Xii INTEODUCTION. 



two powerful muscles, united by two raciating teridonSy 

 and lined by a coating of cartilaginous structure. This is 

 chiefly the structure in granivorous birds ; for in those 

 that subsist on fish or animal food, the muscular coat is 

 thin, and every gradation is observable. Granivorous birds, 

 and occasionally others, often swallow bits of gravel and 

 other hard substances, apparently with a view to increase 

 the triturating power of the gizzard. A supplementary 

 stomach, or accessory pouch, is found near the pyloric 

 orifice in some birds. 



The intestinal canal, wide at first where the gall-ducts^ 

 and pancreatic ducts open, forms a few curvatures, and 

 passes into the great intestine, which ends in a large 

 expansion, the cloaca. This is a pouch in Avhicli the 

 rectum, the ureters, the spermatic ducts in the male, and 

 the oviduct in the female, all terminate. It opens externally 

 by the anus. As a general rule, the intestinal canal is 

 much shorter than in mammals, averaging froiil three to 

 five times the length of the body. Two small coeca are 

 found in the majority of perching birds just at the com- 

 mencement of the great intestines. These are absent 

 entirely in some tiibes, and are of considerable size in 

 others ; and these difterences cannot be satisfactorily ex- 

 plained with reference to the food and habits of the 

 birds. The foeces in most are enveloped in a thin pellicle. 



The liver is large and bilobed, and a gall bladder is 

 present in most birds, but absent in a few tribes. The 

 pancreas is large and the spleen small. The kidneys are 

 laro-e, soft, and convoluted. The urine itself is thick, 

 pultaceous and white, and is, in general, not voided by 

 itself, but mixed with the faeces. Two bodies, called the 

 succenturiate kidneys, are found in most birds under the 

 anterior part of the two kidneys. They are of a yellow 



