\v THE SKIN 171 



perspiration is called insensible perspiration, because it is 

 not evident to the senses. In hot weather, or in consequence 

 of exertion, the perspiration is poured out faster than it can 

 evaporate and collects in drops. Such perspiration is called 

 sensible perspiration. As the perspiration evaporates, 

 water is changed from the liquid to the gaseous condition. In 

 this change heat is absorbed by the water, and the surface from 

 which the evaporation takes place is cooled by the heat thus 

 taken from it. IJy the perspiration a large quantity of heat is in 

 this way lost from the body. Some heat is also lost by radia- 

 tion, and some by conduction or contact with colder bodies, but 

 the heat absorbed by the evaporation of the sweat is the most 

 important means of loss. The skin is a source of loss to the body 

 of water, of heat, and of a small quantity of carbonic acid. The 

 amount of water lost varies very greatly, but is usually nearly 

 a pint in twenty-four hours. The amount of sweat secreted, 

 and so of heat and water lost, depends on many circumstances. 

 The activity of the sweat glands, like that of other glands, such 

 as the salivary, is influenced by the nervous system. The 

 sweat glands are supplied by nerves, and certain nervous im- 

 pulses passing from the brain or spinal cord along these nerves 

 cause the glands to pour out sweat abundantly. Usually, not 

 always, the activity of the glands is assisted by a greater flow 

 of blood to the skin brought about also by the nervous system 

 through the vaso-motor nerves. The two usually go hand in 

 hand ; an emotion of shame which leads to blushing leads 

 also to profuse perspiration of the face. Hut an increased flow 

 of blood to the skin is not the cause of sweating, and, indeed, 

 is not necessary for sweating ; in what are called " cold sweats " 

 the glands secrete freely though the supply of blood to the skin 

 is very small. In hot weather the blood-vessels of the skin are 

 dilated, and so it receives more blood, and at the same time 

 the secretion of sweat is more active. The object of the in- 

 creased perspiration is to remove heat from the body, so as to 

 prevent the body from becoming hotter than usual. In cold 

 weather the vessels of the skin are constricted, and so it 

 receives less blood, and at the same time the secretion of 

 sweat, and thereby the loss of heat, is reduced. 



Nails. A nail consists of rpidi-rmis and is formed by the 

 outer or corneous cells, instead of being shed as in other parts 



