INFLUENCE OF NERVES ON THE SECRETION OF SWEAT. 447 



motion, owing to the rapid evaporation, the formation of drops of sweat is prevented, 

 or at least retarded. [The complementary relation between the skin and kidneys 

 is well known. In summer, when the skin is active, the kidneys separate less 

 water ; in winter, when the skin is less active, it is cold and comparatively 

 bloodless, while the kidneys excrete more water, so that the action of these two 

 organs is in inverse ratio.] 



The influence of nerves upon the secretion of sweat is very marked. 



I. Just as in the secretion of saliva ( 145), vaso-motor nerves are usually in 

 action at the same time as the proper secretory nerves ; the vaso-dilator nerves 

 (sweating with a red congested skin) are most frequently involved. The fact that 

 secretion of sweat does occasionally take place when the skin is pale (fear, death- 

 agony) shows that, when the vaso-motor nerves are excited, so as to constrict the 

 cutaneous blood-vessels, the sweat-secretory nerve-fibres may also be active. 



Under certain circumstances, the amount of blood in the skin seems to determine the 

 occurrence of sweating ; thus, Dupuy found that section of the cervical sympathetic caused 

 secretion on that side of the neck of a horse ; while Nitzelnadel found that percutaneous 

 electrical stimulation of the cervical sympathetic in man, limited the sweating. 



[We may draw a parallel between the secretion of saliva and that of sweat. Both are formed 

 in glands derived from the outer layer of the embryo. Both secretions are formed from lymph 

 supplied by the blood-stream, and if the lymph be in sufficient quantity, secretion may take 

 place when there is no circulation, although in both cases secretion is most lively when the 

 circulation is most active and the secretory nerves of both are excited simultaneously ; both 

 glands have secretory nerves distinct from the nerves of the blood-vessels; both glands may be 

 paralysed by the action of the nervous system, or in disease (fever), or conversely, both are 

 paralysed by atropine and excited by other drugs, e.g., pilocarpin. In the gland cells of both, 

 histological changes accompany the secretory act, and no doubt similar electromotor pheno- 

 mena occur in both glands. ] 



II. Secretory nerves, altogether independent of the circulation, control the 

 secretion of sweat. Stimulation of these nerves, even in a limb which has been 

 amputated in a kitten, causes a temporary secretion of sweat, i.e., after complete 

 arrest of the circulation (Goltz, Kendall and Lucksinger, Ostroumoiv). In the intact 

 condition of the body, however, profuse perspiration, at all events, is always 

 associated with simultaneous dilatation of the blood-vessels (just as, in stimulation 

 of the facial nerve, an increased secretion of saliva is associated with an increased 

 blood-stream 145, A, I.). The secretory nerves and those for the blood-vessels 

 seem to lie in the same nerve-trunks. 



The secretory nerves for the hind limbs (cat) lie in the sciatic nerve. Luch- 

 singer found that stimulation of the peripheral end of this nerve caused renewed 

 secretion of sweat for a period of half an hour, provided the foot was always wiped 

 to remove the sweat already formed. If a kitten, whose sciatic nerve is divided on 

 one side, be placed in a chamber filled with heated air, all the three intact limbs 

 soon begin to sweat, but the limb whose nerve is divided does not, nor does it do 

 so when the veins of the limb are ligatured so as to produce congestion of its 

 blood-vessels. [The cat sweats only on the hairless soles of the feet.] As to the 

 course of the secretory fibres to the sciatic nerve, some pass directly from the 

 spinal cord (Vulpian), some pass into the abdominal sympathetic (Lucksinger, 

 Nawrocki, Ostroumow), through the rami communicantes and the anterior spinal 

 roots from the upper lumbar and lower dorsal spinal cord (9th to 13th dorsal 

 vertebrae cat), where the sweat-centre for the lower limbs is situated. 



The sweat-centre maybe excited directly: (1) By a highly venous condition 

 of the blood, as during dyspnoea, e.g., the secretion of sweat that sometimes pre- 

 cedes death ; (2) by overheated blood (45 C.) streaming through the centre ; (3) 

 by certain poisons (see p. 446). The centre may be also excited reflexly, although 

 the results are variable, e.g., stimulation of the crural and peroneal nerves, as well 

 as the central end of the opposite sciatic nerve, excites it. [The pungency of 

 mustard in the mouth may excite free perspiration on the face.] 



