, 

 V/ 



x 



266 HISTORY OF THE HUMAN BODY 



muscles, forms a valve capable of closing, thus converting the 

 stomach temporarily into a closed sac. 



The remainder of the canal may be comprehendingly desig- 

 nated intestine, the regional differentiation of which does not 

 appear as early and is never so marked as in the anterior 

 portion. A little below the pylorus it develops from its mucosa 

 two enormous glands or gland-complexes, liver and pancreas, 

 which grow out far beyond the limits of the walls of the canal, 

 retaining their connection with their place of origin through 

 ducts. The portion of the intestine between the pylorus and 

 the orifices of these ducts receives the special name of duode- 

 num. A second early and better marked subdivision of the in- 

 testine is a terminal enlargement which generally receives the 

 openings of the urinary and reproductive systems, and is hence 

 termed the cloacal chamber, or simply cloaca. This portion 

 retains its importance in fishes, amphibians and the Sauropsida, 

 but in mammals it plays a subordinate role, appearing as a 

 distinct organ in the lowest forms alone, the monotremes. 



In certain definite portions of its length the alimentary canal 

 shows a tendency to throw out diverticula, sac-like or tubular 

 in shape and designed apparently to increase the general sur- 

 face. Of those the most important are the lateral pharyngeal 

 pouches previously mentioned; the pyloric cceca, found in 

 fishes, and often very numerous; colic cceca, at the beginning 

 of the large intestine in mammals ; and cloacal caeca, found in 

 birds. 



Following this general sketch of the alimentary canal, its 

 development and its differentiation, the separate portions may 

 be considered in greater detail [Cf. Fig. 72]. The most an- 

 terior of these are the mouth cavity and pharynx, usually 

 fused into one, the stomato-pharyngeal cavity, although in 

 mammals the development of the soft palate forms an incom- 

 plete separation between the two. This cavity opens to the 

 exterior through the mouth opening or stoma, which appears 

 to be of two types in accordance with its surroundings and 

 equipment. 



The first of these, the cydostoma, is that seen in the lam- 



