THE SKIN AND THE SKIN-GLANDS 1303 



variable. The secretion is under the control of the central nervous 

 system and is almost entirely adapted to the regulation of the body 

 temperature. The nervous mechanism can be set into activity either 

 centrally or reflexly. The most usual factor is a rise of the body 

 temperature. If a man sit in a warm room, e.g. of a Turkish bath, 

 the secretion of sweat commences as soon as the temperature of the 

 body has attained a height of 0-5 to 1 C. above normal. In the case 

 of muscular exercise the temperature will generally be found to be 

 raised if it be taken at the instant that sweating has commenced. 

 The effect of rise of temperature may, however, be either local or central, 

 so that one arm enclosed in a hot-air bath may sweat while the rest 

 of the body is dry. Under ordinary circumstances the central 

 stimulation by the warm blood is the predominant factor. This is 

 shown by an experiment of Luchsinger. In the cat sweating is to be 

 observed only on the hairless pads of the front and hind paws. If one 

 sciatic nerve be cut and the animal be placed in a warm chamber, 

 sweating will commence as the temperature of the animal rises in the 

 three intact paws, while the paw with the nerve cut will be quite dry. 

 Sweating moreover, as has been shown by Kahn, may be induced in 

 the cat's paws by warming the blood passing through the carotid 

 arteries on its way to the brain, at a time when the temperature of the 

 blood circulating through the rest of the body, including the paws 

 themselves, has undergone no alteration. Sweating may also be 

 aroused by asphyxia, and this result is found even in the spinal cat, 

 i.e. after separation of the spinal centres from the medulla. The 

 secretion of sweat resulting from stimulation of the sweat-nerves, 

 although generally associated with increased vascularity of the skin, 

 is not in any way dependent thereon. Thus even in the amputated 

 limb stimulation of the sciatic nerve may cause the appearance of 

 drops of sweat on the pad of the foot. If the sciatic nerve be stimu- 

 lated in the intact animal, the secretion of sweat which is produced 

 is associated with constriction of the vessels of the skin, due to simul- 

 taneous stimulation of the vaso-constrictor nerves running in the 

 sciatic nerve. 



As Langley has shown, the sweat-nerves run entirely in the sympa- 

 thetic system. Leaving the cord by the white rami communicantes 

 from the second dorsal to the third or fourth lumbar nerves, they 

 pass into the sympathetic chain. Here the first relay of fibres ends in 

 connection with the cells of the sympathetic ganglia, and a fresh relay 

 of fibres, which are non-medullated, pass from the cells along the 

 grey rami into the various spinal nerves, to be distributed to the whole 

 surface of the skin. The secretion of sweat by the sweat-glands may 

 be roused by the injection of pilocarpine even after division of the 

 sweat-nerves, so that this drug must act peripherally on the glands. 



