132 



THE INTERNAL ENVIRONMENT OF THE BODY 



Unguis 



,Subunguis 

 Unguis (noil) /^^^ ^>>^^ 



A. CARNIVORAN CLAW 

 (Cat) 



Pad- 



Subunguis 



B. HUMAN NAIL 



Unguis 



Pad 



Unguis /Subunguis 



C. HORSES HOOF 



Part III 



-Pad 



Subunguis 



Unguis 



C^ — Pad 



Subunguis 

 Unguis /Subunguis 



Pod 



Fig. 8.5. Diagrams of claws, nails and hoofs seen in section and from beneath. 

 All of these are modified scales, an unguis or scale above and a subunguis or 

 cushion below. Thus, the front of a horse's hoof is a modified nail essentially 

 similar to the claw of a lizard or a human fingernail. ( Redrawn after Walter and 

 Sayles: Biology of the Vertebrates, ed. 2. New York, The Macmillan Co., 1949.) 



actually fused hairs. Lizards, turtles, and birds have claws as do many mam- 

 mals, but nails belong solely to a few mammals. A claw fits like a hood over a 

 terminal joint and beneath it is a pad of softer tissue. A nail is a thin horny 

 plate growing on the upper side of the end of a finger or toe. The human 

 fingernail is like a broad flattened claw on the upper surface of the fingertip. 

 None of these structures is molted but broken nails are regenerated. The hoof 

 of a horse is a claw which has become a greatly thickened sheath for the 

 toe-tip. 



Horns and Antlers. The horns of cattle, sheep, goats, and Old World ante- 

 lopes are outgrowths of bone covered by thick layers of cornified epidermis 

 and, like claws and nails, are tough and resistant to chemicals. Horns are not 

 shed and are never branched. 



The antlers of deer, reindeer, moose, and elk are annual growths of bone. 

 Deer shed their antlers when they are about two years old and every year after 

 that. At first the bony outgrowth is covered with hairy skin, later the skin is 

 resorbed and the spike of bone breaks off. In the second year the antler de- 

 velops in the same way, is shed, and in each following year the process is 

 repeated with new branches added (Fig. 8.6). Growing antlers are said to be 

 "in the velvet" because their skin is thickly covered with short hairs. They are 

 hot and feverish to the touch due to the large blood supply and the almost 

 explosive expenditure of heat in their rapid growth. Giraffes, which are close 

 relatives of the deer family, do not shed their stubby antlers, that remain in the 



