142 VEKTEBRATA: AYES OR BIRDS. 



small intestine by lymphatic vessels which end in two 

 thoracic ducts that open into the jugular veins on each 

 side of the base of the neck. 



The circulatory system of Birds is essentially the same 

 as that of Mammals. The blood is hotter than that of 

 any other vertebrates, and is rich in corpuscles, and the 

 corpuscles are elliptic in shape, and nucleated (Figs. 36 

 and 37, page 36.) Birds have only a single aortic arch, 

 namely, the right. 



The respiratory system of Birds is extensive, and varies 

 considerably from that of the Mammals. The cavity of 

 the chest is not limited by a diaphragm, and the lungs are 

 not suspended as free sacs in the cavity of the trunk, but 

 are attached in the form of flattened spongy masses to the 

 posterior side of the thorax, reaching even to the pelvis. 

 Air-sacs on the surface of the lungs and in various parts 

 of the body, and even the bones themselves, which are 

 hollow, are in communication with the lungs, and thus 

 greatly enlarge the means of respiration, as well as di- 

 minish the specific gravity of the animal, thus the better 

 adapting it for locomotion through the air. So completely 

 are the bones in communication with the lungs, that it is 

 said, that if the windpipe be tied, and the humerus be cut 

 off, and the end left exposed, the bird can inspire and ex- 

 pire through the opening in the end of the bone! 



The motatory apparatus of Birds, as in Mammals, con- 

 sists of a skeleton, muscles, tendons, and ligaments. 



As regards the skull of Birds, it is articulated with the 



O ' 



first vertebra of the neck by only one occipital condyle, as 

 in Reptiles; and as in the latter class, each ramus or 

 branch of the lower jaw is composed of several pieces, 

 and is connected with the skull bj- what is called the os 

 quadratum or quadrate bone. 



