20 



REMARKS ON THE STRUCTURE OF BIRDS, 



Birds constitute the second class of Vertebrate Animals, and 

 are characterized by having an internal skeleton ; a twofold, 

 that is, a general, and a pulmonic, circulation ; as well as a 

 double respiration, the air passing beyond their lungs into cells 

 distributed through various parts of the body, and even into 

 some of the bones ; by being warm-blooded, oviparous, fur- 

 nished with four extremities or limbs, of which the two 

 anterior are converted into wing^s ; and bv havino: their skin 

 covered with feathers. The last circumstance is peculiarly 

 distinctive of the class. They never approach in magnitude 

 to many s}:)ecies of Mammalia, Reptiles, and Fishes; and, 

 being organized for flying, are proportionally lighter than 

 animals of equal dimensions belonging to those classes. Their 

 specific gravity, in fact, is less than that of water, so that they 

 all float on the surface of that element, and many of them are 

 adapted for swimming upon it, or for plunging or diving into 

 it. By far the greater number, however, are confined to the 

 air, most of them obtaining their food by walking on the 

 ground or on trees, while a few procure their prey on w^ing, 

 and are incapable of walking without difficulty. 



Although it is unnecessary to enter here into a minute ex- 

 planation of all the organs of birds, I consider it exj)edient to 

 describe as much of their structure as may enable the reader 

 not previously acquainted with their anatomy to follow me 

 through the various details which I shall have to present. I 

 will therefore give an outline of the osseous, digestive, and 

 dermal systems, accompanied by illustrative figures. 



The Skeleton of birds is composed nearly of the same parts as 

 that of the Mammalia, but with modifications suited to the 

 peculiarities of their kind of life. As in man, it is divided 

 into the head, the neck, the trunk, the anterior, and the pos- 

 terior extremities. 



The Head, which is generally small, is at an early age com- 



