356 EXCRETION BY THE SKIN AND KIDNEYS. 



tant. It is well known, for example, that an abundant production of per- 

 spiration is frequently the result of mental emotions. Bernard has shown 

 that the nervous influence may be exerted through the sympathetic system. 

 He divided the sympathetic in the neck of a horse, producing as a conse- 

 quence an elevation in temperature and an increase in the arterial pressure 

 in the part supplied with branches of the nerve. He found, also, that the 

 skin of the part became covered with a copious perspiration. Upon stimu- 

 lating the divided extremity of the nerve, the secretion of sweat was arrested. 

 The local secretion of sweat after division of the sympathetic in the neck of 

 the horse was first observed by Dupuy, in 1816. 



The stimulation as well as the division of certain nerves induces local 

 secretion of sweat, but this is nearly always associated with dilatation of the 

 blood-vessels of the part ; still, sweat is frequently secreted when the surface 

 is pale and bloodless, showing that dilatation of the blood-vessels is not an 

 indispensable condition. The action of the so-called vaso-dilator nerves will 

 be treated of in connection with the physiology of the nervous system. In 

 experiments upon the cat, excito-secretory fibres have been found to exist in 

 the cerebro-spinal nerves going to the anterior extremities. The fibres for 

 the posterior extremities are in the sheath of the sciatic nerve. In all in- 

 stances the action of these nerves is direct and not reflex. Experiments upon 

 the cat have been very satisfactory, as this animal sweats only on the soles of 

 the feet, and the secretion can be readily observed. 



The so-called sweat-centres are in the lower part of the dorsal region of 

 the spinal cord, for the posterior extremities, and in the lower part of the 

 cervical region of the cord, for the anterior extremities. According to Adam- 

 kiewicz, both of these centres are subordinate to the principal sweat-centre, 

 which is situated in the medulla oblongata. Ott has collected a number of 

 cases of disease of the cord in the human subject, which go far to confirm 

 the results of experiments on the inferior animals, with regard to the action 

 of excito-secretory nerves and sweat-centres. 



When the skin is in a normal condition, after exercise or whenever there 

 is a tendency to elevation of the animal temperature, there is a determination 

 of blood to the surface, accompanied with an increase in the secretion of 

 sweat. This is the case when the body is exposed to a high temperature ; 

 and it is by an increase in the transpiration from the surface that the animal 

 heat is maintained at the normal standard. 



Quantity of Cutaneous Exhalation. The quantity of cutaneous exhala- 

 tion is subject to great variations, depending upon conditions of temperature 

 and moisture, exercise, the quantity and character of the ingesta, etc. Most 

 of these variations relate to the action of the skin in regulating the tempera- 

 ture of the body ; and it is probable that the elimination of excrementitious 

 matters by the skin is not subject, under normal conditions, to the same 

 modifications, although positive experiments upon this point are wanting. 

 When there is such a wide range of variation in different individuals and in 

 the same person under different conditions of season, climate etc., it is pos- 

 sible only to give approximate estimates of the quantity of sweat secreted 



