256 THE SOLIDS. 



Epithelium, epidermis, nails, and some other structures of 

 the body, never seem to attain to a stationary state ; they are 

 throughout the whole of life undergoing a process of deve- 

 lopment ; this in the case of the nails of man renders the 

 interference of art necessary to remove from time to time 

 the redundant growth. 



It is probable, however, that if the nails were not cut, but 

 allowed to grow at will, they would not exceed a certain 

 length ; amongst the Chinese, who do not pare their nails, 

 they are usually about two inches long. 



Nails doubtless sustain a loss of substance beyond that which 

 they experience from occasional cutting, as from friction and 

 the desquamation of the cells from the inferior and anterior 

 portion of each nail, and which may be inferred to take place 

 from the fact that the matter which accumulates beneath the 

 extremities of the nails is to a great extent made up of epi- 

 dermic or nail cells. 



It has been estimated that the entire body of a nail, from 

 the root to its free margin, is developed in from two to 

 three months. 



The third month of intra-uterine life is the earliest period 

 at which the nails can be detected; they then consist of nu- 

 cleated cells, and rather resemble soft epidermis than the 

 hard and horny texture of fully developed nails. 



A nail which has been once entirely destroyed is always 

 regenerated in a very imperfect manner, it being usually 

 seamy and irregular in consequence of the disturbance and 

 injury sustained by the subjacent dermis, and the markings 

 of which are impressed upon the nail. 



Nails are subject to deformity in certain chronic maladies 

 of the heart and lungs, especially in cyanosis and phthisis. 

 It has been suggested that this may depend upon the state 

 of the circulation in the vessels of the dermis. 



The various modifications of nail met with throughout the 

 animal kingdom, the claws of birds and carnivora, the hoofs 

 and horns of ruminants, have essentially a similar structure 

 to the nails of man. The hoof of the horse and of some 

 other animals is traversed from above downwards by the 

 spiral ducts of the sebaceous glands. 



