THE SKIN 165. 



the size of the blood-vessels. At the same time the ducts of the 

 sweat glands are contracted, and consequently the secretion of 

 the perspiration is slow. But when the body becomes overheated 

 by exercise, or when the surface of the body is exposed to great 

 heat, the muscles of the skin relax, the supply of blood increases, 

 the gland ducts open wider, and the perspiration is given off freely. 

 The evaporation of this moisture tends to reduce the temperature 

 of the skin, and consequently that of the body. 



We now see how necessary it is that the skin should be 

 cleansed from any matter which may tend to clog its pores, for if 

 the perspiration cannot be freely discharged through the pores, it 

 is reabsorbed into the blood, thus throwing extra work on the 

 other excretory organs, especially the kidneys, which excrete water 

 and urea. The lungs, skin, and kidneys are all similar in their 

 action ; but the work of the skin is more closely allied to that 

 of the kidneys than to that of the lungs. Hence, when the action 

 of the skin is retarded by exposure to cold, the extra work thrown 

 on the kidneys frequently causes a disease in these organs, they 

 being unable to perform their own function in addition to that of 

 the skin. The analogy between the functions of the skin and the 

 kidneys may also be noticed from the fact that in summer, when the 

 skin is active, the excretion of the kidneys diminishes ; and vice versa. 



The sudoriparous glands are distributed all over the body, but 

 not equally. In the lower limbs and back they are estimated at 

 about 600 to every square inch of surface, while in the palms of 

 the hands there areas many as 2,500 in the square inch. 



Generally speaking, the fluid portion of the sweat evaporates as 

 fast as it reaches the surface of the body, and consequently we 

 have no visible evidence of the excretion unless we apply the skin 

 to a very cold surface, when the vapour given off is condensed. 

 Hence the term insensible perspiration. But in summer, 

 especially during activity, the fluid collects in globules on the skin, 

 and the perspiration becomes sensible. It may also become 

 sensible under the influence of some kinds of mental emotion. 



Nails and hairs are out-growths of the epidermis or cuticle, 

 and both agree with it in general structure. 



A nail is simply a thick layer composed of the thin horny 

 scales of the outer portion of the cuticle. At the ' root ' of the 

 nail the skin is folded back on itself, so that two layers are facing 

 each other. Thus the nail at this point grows by additions of 

 epidermic cells above, below, and behind ; the new growth above 

 and below adding to the thickness of the nail, while additions 

 received behind continually push it forward. The nail continues 

 to receive additions in its thickness on the under surface as long 

 as it remains attached to the skin below it. 



