THE SKIN 309 



function it is to determine the secretion of sweat, and these are 

 quite distinct from those which regulate the vascular supply. 

 If the peripheral end of the divided sciatic in the cat be stimulated, 

 the foot-pads sweat ; the proof that this reaction is a specifically 

 nervous one is easy, apart from the fact that stimulation of the 

 sciatic causes a violent constriction of the bloodvessels in the 

 leg, for the sweating occurs when the leg has been cut off or the 

 aorta tied, and it is absent under the influence of atropine. The 

 effect of atropine on the sweat glands is very closely allied to its 

 action on the salivary glands (p. 169) ; it paralyses the secretory 

 nerves which produce sweat. 



As with the salivary glands, so in the present case secretion 

 is not due to any increased supply of blood. It is true that in 

 normal sweating, as is so readily seen in man, the skin is flushed 

 as the increased secretion takes place, but the increased blood- 

 supply which the flushing indicates is merely the necessary 

 adjuvant, not the cause of the secretion ; it supplies the glands^ 

 with the extra material they now require, the secretory nerves 

 causing the gland-cells to utilise the increased supply. 



The secretion of sweat may be induced in man, the cat, and 

 the dog, though not in the horse, by the injection of pilocarpine. 

 In this case the action is peripheral — that is to say, on the glands 

 themselves — since it occurs when the sciatic nerves are cut pre- 

 viously to the injection. 



As we have seen, secretion is ordinarily brought about by 

 specific efferent nerves, and these originate in the central nervous 

 system, from which the necessary secretory impulses are directly 

 supplied. But secretion may also be readily induced by the 

 stimulation of afferent nerves, as in the all-important case of 

 a rise in the surrounding temperature. These facts lead at once 

 to the belief that ' sweat centres ' must exist in the central 

 nervous system comparable to those of the respiratory and 

 vascular mechanisms, though they have not as yet been so 

 definitely localised. There seems to be no doubt that the spinal 

 cord contains sweat centres. The existence of a similar centre 

 in the medulla is less certain, though probable, since in some 

 men perspiration over the face and neck results from merely 

 smelling a pungent substance, such as curry-powder, and becomes 

 profuse if the latter is introduced into the mouth. 



The sweat-nerve supply to the fore and hind limbs leaves the 

 spinal cord in company with the inferior roots of the spinal 

 nerves, and by means of the rami communicantes passes to the 

 sympathetic ganglia, and by post-ganglionic fibres reaches the 

 brachial and sciatic plexuses respectively. The sweat fibres 

 for the head and neck are in the cervical sympathetic ; those 

 lor the face in the horse, the muzzle in the ox, the snout in the 



