402 



STOMACH AND INTESTINE. 



it assumes the length, form, and convolutions, 

 proper to the perfect intestinal tube. 



As it already occupies the whole length of 

 the abdominal cavity, any elongation of the 

 canal will of course give it a curved shape. 

 And since, at this period of foetal life, the 

 abdomen opens by a wide vertical fissure in 

 the situation of the future umbilicus, the 

 first bend of the intestine renders it convex 

 forwards, and then protrudes it through this 

 aperture. Here it adjoins the base of the 

 umbilical duct ; which opens into the point 

 or angle of this convexity, so that the bowel 

 appears like a bifurcation of the duct itself. 

 The two forks of this bifurcation are soon 

 produced into a spiral coil of intestine ; 

 which still lies outside the abdominal cavity, 

 and only recedes into it at about the middle 

 of the third month of uterine life. 



At this stage of its evolution, the intestinal 

 canal may be conveniently described as con- 

 sisting of three portions : an anterior, which 

 extends from the beginning of the tube to the 

 umbilical coil ; a middle, which is formed by 

 this coil itself; and a posterior, which reaches 

 from the latter segment to the end of the 

 canal. 



The anterior of these three portions may 

 again be subdivided into three similar seg- 

 ments. The first, which gradually elongates 

 from the blind end that was formerly the 

 fovea cardiaca, is developed during the evolu- 

 tion of the thorax, so as to form the oeso- 

 phagus. And it finally opens into the cavity 

 of the mouth ; which is itself developed from 

 an involution of the skin, and from the united 

 ends of the anterior visceral arch. The second 

 or middle segment dilates, turns on its left 

 side, and then bends transversely to the axis 

 of the body, to form the stomach. The 

 pyloric valve is only visible some time after 

 this change has occurred. And the third or 

 lowest portion of this anterior segment is con- 

 verted into the duodenum. 



The middle, umbilical, or extra-abdominal, 

 part of the canal, is developed into the jeju- 

 num and ileum, the ccecum, the vermiform 

 appendix, and part of the colon. In this 

 process, the change of form undergone by the 

 small intestine is limited to a mere increase 

 in its length and in the degree of its con- 

 volution : an alteration which is accom- 

 panied by a further elongation of its mesen- 

 tery. The upper boundary of the large 

 intestine is first seen as a constriction and 

 change of calibre, which occupy a point 

 some distance below the insertion of the 

 umbilical duct. Such a situation of the 

 future ccecum conclusively shows, that the 

 vermiform appendix is not that permanent 

 intestinal end of the duct, which Oken 

 supposed it to be. This commencement 

 of the large intestine next enlarges into 

 a projecting pouch of uniform width. But 

 the lower end of this pouch soon ceases to 

 enlarge, and remains as the vermiform appen- 

 dix. While its upper part, increasing in size, 

 becomes the co3cum. The valve appears at 

 about the tenth week. But the proper shape 



and size of the ccecum are only acquired 

 towards the end of foetal life. 



The colon is developed from the lower 

 part of the second, and the upper part of 

 the third portion of the rudimentary intes- 

 tine. The ascending colon is at first a simple 

 straight tube, which, commencing in the 

 pouch just alluded to, runs forwards along 

 the spinal column, lying to the left of the 

 numerous coils of the small intestine. The 

 succeeding backward bend of this tube has at 

 first a median position, which renders it par- 

 allel with (and close to) the ascending colon. 

 But this part of the canal soon elongates ; and, 

 passing outwards towards the left side, forms 

 the transverse and descending portions, as well 

 as the sigmoid flexure, of the colon. Finally, 

 the blind end which corresponds to the rec- 

 tum is continually moved downwards by a 

 gradual lengthening of the tube; so that it 

 meets, and at last opens into, a cavity, which 

 is sent inwards from the skin to form the 

 future anus. The sacculation of the large 

 intestine only occurs in the latter half of 

 uterine life. The valvulre conniventes appear 

 still later, and are but rudimentary at birth. 



The development of the various microscopic 

 constituents of the canal may be almost as 

 briefly summed up. The cell-growth (which 

 is derived from the mucous lamina), and the 

 fibrous tunic (which is developed from the 

 vascular lamina), are at first very loosely united 

 to each other. Hence they may be easily 

 separated into distinct and comparatively 

 plane strata ; of which the fibrous has 

 about double or treble the thickness of the 

 epithelial one. The cells of the latter affect 

 an elongated or columnar form at a very 

 early date of foetal life (about the sixth 

 week). The various offshoots of tubes and 

 other glands which are contained in the wall of 

 the canal, are developed from a mass of cell- 

 growth, which sprouts from the external 

 surface of the mucous layer, and gradually 

 acquires the definite form and cavitary ar- 

 rangement specific to these minute structures. 

 The larger accessory glands of the liver and 

 pancreas are produced from a similar mass 

 which lies external to the bowel : and they 

 ultimately prolong their ducts so as to 

 open into the cavity of the intestine. 



The fibrous layer, which is at first smooth 

 and homogeneous, soon becomes roughened 

 into little projections, which ultimately take 

 the shape of conical processes. These, as 

 they enlarge, pass upwards into the mucous 

 or epithelial layer. Some of these projections 

 hot only separate the various tubes and 

 glands from each other, but, by a farther 

 advance and enlargement, carry before them 

 the general surface of the cell-growth. They 

 thus form the future villi. While others 

 and by far the majority affect a lateral, 

 instead of a vertical, growth ; uniting with 

 their neighbours by cross ridges, which soon 

 form a network, that extends between the 

 tubes at all parts of their height, so as to 

 constitute a matrix for these and the other 

 structures derived from the mucous lamina. 



