CHAPTER XVII. 
ANATOMY AND PHYSIOLOGY OF THE FOWL. 
Birps, considered as a great class of animals, are one of the 
most clearly defined classes in the animal kingdom. There is a 
great variety of types, representing different forms, sizes, and 
habits, yet in general their structure is very similar. Birds are 
more closely allied to the reptile group than to the mammals. 
There are over eight thousand species in the class. 
The fowl is a warm-blooded, air-breathing, egg-laying, feathered 
vertebrate, with four limbs. Those in front are for flying (which use 
has been greatly diminished during domestication), while the hind 
ones are for walking, scratching, perching, and swimming. Since 
the fowl is a true bird and is organized for flight, it possesses an ex- 
ceedingly light skeleton. Its muscular tissue is capable of great con- 
traction, and its respiratory system is exceedingly well developed. 
The Bird’s Skeleton.—There are four marked points in which 
the bird’s skeleton differs materially from mammals: 
1. The extreme lightness of the bones, making flight possible. 
2. The tendency of bones to fuse, thus giving greater rigidity 
to the body proper. 
3. The adaptation of the limbs to allow of walking as well as 
flight. 
4, The great length of the spinal column, especially the neck, 
this being as long as the rest of the column combined. 
The bird’s skeleton (Fig. 141) is very compact, much more so 
than that of mammals, and the bones are harder and whiter. It 
contains fewer bones than the mammal, many being grown to- 
gether to form one, as the skull bones, the dorsal vertebra, and 
the tarsal and metatarsal bones. The neck is very long and flex- 
ible, giving the bird great latitude in movements of the head. The 
ribs are joined in the middle as well as to the backbone and sternum, 
the latter being highly developed to furnish the seat or location 
of the muscles used in flight. 
Wings.—Parts corresponding to the hands and forearms of the 
human skeleton are found in the wings of the bird, and from them 
are developed respectively the primary and secondary wing feathers. 
The wings are carried folded on the back. 
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