1168 



DIGESTIVE SYSTEM 



intestine a second tier of arches, formed in a similar manner, is often noted, and below the middle 

 of the jejuno-ileum more than two tiers of arches may be present the complexity of the arches 

 increasing, while the size of the vessels diminishes. From the convex border of the most dis- 

 talh' placed arches there pass to the intestine straight branches, so-called vasa recta. Near the 

 beginning of the jejunum these are numerous and large, and have a length of about 4 cm., 

 and are quite regular. After the first third of the intestine is passed the vasa recta become 

 smaller and shorter, and toward the lower end of the ileum they become short and irregular 

 and are often less than 1 cm. in length. (Dwight.) The blood is returned by means of the 

 superior mesenteric vein, which, with the splenic vein, forms the portal. The vascular ar- 

 rangement in the intestinal wall is shown in fig. 928. 



The lymphatic vessels form a continuous series, which is divided into two sets — viz., that 

 of the mucous membrane and that of the muscular coat. The lymph-vessels of both sets form 

 a copious plexus (fig. 928). The efferent lymphatic vessels form the so-called lacteals, which 

 pass through the mesenteric lymph-nodes, finally reaching the cisterna (receptaculum) chyli. 



The nerves. — The small intestine is supplied by means of the superior mesenteric plexus 

 which is continuous with the lower part of the coeliac (solar) plexus. The branches follow the 

 blood-vessels, and finally form two plexuses: one (Auerbach's or myenteric) which lies between 

 the muscular coats; and another (^leissner's) in the submucous coat. The nerve fibres are 

 chiefly from the sympathetic, partly from the vagus. 



Development of the small intestine. — As the intestine is being separated from the yolk- 

 vesicle it forms at first a relatively straight tube, and as the tube elongates there is formed a 

 single primary loop, situated in the sagittal plane of the embryo, which loop extends into the 

 coelom of the umbilical cord; to its summit is attached the constricted attachment of the yolk- 

 vesicle, the yolk-stalk (fig. 929). This primary loop of the intestine, as it elongates, turns on 

 an axis, so that its caudal portion turns toward the left and its cephalic portion toward the 

 right. We may then speak of a right and a left half of the loop. Near the top of the left half 

 of the loop there is noted an enlargement which marks the caecum, the greater part of the left 



Fig. 929. — Model of Stomach and Intestine of Human Embryo 19 mm. Long. The 

 figures on the intestine indicate the primary coils (X 16). (Mall.) 



half of the loop forming, therefore, the large intestine, while the right half of the loop forms the 

 small intestine. In the further growth of the loop the right half elongates more rapidly than 

 the left half, so that the caecum is no longer found in the middle of the loop. In an embryo 

 of the fifth week, as noted by Mall, whose account is here followed closely, 'the right half of 

 the loop has a number of small bends in it, which are of groat importance in the further develop- 

 ment of the intestine.' These small bends or loops he has marked with the numbers 1, 2, 3, 4, 

 5, 6. (See figs. 929, 930, 931.) The first of these bends is primarily not clear, appearing as a 

 portion of the pyloric end of the stomach; however, it is recognised by the fact that the ducts 

 of the liver and pancreas terminate in it, marking it as the duodenum. The omphalo-mesenteric 

 veins and arteries, th(; futin-e superior mesenteric vessels, pass through the middle of the 

 mesentery of the large i)rimary loop and pass over the sixth l)end or secondary looj), to which 

 is also attached the yolk-stalk. With the elongation of the intestine these six bends or loops 

 become accentuated and ac(juire secondary lo()i)s or coils, nearly all of which are still found in 

 the ca'lom of the umbilical cord, but even with this more complicated coiling of the intestine 

 the six primary divisions may bo clearly made out. (See fig. 929.) 



The large intestine, the left half of the large primary loop, lies in the sagittal plane of the 

 embryo and does not grow as rapidly as the small intestine, and while this is acquiring the 

 secondary coils, the wliole mass rotates about the large intestine as an axis. 'By this process the 

 small intestine is gradually turned from tlie right to the; left side of the l)ody, and in so doing is 

 rolled under the superior mesenteric artery. This takes place while the large intestine has an 

 antero-post(;rior direction and before there is a transverse colon.' (Mall.) With the return 

 of the small intestine from the umbilical co'loin to the peritoneal cavity, which occurs apparently 

 quite suddenly anrl during the middle of the fourth month, the caecum comes to lie in the right 

 half of the abdominal (;avity, just below the liver; the greater portion of the remainder of the 

 large intestine then lies transvensely across the abdominal cavity as the transverse colon. The 

 six groups of loops of the small intestine may still be recognised^ the loops of the upper part 



