302 THE VERTEBRATE ANIMALS 



This is one of the greatest adaptations to the active life led by a 

 bird. Man breathes from twelve to fourteen times per minute. 

 Birds breathe from twenty to sixty times a minute. The lungs are 

 not large, the bronchial tubes being continued through the lungs 

 into hollow spaces filled with air, which are found between the 

 organs of the body. Only the lungs, however, are used for breath- 

 ing. Because of the increased activity of a bird, there conies a 

 necessity for a greater and more rapid supply of oxygen, an increased 

 blood supply to carry the material to be used up in the release of 

 energy, and a means of rapid excretion of the wastes resulting from 

 the process of oxidation. A bird may be compared to a high-pres- 

 sure steam engine ; in order to release the energy which it uses in 

 flight, a large quantity of fuel which will oxidize quickly must be 

 used. Birds are large eaters, and the digestive tract is fitted to 

 digest the food quickly and to release the energy when needed, by 

 having a large crop in which food may be stored in a much softened 

 condition. As soon as the food is part of the blood, it may be sent 

 rapidly to the places where it is needed, by means of the large four- 

 chambered heart and large blood vessels. 



The high temperature of the bird is a direct result of this rapid 

 oxidation; furthermore, the feathers and the oily skin form an 

 insulation which does not readily permit of the escape of heat. 

 This insulating cover is of much use to the bird in its flights at 

 high altitudes, where the temperature is often very low. 



The Nervous System and the Senses. The central nervous system 

 is well developed. A large forebrain is found, which, according to a series 

 of elaborate experiments with pigeons, is found to have to do with the 

 conscious life of the bird. The cerebellum takes care of the acts which are 

 purely mechanical. 



Sight is probably the best developed of the senses of a bird. The keen- 

 ness of vision of a hawk is proverbial. It has been noticed that in a bird 

 which hunts its prey at night, the eyes look toward the front of the face. 

 In a bird which is hunted, as in the dove, the eyes are placed at the side 

 of the head. In the case of the woodcock, which feeds at night in the 

 marshes, and which is in constant danger from attack by owls, the eyes 

 have come to lie far back on the top of the head. Hearing is also well 

 developed in most birds; this fact may be demonstrated with any 

 canary. 



The sense of smell does not appear to be well developed in any bird, and 

 is especially deficient in seed-eating birds. 



