584 VETERINARY PHYSIOLOGY 



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 (p. 269). When poured out, sweat evapor- 

 ates, and in doing so causes loss of heat. 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 when it evaporates 

 causes loss of heat. Hence the importance of grooming after 

 exercise. In the horse the salts of evaporated sweat may 

 accumulate on the coat if evaporation is allowed. 



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. The secretion of sweat and 

 the condition of the blood vessels play an important part in 

 regulating the temperature of the body (p. 269). 



3. Nervous Mechanism of Sweat Secretion. — That the sweat 

 glands are under the control of the central nervous system 

 may be demonstrated in the cat. The sweat glands are chiefly 

 in the pads of the feet, and, 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 stimu- 

 lated, a secretion of sweat is produced. The secreting fibres 

 for the sweat glands run in the true sympathetic system. They 

 leave the cord by the anterior roots in the thoracico-abdominal 

 region, pass to the sympathetic ganglia, where they have their 

 cell stations. From these, non-medullated fibres pass back by 

 the grey ramus into the somatic branch of the nerve and so 

 onwards to plexuses round the sweat glands. 



The centres presiding over these nerves are distributed 

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

 stimulation, as when pepper is taken into the mouth ; and 

 (6) of direct stimulation (i.) by a venous condition of the blood, 

 as in the impaired oxygenation of the blood which so fre- 

 quently precedes death as the respirations fail, and (ii.) by a rise 

 in the temperature of the blood supplied to them. 



Even after the nerves of the sweat glands are cut, the 

 glands may be stimulated by certain drugs, e.g. pilocarpine. 



