STRUCTURE OF THE VERTEBRATES 



121 



Horn steers of the western plains were almost an end product 

 in horn development, some of them measming eleven feet from 

 tip to tip. After death the core shrinks and the epidermal cover 

 is easily removed. The prong-horn antelope (Antilocapra) is an 

 exception. This animal has a tj'pical bony core in the horn, but 

 the ectodermal cover is periodically shed. 



Fig. 58. Prong-horn Antelope (Antilocapra). An 

 American artiodactyl with horns apparently inter- 

 mediate between true horns and antlers. The bony 

 core is permanent, but the ectodermal cover is shed 

 annually. The diagram shows the horn at time of 

 shedding, with the skin surrounding the core. 



Antlers, on the other hand, are shed each year and replaced 

 with larger ones until the deer, moose or elk approaches senility. 

 The antlers are lost in the early spring and the new ones begin 

 their development soon after. Bone building cells congregate at 

 the stub of the shed antler and growth begins. The food supply 

 is gained from the vessels of the skin which is carried out over 

 the growing antler. When full size is reached the skin constricts 

 at the base of the antler, dies, and is rubbed off. The antler, 

 consequently, is bone, and logically has its place with the skeletal 

 svstem. 



Fig. 59. Growth and Structure of Antlers. During the growth season the 

 antler is covered with skin which is shed when full size has been reached. 

 The antlers are shed annually, and the skin covers the growth base on 

 the frontal bone (D). The growth process repeats itself during the spring 

 and earlv summer. 



