EXCRETION OF MATTEE FEOM THE BODY 429 



with ducts opening on the surface of the skin. The secreting 

 epithelium somewhat resembles that of the convoluted tubules 

 of the kidney. It has been calculated that a man possesses 

 about two and a half million sweat glands, and that if spread 

 out they would present a surface of very great extent. 



2. Functions. 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, 

 evaporation 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. 



3. 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. 

 Those to the hind leg come from the upper lumbar region, 

 those for the forelimb from the lower cervical nerves, and 

 those for the head in the sympathetic of the neck and 

 partly in the fifth cranial nerve. 



The centres presiding over these nerves are distributed down 

 the medulla and cord. They are capable of (a) reflex stimula- 

 tion, as when pepper is taken into the mouth ; and (b) of direct 

 stimulation by a venous condition of the blood, as in the 

 impaired oxygenation of the blood which so frequently precedes 

 death as the respirations fail. 



But even after the nerves to the sweat glands are cut, the 



