260 VERTEBRATE ANIMALS. 



crop is returned to the mouth, and supplied to the young in a 

 state suitable for digestion. The gullet, after leaving the 

 crop, shortly opens into a second cavity, called the " proven- 

 triculus," which is the true digesting stomach, and is richly 

 supplied with glands which secrete the digestive fluid or gas- 

 tric juice (p). This, in turn, opens into a muscular cavity 

 which is called the "gizzard" (g), and which leads into the 

 commencement of the small intestine. The characters of the 

 gizzard vary with the nature of the food. In the birds of 

 prey, which live on a easily-digested animal diet, the walls of 

 the gizzard are thin and membranous. In the grain-eating 

 birds, such as the fowls, whose hard food requires to be 

 crushed before it can be properly digested, the walls of the 

 gizzard are extremely thick and muscular, and the inner hh- 

 ing is hard and homy. In these birds the gizzard constitutes 

 a kind of grinding apparatus, Uke the stones of a mill ; while 

 the " crop " may be compared to the " hopper " of the mill, 

 since it supplies to the gizzard " small successive quantities 

 of food as it is wanted (Owen). The grinding action of the 

 gizzard is further assisted by the small pebbles and gravel 

 which, as is well known, so many birds are in the habit of 

 swallowing. These pebbles take the place of teeth, and there 

 can be no doubt that they are in many cases essential to 

 health, the bird being otherwise unable to triturate its food 

 properly. The intestinal canal extends from the gizzard to 

 the cloaca (cl), and is comparatively short. The secretions of 

 the liver and pancreas are poured into the commencement of 

 the small intestine. The commencement of the large intestine 

 is furnished in most birds with two bhnd tubes or caeca (A). 

 These vary considerably in length in different birds, and are 

 sometimes wanting; while their exact function is still ques- 

 tionable. The large intestine is seldom more than a tenth 

 part of the length of the body, and is generally conducted 

 straight from the cseca to the cloaca. The cloaca is a com- 

 mon cavity which in birds, as in Reptiles, receives the termi- 

 nation of the intestine and the ducts of the generative and 

 urinary organs (cZ). 



Hespiration is effected in Birds more completely, exten- 

 sively, and actively, than in any other class of the Vertebrata, 

 and, as the result of this, their average temperature is higher 

 than in any other Vertebrates. This extensive development 

 of the respiratory process is due to the fact that air is admit- 

 ted in Birds not only to the lungs, but also to the interior of 

 a greater or less number of the bones, and to a series of air- 



