586 MANUAL OF ZOOLOGY. 



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. 

 326, k). They are sometimes wanting (Parrots, &c.), or there 

 may be only one ; and their exact function 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. 326, 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 expulsion of the faeces, the gene- 

 rative products, and the urinary secretion. 



Respiration is effected in Birds more completely and actively 

 than 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 

 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, and which are present in 

 all birds except the Apteryx. 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 its highest perfection. The cavities of the 

 thorax and abdomen are not separated from one another by a 

 complete partition, the diaphragm being mostly 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 back of the thorax, extending along each side of the spine, 

 from the second dorsal vertebra to the kidney. They differ 

 from the lungs of the Mammals in not being freely suspended 

 in a pleural membrane. The pleura, on the other hand, is 

 reflected only over the anterior surface of the lungs. The 

 bronchi, or primary divisions of the windpipe (fig. 327), 

 diminish in size as they pass through the lung, by giving off 

 branches, which, in turn, give off the true air-vesicles of the 

 lung. When the bronchial tubes reach the surface of the lung, 

 they open, by a series of distinct apertures, into a series of 



