38 GENERAL HISTORY OF BIRDS. 



is developed in which the food temporarily remains to become softened. Dis- 

 tally, the extremity of the oesophagus is dilated to form an enlargement kno^n 

 as the j)roventriculus, and this is followed by a distinct and very muscular 

 gizzard, that takes on a variety of shapes in different families (Fig. 4). In 

 Raptores its muscularity is weak, while in the granivorous Fowls, and others, it 

 is far stronger, except in such birds as Centrocercus, a large North American 

 grouse that lives upon leaves and buds of a prairie shrub {Artemisia). 



The intestine is long, smooth, and of nearly uniform calibre, there being 

 no such distinction as large and small intestine. In its first division it surrounds 

 the pancreas like a frame, after which its behavior and course is markedly 

 different in different groups of birds. Anatomists recently have given con- 

 siderable attention to this, and employed the facts in taxonomy. Birds have 

 two kinds of caeca associated with their abdominal viscera. The umbilical ccecum. 

 is unimportant, while the ones attached to the intestinal tract have, from their 

 great variation, come into effective use in classification. They are absent in 

 Cathartes and the Columhce, while some of the Anseres possess them over a foot 

 in length. 



The globular enlargement of the rectum is termed the cloaca, and is the 

 cavity that contains the essential organs of generation in the two sexes; the 

 openings of the ureters ; and the peculiar anal glands called the hursa Fahricii. 

 In birds, a gall bladder may be present, but many species are without that 

 cyst; the pancreas is present as already stated; and the liver is usually of 

 average size. Hepatic ducts are apt to vary, being governed by the fact of 

 the presence or absence of the gall bladder, and that no ductus communis chol- 

 edochiis ever exists in birds. Three separate ducts may pass from the hepatic 

 apparatus to the intestine; it is the rule when the gall bladder is present. 



THE VASCULAK SYSTEM. 



The vascular system is very perfectly developed in birds. There is an 

 arterial, venous, and lymphatic system, and the hot, red blood circulates with 

 great pei-fection all parts of the body. The heart is four-chambered, there 

 being an auricle and ventricle upon either side, and they communicate. 

 Further, the usual pulmonary circulation is found, as well as the hepatic portal 

 system, but there is no renal portal system in this group of vertebrates. 

 Morphologically, the carotid arteries are extremely interesting, and the re- 

 markable variations they are subject to in different groups of birds has led to 

 their being employed with advantage in classification, as it has been found 

 that these variations correspond with marked certainty to the natural divisions 

 into which birds normally fall. 



The lymphatic system presents for examination but few lymphatic glands 

 beyond the spleen which lies upon the right hand side of the proventriculus; 



