CHARACTERS OF AVES. , 493 



Hum, and its walls are extremely thick and muscular. This 

 constitutes 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). 

 Supplementing the action of the muscular walls of the gizzard, 

 and acting in the place of teeth, are the small stones or peb- 

 bles, 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 are absolutely essential to 

 existence, 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 al- 

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

 the length of which varies a good deal in different birds (fig. 

 215, /). They are sometimes wanting; and their exact func- 

 tion is uncertain ; 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. 215, d). 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 expul- 

 sion of the faeces, the generative products, and the urinary 

 secretion. 



Respiration is effected in Birds more completely and actively \ 

 tharr in any other class of the Vertebrata, and as the result of 

 this, their average temperature is also higher. This extensive 

 development of the respiratory process is conditioned by the 

 fact that, in addition to true lungs, air is admitted into a greater 

 or less number of the bones and into a number of cavities 

 the so-called air-receptacles which are distributed through 

 various parts of the body. By this extensive penetration of 

 air into various parts of the body, the aeration of the blood is 

 effected not only in the lungs, but also over a greater or less 

 extent of the systemic circulation as well ; and hence in Birds 

 this process attains 4t^m'gh^slerjetioja. The cavities of the 

 thorax and abdomen are not separated from one another by a 

 complete partition, the diaphragm being only present in a 

 rudimentary form. The lungs are two in number, of a bright- 

 red colour, and spongy texture. They are confined to the 



