FORE-GUT OF VERTEBRATA. 



557 



once into the stomacli, whicli can only be distinguislied from it by 

 the differences in the characters of its mucous membrane. As a 

 rule, the stomach (Fig. 313) forms a csecal sac, which is directed back- 

 wards, and from which a narrow portion (pyloric tube) which bends 

 forwards, can be distinguished; this leads to the mid-gut (i). This 

 is the case in all Selachii and Ganoi'dei, and in many Teleostei, 

 while the rest vary greatly in the absence, or the great development 

 backwards, of the caecal sac. 



Among the Amphibia we find a lower stage in Proteus, for the 

 enteric tube, which has a perfectly straight course, has no stomachal 

 enlargement at all. In the other Urodela, however, the stomach 

 forms a wider portion of the enteron ; and this is the case also in the 

 Anura, where the stomach is sometimes, 

 indeed, placed transversely (Bufo). 



Among the Reptilia, the fore-gut is of 

 a lower stage in the Ophidii and Saurii, 

 owing to the greater width of the oeso- 

 phagus and the straight course of the 

 stomach. However, there is an arrange- 

 ment in the Saurii which calls to mind 

 the pyloric tube of the Selachii, and from 

 this the stomach might gradually acquire 

 a transverse position. In the Chelonii and 

 Crocodilini the oesophagus is more sharply 

 separated from the stomach, which in the 

 former has a large and a small curvature, 

 owing to the great elevation of the pyloric 

 portion. Owing to the approximation of 

 the cardiac end of the stomach to the 

 pylorus, this portion is rounded in the 

 Crocodile, and is also distinguished by 

 a tendinous disc on each face of its 

 muscular wall ; in this point it resembles 

 the stomach of Birds. 



In the fore-gut of Birds there is a 

 greater division of labour. The influence 

 of adaptation to the mode of life, and here 



especially to the mode of nutrition, is most clearly shown by the varia- 

 tions in the different arrangements. The oesophagus, which is of the 

 same length as the neck, is either of equal calibre along its whole 

 course, or is provided with a widened portion (Fig. 314, A), or with 

 a cascal diverticulum (jB), which looks Like an appendage. Portions 

 (i) of this kind, which are characterised by modifications of the 

 glandular organs of the mucous membrane, form a crop (ingluvies). 

 This is best developed in carnivorous and graminivorous Birds ; in 

 the former, indeed, it generally forms a spindle-shaped enlargement, 

 while in the latter it forms a imilateral diverticulum, which is dif- 

 ferentiated into a csecal appendage, in many provided with a narrow 

 connecting piece. 



Fig. 314. 

 Eaptorial 



A Fore-gut of a 

 Bird (Buteu). 



B Of a Fowl, oe Oilsophagua. 



i Crop, pv Glandular stomach. 



V Muscular Btomaoh. d Duode- 

 num. 



