EXCRETION OF MATTER FROM THE BODY 409 



(2) The Sensory functions have been studied under the 

 Special Senses (p. 98 et seq.). 



(3) The Respiratory action of the skin in mammals is of 

 little importance. 



(4) The Excretory Function of the Skin. Three sets of 

 glands develop in the skin sweat glands and sebaceous 

 glands, which are common to both sexes and are constantly 

 active and mammary glands, which are active in the female 

 during the period of suckling. 



A. Sweat Secretion 1. Sweating. The simple tubular 

 sweat glands are exceedingly numerous. It has been calcu- 

 lated that a man possesses about two and a half million, and 

 that if spread out they would present a surface of very great 

 extent. 



From these glands a considerable amount of sweat is 

 poured out, but to form any estimate of the daily amount 

 is no easy matter, since it varies so greatly under different 

 conditions. Probably about 1000 c.cm. is an average amount. 

 When poured out, sweat usually evaporates, and is then called 

 insensible perspiration, but when large quantities are formed, 

 or when, from coldness of the surface, or of the air, or from 

 the large quantity of watery vapour already in the air, eva- 

 poration is prevented, it accumulates, and is called sensible 

 perspiration. 



A free secretion of sweat is usually accompanied by a 

 dilatation of the blood vessels of the skin, but this may be 

 absent, and it may occur without any sweat secretion e.g. 

 under the influence of atropine. 



2. Nervous Mechanism of Sweat Secretion The sweat 

 glands are under the control of the central nervous system. 

 This may be very conveniently studied in the cat, in which 

 animal the sweat glands are chiefly in the pads of the feet. 

 If a cat be put in a hot chamber it sweats on the pads of all 

 its feet. But if one sciatic nerve be cut the foot supplied 

 remains dry. If the cat be placed in a warm place and the 

 lower end of the cut sciatic stimulated, a secretion of sweat 

 is produced. These sweat-secreting fibres all pass through 

 the sympathetic ganglia, and back into the spinal nerves. 



