562 



COMPAEATIVE ANATOMY. 



piece (Fig. 313, r; 316, G c). In the Selachii it is provided with a 

 special glandular appendage (Fig. 316, Gx). It is onlyin the Amphibia 

 that, owing to its greater length and width, it becomes of some im- 

 portance, but in them, as in the Eeptilia, it retains its straight 

 course in correspondence with its shortness. In consequence of this 

 straight course it has got the name of " rectum." It is generally 

 separated from the mid-gut by a transverse fold or valve. Many 

 Eeptiles are provided with a caecal appendage, which, in the Ophidii 

 is feebly, and in the Saurii is better developed. The casca in Birds 

 are much more independent. In this group, also, the hind-gut is 

 short and straight (Fig. 320). The c^cum is generally paired, and 

 is absent in a few famihes only (e.g. Woodpecker, Psittacus, etc.). 

 They vary greatly in the extent to which they are developed, so that 

 they may form short papilliform appendages, or very long tubes 

 (Apteryx, Gallinse, Anseres). 



The hind-gut is longest in the Mammalia, where it forms the 

 large intestine, and is distinguished, as such, from the mid-gut, or 

 small intestine. Owing to its greater length it is arranged in coils, 

 so that the terminal portion, only, has the straight course taken by 

 the hind-gut of other Vertebrata. The anterior portion ordinarily 

 forms a loop which bends from the right side of the abdominal cavity 

 forwards, and then to the left, and then again backwards to be 

 continued into the rectum. This loop is sometimes broken up into 

 secondary loops. 



At the boundary between it and the small intestine caecal struc- 

 tures are likewise developed, but these are rarely arranged in 

 pairs (Fig. 318, c d), and are commonly single. The size of this 



cEecum may be shown to depend 

 on the food. In the Carnivora 

 it is short, and sometimes com- 

 pletely absent (Ursina, Muste- 

 lina) : it is very large in the 

 Herbivora, where its length is 

 compensated for by that of the 

 colon. 



The csecum itself may be 

 affected by differentiations. Its 

 terminal portion is frequently 

 diminished in size (e.g. in various 

 Prosimite and many Rodents) 

 (Fig. 318, c). In various Primates, 

 and in Man, the terminal portion, which, at first, is as wide as the 

 rest, is not developed in proportion to it ; it thus becomes more and 

 more distinct from the other portion, which continues to grow 

 wider, until at last it forms a mere appendage to it — the appendix 

 vermiformis. 



The hind-gut primitively opens into the same space as the urinary 

 and generative ducts, the cloaca. This arrangement, which ob- 

 tains in the Selachii, Amphibia, Eeptilia, and Aves, is permanent in 



rig. 318. Caecum and colon of Lago- 

 mys pusillus. a Small intestine, 

 b Opening of the larger (c), and of the 

 smaller (d) cseoum. e f g Diverticula of 

 the colon (after Pallas) . 



