162 VERTEBRATE ZOOLOGY 



The body may be divided into four regions,— the head, neck, 

 trunk, and tail, all of which are specialized for flight. 



The Head. This body division articulates with the vertebral 

 column by a single condyle and consequently has great range of 

 movement. It has a high, arched cranial or posterior portion and 

 is prolonged anteriorly to form the conical beak, which is made 

 up of the upper and lower jaws and is the organ of prehension. 

 Note that the upper jaw possesses considerable flexibility and is 

 not so rigidly joined with the cranium as it is in mammals and 

 in the turtle. The mouth is large and is without lips. The nostrils, 

 or external nares, are a pair of slits near the base of the upper beak 

 and just in front of a swollen area called the cere. The eyes are 

 large and round, and each has three lids, an upper and a lower lid, 

 —both movable, the latter more so than the former,— and a nicti- 

 tating membrane, a third lid. This membrane is situated at the 

 anterior corner of the eye, where it is entirely concealed when not 

 in use, and moves back and forth. With fine forceps pull it over 

 the eye. The opening of the ear is just behind the eye and leads 

 to the tympanic cavity ; note the feathers which guard the open- 

 ing. There is no external ear. 



The head of all modern birds is remarkable in that it contains 

 no teeth and no strong masticatory muscles ; a considerable weight 

 is thus spared it. This is a feature which is also correlated with 

 the great powers of flight which most birds possess. Flying is one 

 of the most difficult and highly developed methods of locomotion 

 among animals, and makes necessary a careful adjustment of the 

 weights to be carried. It is very much to the advantage of a flying 

 vertebrate that the heavier portions of the body be brought as 

 near the center of gravity as possible, and that the various organs 

 be lightly constructed. 



Thus the teeth, which are composed of the densest and one of 

 the heaviest tissues of the body, have entirely disappeared in the 

 course of the evolution of the modern bird. But a grain-eating bird 

 like the pigeon must chew its food, because only in a finely re- 

 duced condition is the food digested quickly enough to furnish the 

 needed amount of heat and energy. The function of chewing, 

 however, instead of being done in the mouth,— as it is in the mam- 



