57 



AVES. 



Birds are oviparous Vertebrata with a double circulation 

 and respiration, organized for flight. 



The lungs, which are not separated from the abdomen by 

 a perfect diaphragm, are undivided, fixed to the ribs, and sur- 

 rounded by a membrane pierced with large holes, so that the 

 air passes into the cavity of the chest, the abdomen, the arm- 

 pits and even into the cavities of the bones, which are hollow. 



Their anterior extremities cannot serve either for standing 

 or prehension ; they are consequently bipeds, and take their 

 food by means of the mouth. The two clavicles are joined 

 together and form what is called the fourchette. This, with 

 the projections of the scapula, keeps the shoulders asunder. 

 The wing supported by the humerus, the fore arm and the 

 enlarged hand, has throughout its length a range of elastic 

 feathers which offer a large surface of resistance to the air. 

 The feathers of the hand, always ten in number, are called 

 primaries ; of the fore arm, varying in number, secondaries ; 

 of the humerus, which are less strong, scapularies ; those of 

 the thumb bastards. The bony tail is very short, but sup- 

 ports a range of strong feathers from twelve to eighteen in 

 number. The weaker feathers ranged along the base of the 

 * quills are called the wing and tail coverts. 



The legs have a femur, a tibia and fibula, a tarsus and 

 metatarsus represented by a single bone terminated by three 

 pullies and to which are attached commonly three toes and a 

 thumb. There are also certain muscles attached to the pel- 

 vis and passing over the knee and heel to the toes, so that 

 the weight of the body suffices to flex them and thus allows 

 the bird to sleep perched upon its feet. 



H 



