HANDBOOK OF PHYSIOLOGY. 



of the corium, or true skin, by which the nail is secreted is called the 

 matrix. 



The back edge of the nail, or the root as it is termed, is received into 

 a shallow crescentic groove in the matrix, while the front part is free 

 and projects beyond the extremity of the digit. The intermediate por- 

 tion of the nail rests by its broad under surface on the front part of the 

 matrix, which is here called the bed of the nail. This part of the matrix 

 is not uniformly smooth on the surface, but is raised in the form of 

 longitudinal and nearly parallel ridges or laminae, on which are moulded 

 the epidermal cells of which the nail is made up. 



The growth of the nail, like that of the hair, or of the epidermis gen- 

 erally, is effected by a constant production of cells from beneath and 

 behind, to take the place of those which are worn or cut away. Inas- 

 much, however, as the posterior edge of the nail, from its being lodged in 

 a groove of the skin, cannot grow backward, on additions being made 

 to it, so easily as it can pass in the opposite direction, any growth at its 

 hinder part pushes the whole forward. At the same time fresh cells 

 are added to its under surface, and thus each portion of the nail becomes 

 gradually thicker as it moves to the front, until, projecting beyond the 

 surface of the matrix, it can receive no fresh addition from beneath, and 

 is simply moved forward by the growth at its root, to be at last worn 

 away or cut off. 



FUNCTIONS OF THE SKIN. 



The function of the skin to be considered in this chapter is that of 

 the excretion of the sweat. The fluid secreted by the sweat-glands is 

 usually formed so gradually that the watery portion of it escapes by 

 evaporation as fast as it reaches the surface. But during strong exer- 

 cise, exposure to great external warmth, in some diseases, and when 

 evaporation is prevented, the secretion becomes more sensible, and col- 

 lects on the skin in the form of drops of fluid. 



The perspiration, as the term is sometimes employed in physiology, 

 includes all that portion of the secretions and exudations from the skin 

 which passes off by evaporation; the sweat includes that which may be 

 collected only in drops of fluid on the surface of the skin. The two 

 terms are, however, most often used synonymously; and for distinction, 

 the former is called insensible perspiration; the latter, sensible perspira- 

 tion. The fluids are the same, except that the sweat is commonly mingled 

 with various substances lying on the surface of the skin. The contents 

 of the sweat are, in part, matters capable of assuming the form of vapor, 

 such as carbonic acid and water, and in part, other matters which are 

 deposited on the skin, and mixed with the sebaceous secretions. 



The secretion of the sebaceous glands and hair-follicles consists of 

 cast-off epithelium cells, with nuclei and granules., together with an oily 



