454 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



additions to the bottom, pushing outward, not by addi- 

 tions to the top like a tree. The follicle in which it is 

 formed acts as a mold, determining its shape. The 

 quills of the porcupine are only large hairs. 



Nails. — If we tear off a finger nail, we find beneath 

 an exquisitely tender surface of dermis covered with a 

 layer of very active epithelium. At the base of the nail 

 is a pocketlike infolding, where the epithelium is espe- 

 cially active. Over the whole surface the epithelial cells 

 harden into horn, but this process is especially active 

 in the pocket (Fig. 309). Therefore the nail, by its 

 more rapid formation in the pocket at its base, is pushed 

 forward continuously, and, if not worn away or pared 

 away, will grow indefinitely. In some countries, as in 



cX-f 



Fig. 309. — Section of the end of the 

 finger, showing how the nail is 

 formed : c, cuticle ; ml, Malpighi- 

 an layer ; he, the horny cuticle ; 

 b, the bone. 



Fig. 310. — Section through terminal 

 joint : clcr, claw core ; kc, horny 

 cuticle ; ml, Malpighian layer ; 

 c, unhardened cuticle. 



Japan, they are sometimes protected and become enor- 

 mously long, as a badge of a leisure class. 



Claws. — These differ from nails only in the fact that 

 they grow on all sides instead of one side of a peculiarly 

 shaped terminal joint of a finger or toe. The bone of 

 the terminal joint is the claw core and determines the 

 shape of the claw. The core is covered with dermis and 

 with an active layer of epithelium, which hardens into 



