THE M.niKXTAUY ClXAL 



993 



The simple alimentary tube. — The alimcntarv canal first aj>pears as a simple 

 tube lyin.i4al)()Ut ihv niciliau line in front of the ])(^sterior alidoniinal parietes, and 

 placed vertically. This tube is corniected with the posterior parietes by a simple 

 fold of ])eritoneum — a species of fr<nieral mesentery (fig. ol)2). 



The fact that the abdominal cavity is at an early period open in front, and that 

 the rudimentary intestine protrudes beyond the limits of the future cavitv may be 

 disreganlcd. The cavity, such as it is, is lined by a membrane wliicji is later 

 recognisable as the perit<^»neum, and tiie general mesentery is produced l)v the 

 growth and protrusion forwanls of the elementary Ijowel from the tissues liehind, 

 i.e.. outside the peritrMieum. 



The simple straight tube, suspended on the ])osterior mesial line by its fokl of 

 peritoneum, becomes in due course differentiated (fig. 592). 



Theu[)per part becomes the stomach; the bowel immediately beyond forms the 

 'luodenum; then follow the small intestine; and lastly the colon and rectum. 



The point of separation of the small bowel from the colon is indicated bv the 

 appearance of the ciecum. 



It will be understood also that the up]ier part of the mesial peritoneal fold is 

 called the meso-gaster ( m-g, fig. 593); the next part the meso-duodenum (^m-d); 



Fig. 593.— Di.iGKAM of the Primitive Alimentary Canal. 



and the succeeding portions, in order, the mesentery, the meso-colon, andthemeso- 

 rectuin (fig. 598, mks, m-c, m-r). The stomach liecomes more distinctly marked; 

 the tul)e below lengthens and forms an intestinal loop, which in the embryo i)ro- 

 jects beyond the site of the future umbilicus, taking the mesentery with it. 



This elementary condition of the intestinal canal jx-rsists in certain fully 

 develoi)ed animals. As an example may l)e taken the alimentary canal of the 

 salamander, as depicted in fig. 594. In this figure the vi.sci'ra are shown undis- 

 turbed. In fig. 595, the })eritoneum has been drawn forwards so as to show the 

 ])arts in one median plane, and well represents tiie simjile arrangement. In many 

 mammals among the Edenttites the simple vertical median fold of peritoneum is 

 retained. This is shown in the great anteater, and in Hoffman's sloth (fig. 590). 

 wliere the intestinal canal, from the pylorus to the rectum, is still quite simple, 

 altliough it is of considerable h-ngth, and is, as a conse<iuence, thrown into 

 numerous coils. The whole of this long tube is su)>i)orte(l by a single median fold 

 of peritoneum attached to the midtlle line behind, and serving the purpose of 

 meso-duodenum, mesentery, meso-colon. and meso-rectum in one. 



Where this fold is attachi'd behind, ruiLs the aorta; and In'twi-en the layers of 

 the fold vessels pass forwards to su})ply the visci-ra. (Fig. 592, B, a. represents the 



