, is thrown into coils, which become more and more complex as the length increases, until the adult condition is attained. The terminal portion of the large bowel retains its position on the left side, and passes down to the anus.
Formation of Gastric and Intestinal Glands, etc.—The epithelial lining of the intestinal tube is composed, at first, of a single layer of cells, and the inner surface is smooth. In the second month the epithelium increases rapidly, and as a result its surface is thrown into folds and furrows, arranged irregularly. Mesenchymal tissue passes into the interior of the folds, and also blood capillaries. The folds appear first in the stomach, especially
Superior mesen-. teric artery
Inferior mesenteric artery
the stomach the folds
are arranged so as to surround small isolated
Inferior mesen depressions, which afterwards become the foveola gastrica. In the small intestine isolated elevations are found, in place of con tinuous folds, and at a later stage new eleva tions are formed be tween the primary ones. The figure to the right shows the rotation of the intestinal loop around the These papillary eleva superior mesenteric artery. In both figures the parts are supposed to be tions form the villi.
FIG. 972. Two DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF THE
viewed from the left side.
In the large intestine. the arrangement resembles that in the stomach. The glands of the stomach and intestine, viz., the gastric and duodenal glands, and the intestinal glands in the small intestine are formed by an active proliferation of the epithelium at the bottom of the furrows, and at first the cells forming them are everywhere of a similar character, and
become differentiated later on. In the stomach the formation of the glands begins about the end of the third month.
The intestinal glands of the large intestine represent merely furrows between adjacent elevations, and are not due to an active proliferation of cells at the base of the furrows, and hence a distinction may be drawn between the two, and be expressed by using the term intestinal glands for the depressions of the small intestine and intestinal follicles for those of the large intestine.
According to v. Nagy, whose description has been followed above, the glandular epithelium of the gastric glands begins to assume its differentiated form in different parts, i.e. cardiac and pyloric glands, towards the fifth month of development.
Cæcum and Vermiform Process.-The cæcum first appears in the embryo, at about the fifth week, as a small outgrowth of the wall of the primitive gut (midgut), not yet differentiated into small and large intestines. At this time the outgrowth is of the
FIG. 973. THE ABDOMINAL VISCERA IN THE NEWLY BORN CHILD. The liver and the jejunum and ileum have been removed. The vertical stomach, the large supra-renal gland, the high position of the cæcum, and the whole arrangement of the large intestine are typical of the condition found at birth, and differ, as can be seen, largely from the adult condition.
same size throughout, and is practically equal to the intestines in diameter. About the eleventh week, whilst the large and small bowels are still of the same width, it has increased very considerably in length (being equal to about five times the diameter of the small intestine, and thus being relatively as long as in the adult); but even at this early date the basal portion, for about one-fifth of its length, is quite as wide as the intestine, whilst the remaining four-fifths of the outgrowth-the future vermiform process-is only about one-half or one-third the diameter of the gut. From this it is seen that the distal portion of the outgrowth, which subsequently becomes the vermiform process, begins to lag behind even at this early period of its development.
The basal portion continues to expand with the gut; the distal part grows rapidly enough in length, but otherwise enlarges very slowly, so that, towards the end of foetal life, the cæcum has attained a conical shape, the wider end joining the ascending colon, the narrow end tapering gradually and passing into the vermiform process. This form, known as the infantile type of cæcum, is retained for some time after birth, or even may (in 2 or 3 per cent. of cases) persist throughout life.
As early as the sixth or seventh month of foetal life the wall of the terminal portion of the small intestine adheres to the medial side of the cæcum for some distance below the ileo-cæcal orifice. And that connexion, which is rendered more intimate by the passage
of two folds of peritoneum, one on the front, the other on the back, between the two parts, profoundly modifies the subsequent growth of the cæcum, and determines very largely its adult form. For, when the cæcum begins to expand, the medial aspect is prevented, by its connexion with the termination of the ileum, from enlarging as freely as the rest of the wall; in consequence of this the lateral part grows and expands much more rapidly, producing the lop-sided appearance already referred to, and soon comes to form the lowest part or fundus of the cæcum, and the greater part of its sac; whilst the original apex, with the vermiform process springing from it, anchored, as it were, to the end of the ileum, is thrust to one side, and finally lies on the medial and posterior aspect of the cæcum, a little way below, and usually posterior to, the end of the ileum.
The position of the cæcum varies at different periods of fœtal life. About the eleventh or twelfth week it lies immediately beneath the liver, and to the left of the median plane it then gradually travels to the right, crossing the descending part of the duodenum, and is found lying on the right side, just beneath the liver, at the fourth month. From there it descends slowly to its adult position, which it usually approaches towards the end of fœtal life, but it may not actually reach it until some time after birth. An imperfect descent gives rise to the lumbar position of the cæcum, or an excess in this direction to the pelvic position (referred to on p. 1213).
Rectum. The rectum and anal canal are formed from the posterior portion of the hindgut, and from the proctodeum.
The primitive closed cloacal portion of the hindgut becomes divided by a vertical septum into ventral and dorsal portions. The ventral, with the allantois growing from it, forms the sinus urogenitalis, the dorsal forms the rectum.
The proctodeum is separated from the rectum by the proctodeal membrane, but that membrane disappears, and thus the rectum comes to open on the surface.
The rectum and anal canal at first form a single continuous straight tube, which passes downwards in front of the comparatively straight anterior surface of the sacrum to the anal orifice.
That is the condition which the parts present at birth. After birth, the bony pelvis undergoes great enlargement. The sacrum and coccyx become curved, and the anteroposterior diameter of the pelvis minor increases very considerably.
The urinary bladder and, in the female, the uterus-both organs at birth lying mainly in the abdomen-descend into the pelvis minor. The anal orifice appears to be moved further forwards in the perineum, through the bending of the sacrum and coccyx, and the rectum is pushed back into the hollow of the sacrum. Hence the "flexura sacralis is formed.
The "flexura perinealis" is formed by the junction of the curved rectum with the straight vertical or backwardly directed passage formed by the intestine as it passes through the tissues of the pelvic floor.
The increase in the thickness of the pelvic floor gives to the anal canal the length which it attains in the adult.
DEVELOPMENT OF THE PERITONEUM.
At first the primitive alimentary canal is suspended from the dorsal wall of the embryo, along the median plane, by a simple dorsal mesentery, which extends along the whole length of the tube, and is common to all its divisions-a condition found in the adult stage of many reptiles. There is also present, in the upper part of the cavity, after the stomach and liver descend into the abdomen, a ventral mesentery (Fig. 972), which connects the stomach and duodenum to the back of the liver, and, passing on, connects the front of the liver to the anterior abdominal wall and diaphragm. The portion of this ventral mesentery, between the stomach and liver, becomes the lesser omentum; its anterior portion, between the liver and the abdominal wall, forms the falciform ligament (Fig. 972); and, in its inferior margin, the umbilical vein runs from the umbilicus to the liver.
The portion of the dorsal mesentery lying behind the stomach is known as the mesogastrium. At first it is relatively short; but with the growth of the posterior wall of the stomach, and the turning of that organ over on its right side, the mes gastrium becomes elongated, and is folded on itself, forming more or less of a pouct directed downwards and to the left. The wall of this pouch becomes in part the greater omentum, and the cavity enclosed by it forms the greater part of the omental burs In the rotation of the stomach and the accompanying passage of the lesser omentum frot an antero-posterior to a more or less transverse direction, a portion of the cavity of the
abdomen is, as it were, caught in behind the stomach and lesser omentum. This portion of the cavity becomes the upper part (vestibule) of the omental bursa, and at first it communicates with the general cavity by a wide opening to the right of the lesser omentum; but the growth of the liver, encroaching upon the opening, and other causes, reduce it to a relatively small size, and it forms the foramen epiploicum in the adult.
The greater omentum is, as pointed out above, a bag-like growth of the lower part of the mesogastrium, which passes downwards and to the left
Mesentery of descending colon
in front of the transverse
It would appear that the growth of the inferior part of the omental bursa,
Pelvic sigmoid) mesocolon
FIG. 974. Two DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF THE
the first figure the rotation of the intestinal loop and the continuous
and of the greater omentum, is primarily due to a proliferation of the cells over a limited area of the mesogastrium, and a resulting folding of this layer downwards and to the left.
In the upper part of the mesogastrium the spleen is developed, and the portion of this fold which intervenes between the stomach and spleen forms the gastro-lienal
Of the primitive mesentery, the portion connected with the stomach - the mesogastrium becomes modified in the manner just described. The next division - the mesoduodenum-disappears completely, owing to the turning over of the duodenal loop on to its right side, and its subsequent adhesion to the posterior abdominal wall, accompanied by the absorption of its mesentery. The mesenteries of the small and large intestine are continuous at first (Fig. 972). When the rotation of the intestinal loop takes place around the superior mesenteric artery (see above), the beginning of the large intestine, with its mesentery, is carried to the right across the duodenum, and a fan-shaped portion of the general mesentery, lying within the concavity of the loop, is partially cut off; this, later on, forms the mesentery proper in the adult. At first it is continuous by its right border with the mesentery of the
FIG. 975.-DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF THE
A, shows the beginning of the greater omentum and its independence of
great sac; H, in omental bursa.
ascending colon, a part of the primitive mesentery (which is similarly continued int the mesentery of the transverse, descending, iliac, and pelvic colons). Subsequently, shown by the darkly shaded parts in Fig. 974, the back of the mesenteries of the ascend ing, descending, and iliac portions of the colon adheres to the posterior abdominal wa.. and these mesenteries become lost; whilst the mesenteries of the transverse and pelvi portions of the colon remain free, and persist in the adult.
At the same time, the mesentery proper (which was at first attached only at its narrow neck, between the duodenum and transverse colon, and below this was continuous on the right with the ascending mesocolon) now acquires a new attachment to the posterior abdominal wall through the absorption of the ascending mesocolon (Fig. 974, and the adult condition is attained.
DEVELOPMENT OF THE LIVER AND PANCREAS.
The glandular tissue of the liver and pancreas, and the epithelial linings of the ducts of these organs, including the gall-bladder and cystic duct, are formed from protrusions of
FIG. 976.-DIAGRAM illustrating the arrangement of the blood-vessels (on left) and of the hepatic cells an bile-ducts (on right) within a lobule of the liver. The first diagram shows the interlobular veins running around the outside of the lobule, and sending their capillaries into the lobule to join the centra vein. In the second diagram the bile capillaries are seen, with the hepatic cells between them radiating to the periphery of the lobule, where they join the interlobular bile-ducts.
the endothelial wall of the foregut, below the stomach. The connective tissue framework of the glands is formed from the mesodermic tissue into which the protrusions grow. The process of formation is as follows::
1. Liver.-A longitudinal groove appears on the interior of the ventral wall of the fore gut, close to its union with the midgut, at about the third week. This groove appears on the external surface of the gut as a projection, which rapidly increases in size and grows forwards and upwards towards the lower part of the septum transversum. This septum is a mass of mesodermic tissue which lies in front of the foregut, just below the heart, and which is attached to the anterior and side walls of the trunk. It conveys the umbilical and vitelline veins as they pass to the sinus venosus.
The liver bud grows into the lower (caudal) portion of the septum transversum, and sends out strands of cells termed trabeculae, which come into contact with the vessels in the septum, and enclose them.
By the growth of capillary vessels, from the vitelline and umbilical veins, and of the trabeculæ, a spongy network is produced, the framework of which is formed by branching and anastomosing trabeculæ, while the spaces of the network represent portions of the lumen of the vessels, and are filled with blood. This form of vascular network is known as a "sinusoidal circulation."
The trabeculae become hollowed out, and are reduced in size, so that eventually s minute channel is formed in the centre of each of them, surrounded by a single layer of cells. The lumen of the channel forms a bile capillary, and the cells surrounding it form the secreting cells of the liver lobule.
The bile capillaries of adjacent trabeculae meet and unite, and converging together