CLASSIFICATION OF THE ANIMAL KINGDOM. 241 



system, is the elevated temperature of the body, and the heat of the 

 circulating fluids. The skeletons of the animals which belong to this class 

 being formed for flight, of course differ widely in their mechanical construction 

 from those we have yet considered. The bones throughout are eminently 

 fitted for the purposes for which they are intended, uniting the greatest 

 strength and firmness with the lightness indispensable for the due exercise 

 of their powers. The muscular system is also necessarily very highly 

 developed, and far exceeds that of all the classes we have hitherto considered, 

 with the exception of insects, which are also adapted to flight. 



The mouth consists of a variously shaped beak, in no instance provided 

 with teeth, enclosing a stiff bony tongue, which is used for widely different 

 purposes in different species. The oesophagus is provided, in most birds, 

 with a crop for holding the food previous to its entering the stomach, and 

 a second dilatation, called the bulbus glandulosus, situate just at the 

 commencement of the gizzard for the gastric juice. The gizzard itself is 

 of immense strength, and its crushing powers are assisted in some birds 

 by small pebbles which they swallow for the purpose. The length of the 

 intestinal canal varies considerably in different species, and its division into 

 small and large intestines is not very distinct. The auxiliary secreting 

 glands consist of the salivary, placed immediately beneath the mucous 

 membrane of the mouth, the gastric already spoken of, the liver and pancreas, 

 which are of a considerable size, and the spleen, which is always very 

 small. This class also possesses a peculiar external gland for secreting oil 

 for lubricating the feathers, which is especially useful to those species that 

 inhabit the water. The respiratory and circulatory systems are much more 

 highly developed than in any class we have yet considered. 



The heart is divided into four distinct cavities, two auricles, and two 

 ventricles, one of the latter transmitting venous blood to the lungs, while 

 the other drives the arterialized portion to all parts of the body, the 

 atmospheric air being not only confined to the lungs, but resembling somewhat 

 the action of the tracheal tubes in insects, it permeates all the interior of 

 the body, and penetrates even into the bones. The lungs are no longer 

 confined in closed bags as in reptiles, but resemble spongy vascular masses, 

 which are bound down to the dorsal aspect of the thorax, and are incapable 

 of alternate dilatation and contraction, the air being distributed through 

 the substance by the introduction of innumerable branchial tubes, the main 

 trunks of which passing through the lungs, open by wide mouths into the 

 thorax, whence the air is transmitted to all parts of the body, an arrangement 

 wonderfully adapted to the requirements of the feathered tribes, as by the 

 abundant supply of pure air, the blood is at all times highly oxygenized, 

 the specific weight of the body is considerably lightened, and those that 

 are endowed with the gift of song, are enabled by the same means to 



