252 THE TISSUES. 



harder, and more phosphate of lime is deposited in them or within 

 the cells. 2. Like the horny cells of the epidermis, they become 

 flattened and increase longitudinally and transversely. 3. They 

 coalesce more completely, so that they cannot be separately recog- 

 nized. But their nuclei do not disappear as do those of the epider- 

 mis; and herein is a characteristic distinction. 



The nails are constantly suffering loss from friction and other 

 causes. Much of the matter accumulating under them consists of 

 epithelial cells. 



The development of the nails commences in the third month of 

 intra-uterine life ; they not being at first distinguishable from a soft 

 epidermis. The ridges of the bed of the nail are well marked at 

 the end of the fourth month. They cover the whole bed, and have 

 assumed the consistency of a nail at five months, and reach the 

 extremities of the finger at eight months. 



The free edge of the nail of the new-born infant is cast off once 

 at least (Weber says many times), sdon after birth; probably from 

 external violence which it is too delicate to resist. This free edge 

 appears to be a nail of an earlier period, probably of about the 

 sixth month, which has been thrust forward in the course of deve- 

 lopment. Six or seven months after birth the first set of nails is 

 completely replaced by new ones (Kolliker) ; and at two or three 

 years the horny layer is not distinguishable in appearance from that 

 of the adult. 



Nails, when destroyed, are almost always imperfectly regenerated, 

 on account of injury done to the laminae and vessels. A rudi- 

 mentary nail sometimes appears on the second phalanx of a finger 

 in case of loss of the first. In some rare cases, a periodical loss 

 and regeneration of the nails occurs. 



The hoofs and claws of the lower animals are the analogues of 

 the nails, both physiologically and histologically. 



Uses of the Nails. The nails support the pulp of the fingers and 

 toes, and thus conduce to the perfection of touch. They also in- 

 crease the power of the fingers as prehensile organs ; and in a state 

 of nature at least, (i. e. if remaining uncut), they become not ineffi- 

 cient means of attack and defence. 



Pathological States of the Nails. 



Any abnormal condition of the bed of the nail will, of course, 

 affect the growth of the latter. In the lamellated nails of old peo- 

 ple, Kolliker found all the capillaries in the anterior segment of the 



