330 ZOOLOGY. 



intestine. Finally, a gall-bladder is almost always present, 

 which receives from the liver a portion of bile, and pours it 

 into the intestine by a distinct canal. The pancreas is en- 

 closed in the first loop of the small intestine ; it is long, 

 narrow, and more or less divided. 



The spleen is small ; the kidneys are large and irregular in 

 form. They are lodged behind the peritoneum, in little hol- 

 lows along the upper wall of the pelvis, and, unlike the organ 

 in mammals, they have no distinct cortical substance. The 

 ureters, as well as the oviducts, terminate near the anus, in a 

 dilated part of the intestine called cloaca (Fig. 315) ; there 

 exists no urinary bladder, and the urine is voided with the 

 excrements. The urine is composed almost wholly of uric 

 acid, which is not very soluble, and when dried forms a 

 whitish mass. 



438. The nutrient products of digestion leave the intes- 

 tine by lymphatic vessels, which terminate in two thoracic 

 ducts ; these ducts open into the jugular veins an each side of 

 the base of the neck. 



439. The blood of birds is richer in globules than that 

 of mammals, and these corpuscles, instead of being globular, 

 are elliptic (Fig. 37). The circulation in birds is complete, as 

 in mammals, and the anatomical arrangements are the same 

 (Fig. 47). But the walls of the left ventricle are much 

 thicker, the auricles have no well marked appendages, and the 

 right ventricle does not extend to the apex of the heart. 

 These latter differences are unimportant physiologically. The 

 aorta at its commencement divides into three large branches 

 (Fig. 316), of which the first two convey the blood to the 

 head and neck, wings, and muscles of the chest ; whilst the 

 third, curving downwards around the right bronchus, becomes 

 the descending aorta. There exist also some other pecu- 

 liarities in the mode of distribution of the arteries, such as 

 the formation of plexuses in various parts of the body. The 

 venous system terminates in the right auricle by three large 

 trunks, of which one represents the inferior cava, and the two 

 others the subclavian veins of mammals, which in birds enter 

 the auricle without uniting to form a common trunk. 



440. The respiratory apparatus presents modifications 

 more remarkable than those of the circulation. The lungs 

 communicate with large membranous cells spread throughout 

 the body, and extending even to the skeleton. Thus the venous 

 blood in the walls of many organs becomes exposed to the 



