DIGESTIVE TRACT. 39 



blind tube, the intestinal caecum, 1 which is clearly connected with 

 increase of digestive surface. In certain birds there may be two 

 of these caeca, and in the ostrich there is developed on its inside 

 a spiral fold. In the mammals the caecum shows great varia- 

 tions. It is lacking entirely in certain groups (edentates, chei- 

 roptera, some carnivores). In the herbivorous forms, on the 

 other hand, it may equal the body in length. In man and some, 

 apes and rodents all parts of the caecum are not equally devel- 

 oped, the terminal portion, known as the appendix vermiformis, 

 remaining smaller than the rest. 



In the elasmobranchs, dipnoans, amphibians, sauropsida, and 

 the monotremes among the mammals, the rectum does not 

 open directly to the exterior, but into a terminal enlargement, 

 the cloaca, into which the urinary and reproductive ducts also 

 empty ; and from this chamber all contents pass to the exterior 

 through the vent. In the other vertebrates no cloaca is formed. 

 In connection with the cloaca in birds is developed a sac 

 (bursa Fabricii), which comes to lie in the pelvic cavity be- 

 tween the vertebrae and the terminal portion of the hind gut. 

 Its function is unknown. The bursa is said to be of ectodermal 

 origin. 



The alimentary tract is here placed among the entodermal 

 structures, but only the lining coat is derived from that germ 

 layer. Other constituent parts are derived from the mesen- 

 chyme. Beneath the entodermal epithelium and following 

 closely its contour is a layer of loose connective tissue, the 

 sub-mucosa, 2 which carries blood and lymph vessels. Outside 

 of this are the muscular layers, two in number, an inner circular 

 and an outer longitudinal, each of smooth or non-voluntary 

 muscle. These by their action produce peristaltic movements 

 of the contents of the tube. Where it passes through the 

 body cavity the alimentary canal receives a third or peritoneal 

 layer of pavement epithelium derived directly from the splanch- 

 nic layer of the ccelom. 



1 The rectal gland of the elasmobranchs is possibly home logous with the caecum of 

 the amniotes. 



2 Occasionally the sub-mucosa may be divided by a muscular layer, in which case that 

 portion nearest the entodermal epithelium is called the tunica propria, the deeper portion 

 retaining the name sub-mucosa. 



