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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 wesophagus, 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. 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
in the regions of the
curvatures, and later Esophagus
in the duodenum and Stomach
small intestine, and Spleen
then in the large inDiaphragm Spleen
testine, where they are
formed first in the mesogastrium
rectum and last in the
Superior mesen. vermiform process. In Small intestine
the stomach the folds Superior mesen
are arranged so as to teric artery
surround small isolated
Inferior mesen. depressions, which intestine
teric artery afterwards become the
foveolæ gastricæ. In
isolated elevations are Inferior mesenteric artery
found, in place of conThe mesentery
tinuous folds, and at a
later stage new elevaFig. 972.-Two DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF THE tions are formed beINTESTINAL CANAL.
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 rilli. 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
rentiated later on. In the stomach the formation of the glands begins I of the third month. cinal glands of the large intestine represent merely furrows between adjacent d are not due to an active proliferation of cells at the base of the furrows, distinction may be drawn between the two, and be expressed by using the al glands for the depressions of the small intestine and intestinal follicles for arge intestine.
tov. Nagy, whose description has been followed above, the glandular the gastric glands begins to assume its differentiated form in different parts, nd pyloric glands, towards the fifth month of development. nd Vermiform Process.—The cæcum first appears in the embryo, at about ek, as a small outgrowth of the wall of the primitive gut (midgut), not iated into small and large intestines. At this time the outgrowth is of the
HE ABDOMINAL VISCERA IN THE NEWLY BORN CHILD. The liver and the jejunum and ileum een removed. The vertical stomach, the large supra-renal gland, the high position of the cæcum, he whole arrangement of the large intestine are typical of the condition found at birth, and differ, - be seen, largely from the adult condition.
throughout, and is practically equal to the intestines in diameter. About the veek, whilst the large and small bowels are still of the same width, it has wery considerably in length (being equal to about five times the diameter of the stine, and thus being relatively as long as in the adult); but even at this early basal portion, for about one-fifth of its length, is quite as wide as the intestine,
remaining four-fifths of the outgrowth-the future vermiform process—is only half or one-third the diameter of the gut. From this it is seen that the distal the outgrowth, which subsequently becomes the vermiform process, begins to
even at this early period of its development. asal portion continues to expand with the gut; the distal part grows rapidly 1 length, but otherwise enlarges very slowly, so that, towards the end of fætal æcum has attained a conical shape, the wider end joining the ascending colon, w end tapering gradually and passing into the vermiform process.
This form, the infantile type of cæcum, is retained for some time after birth, or even may - per cent. of cases) persist throughout life. rly as the sixth or seventh month of fætal life the wall of the terminal portion all intestine adheres to the medial side of the cæcum for some distance below the 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 fætal 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 fætal 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 movei 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 posters wall of the stomach, and the turning of that organ over on its right side, the mesra gastrium becomes elongated, and is folded on itself, forming more or less of a pouch 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 bursi In the rotation of the stomach and the accompanying passage of the lesser omentum fruti 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
Stomach upon the opening, and
Median plane other causes, reduce it to
Transverse mesocolon a relatively small size, and it forms the foramen
Descending epiploicum in the adult.
obliterated The greater omentum is, as pointed out above, a bag-like growth of the lower part of the mesogastrium, which passes
INO downwards and to the left in front of the transverse colon.
As shown in Fig. mesentery 975, A and B, it is first entirely unconnected with
Ysigmoid) the transverse colon and. Mesentery or descending colon mesocolon; but about the
Fig. 974.-Two DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF THE third or fourth month it
MESENTERIES. becomes united to both, In the first figure the rotation of the intestinal loop and the continuous and the adult condition is
primitive mesentery is shown. In the second figure (to the right), established (Fig. 975, C). which shows a more advanced stage, the portions of the primitive
mesentery (going to the ascending and descending colons) which disIt would appear that appear, through their adhesion to the posterior abdominal wall, are the growth of the inferior shaded dark ; the portions which persist are lightly shaded. part of the omental bursa, 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
ligament, whilst the part behind the spleen becomes the lieno-renal ligament.
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 A B
posterior abdominal wall, ac
companied by the absorption FIG. 975.-DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF THE GREATER OMENTUM (after Hertwig).
of its mesentery. The mesen
teries of the small and large A, shows the beginning of the greater omentum and its independence of the transverse mesocolon ; in B, the two come in contact; and in
intestine are continuous at C, they have fused along the line of contact. (According to Lock- first (Fig. 972). When the wood, the two layers of the fold shown in A, running in between rotation of the intestinal loop the greater omentum and transverse mesocolon, instead of fusing, as takes place around the superior shown in B, are drawn out-unfolded - producing the condition shown in C.) A, stomach ; B, transverse colon ; C, small intestine; mesenteric artery (see above), D, duodenum ; E, pancreas ; F, greater omentum ; G, placed in the beginning of the large great sac ; H, in omental bursa.
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 mesenterý proper in the adult. At first it is continuous by its right border with the mesentery of the 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 wal. and these mesenteries become lost; whilst the mesenteries of the transverse and pelvic 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 ducta of these organs, including the gall-bladder and cystic duct, are formed from protrusions of
veins and ducts
Bile-ducts 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 central 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 foregut, 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 trabeculæ, 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 trabeculæ become hollowed out, and are reduced in size, so that eventually a 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 trabeculæ meet and unite, and converging together