THE BIRDS 267 



bee-eaters, rollers and woodpeckers, have curiously formed feet, 

 somewhat resembling the feet of a chameleon ; perching birds have 

 the hind toe long, making the foot a convenient grasping organ ; 

 and in water birds the digits are united by a web. The arrange- 

 ment and length of the digits vary considerably in different kinds 

 of birds. 



Many birds have the tongue but feebly developed, in others 

 it is encased in horn ; some, however, the woodpecker for instance, 

 have a powerful, well- developed tongue, which is used for seizing 

 insect prey, and is darted out with great rapidity. On the other 

 hand, the pelicans are quite tongueless. 



Birds, like all warm-blooded creatures, have the heart divided 

 into two separate halves, each of which is again divided, forming 

 altogether four cavities two upper chambers, or auricles, and 

 two lower chambers, or ventricles; 



The respiration of birds differs from that of other vertebrates. 

 The lungs are not freely suspended, but are attached to the roof of 

 the body-cavity, and the air is not drawn into the lungs and again 

 expelled, but drawn through the lungs, as it were, into the air- 

 sacs membranous chambers of which there are five pairs attached 

 to the upper part of the body-cavity. In these air sacs the air is 

 stored not only for purposes of respiration, but to regulate the 

 temperature and to serve as a reserve supply to increase the volume 

 of sound during long-sustained singing. Some birds, in addition, 

 possess curious air-pouches in the neck which can be inflated or 

 deflated at will. These, as they are possessed by the male bird 

 only, must be regarded as sexual adornments. The adjutant 

 storks, the emus, and the bustards are distinguished in this way. 

 Not only are birds provided with these numerous air-sacs, but 

 even the bones of the majority are hollow and act as receptacles 

 for air, so that a bird is practically clothed with air ; yet, strangely 

 enough, this pneumaticity does not appear to be of any particular 

 advantage to the bird in its flight, as some of the poorest fliers 

 are also the most permeated with air. 



The digestive system of birds is peculiar. Most birds swallow 

 their food without any attempt at mastication, and it passes into 

 the gullet, which is dilated into a crop. Here the food is stored 

 for awhile and softened by means of water and saliva in conjunc- 

 tion with the body-heat; It then passes on into the stomach, which 



