CHARACTERS OF AVES. 435 



follicles which secrete the gastric juice. The proventriculus, 

 however, corresponds, not with the whole stomach of the Mam- 

 mals, but only with its cardiac portion ; and it opens into a 

 second, muscular cavity, which corresponds to the pyloric 

 division of the Mammalian stomach. The gizzard (g) is situ- 

 ated below the liver, and forms in all birds an elongated sac, 

 having two apertures above, of which one conducts into the 

 duodenum or commencement of the small intestine, whilst the 

 other communicates with the proventriculus. The two chief 

 forms of gizzard are exhibited respectively by the Raptorial 

 birds, which feed on easily - digested animal food, and the 

 Rasores and some of the Natatores, which feed on hardly- 

 digested grains. In the birds of Rapine the gizzard scarcely 

 deserves the name, being nothing more than a wide membran- 

 ous cavity with thin walls. In the granivorous birds, whose 

 hard food requires crushing, the gizzard is enormously devel- 

 oped ; its lining coat is formed of a thick, horny epithelium, 

 and its walls are extremely thick and muscular. This consti- 

 tutes a grinding apparatus, like the stones of a mill ; whilst the 

 " crop " or oesophageal dilatation may be compared to the 

 " hopper " of a mill, since it supplies to the gizzard " small 

 successive quantities of food as it is wanted " (Owen). Sup- 

 plementing the action of the muscular walls of the gizzard, and 

 acting in the place of teeth, are the small stones or pebbles, 

 which, as is so well known, so many of the granivorous birds 

 are in the habit of swallowing with their food, or at other 

 times. In fact there can be no doubt but that the gravel and 

 pebbles swallowed by these birds is absolutely essential to ex- 

 istence, since the gizzard, without this assistance, is unable 

 properly to triturate the food. 



The intestinal canal extends from the gizzard to the cloaca, 

 and is comparatively speaking short. The secretions of the 

 liver and pancreas are poured into the small intestine, as in 

 Mammals. The commencement of the large intestine is almost 

 always furnished with two long " caeca " or blind tubes, the 

 length of which varies a good deal in different birds (fig. 171, k). 

 They are sometimes wanting ; and their exact function is un- 

 certain ; though they are most probably connected partly with 

 digestion and partly with excretion. The large intestine is 

 always very short seldom. more than a tenth part of the length 

 of the body and it terminates in the "cloaca" (fig. 171, cl). 

 This is a cavity which in all birds receives the termination of 

 the rectum, the ducts of the generative organs and the ureters ; 

 and serves, therefore, for the expulsion of the faeces, the gen- 

 erative products, and the urinary secretion. 



