994 



THE ORGANS OF DICESTION 



intestinal tube to whicli an artery is passing.) Tliere will l)e an artery to the 

 stomach (1, tig. 593) and the great nie-st-nterie artery. The latter vessel will supply 

 the rudimentary duodenum, the small intestines, the caecum, and such part of the 

 colon as is later on known as the ascending and transverse segments. The arrange- 

 ment of tlie vessel is shown in tigs. 598, 2, and 596. 



The descending colon and rectum are supi)lied by a separate vessel, the inferior 

 mesenteric, direct from the aorta (3, figs. 593 and 596). 



At the summit of the primitive loop which forms the principal part of the intes- 

 tine will be the vitello-intestinal duct or vitelline stalk (v. d, fig. 593; a, fig. 592, A). 

 It is to this point that the great or superior mesenteric artery is directed. The 

 morphological ending of the superior mesenteric artery is at this spot, while 

 branches pass off from either side of it. The student may be reminded that a trace 

 of the vitelline duct may persist in the form of a process known as Meckel's 

 (Hverticulum, and that this process, when existing, is situated in the lower part of 

 the ileum not far from the csecum (page 967). In the adult the trunk of the 

 artery may be represented by a line drawn from its place of origin to a point on the 

 ileum from one to three feet from the caecum. 



The primitive intestinal loop increases in length, and forms longer and more 

 complex coils. A large part of these coils will lie without the abdomen, the 

 anterior abdominal w'all being still incomplete. 



As the loop lengthens, a species of neck forms which tends to become narrower. 



Fig. 594. — Alimentary Canal of Salamandra maculosa. 



GASTRO-HEPATIC 

 OMENTUVI 



Tlie upper part of this neck or strait is bounded or formed by the duodenum; the 

 lower part by that portion of the large bowel which is at a later period knoAvn as 

 the transverse colon (fig. 593). 



The transverse colon and the duodenum are thus brought near together, and it 

 is notew'orthy that, no matter how complex the relations of the bowel become, 

 these segments of the intestinal tube are never separated. In the narrow neck 

 formed betAveen them runs the trunk of the superior mesenteric artery. The great 

 loop beyond this neck will form the small intestine, the csecum, and the ascending 

 colon. The descending colon retains its sim])le connections with the median line 

 (fig. 593). 



The mesentery does not increase in the same ])roportion as the intestines grow, 

 and hence the bowel is thrown into innumerable convolutions. In the human 

 subject the increase in the breadth of the mesenter}'' is somewhat more noteworthy 

 than its increase in length. 



There is a time, then, when the great mass of the intestine is supported by a 

 simple but extensive mesentery, which is entirely free, and which is attached 

 behind by means of a narrow^ neck bounded by the duodenum and the right end of 

 the transverse colon, and through which the superior mesenteric artery runs. 



The great intestinal loop ^n-ojects at first anteriorly and mesially. The small 

 intestine is above, the large intestine below (figs. 593, 595). 



