57# THE SKIK. [CH. xxxviil. 



those for the upper limbs leave the cord by the sixth, seventh, and 

 eighth anterior thoracic roots, have cell stations in the ganglion 

 stellatum and ultimately pass to the ulnar and median nerves ; 

 they are controlled by a centre in the cervical enlargement of the 

 cord. The secretory fibres for the head pass in the cervical 

 sympathetic, and in some branches of the fifth cranial nerves. 

 These subsidiary centres are dominated by one in the medulla 

 oblongata (Adamkiewicz). These facts have been obtained by 

 experiments on animals (cat, horse). 



The sweat-centres may be excited directly by venous blood, as 

 in asphyxia ; or by over-heated blood (over 45 C.) ; or by certain 

 drugs (see further) ; or reflexly by stimulation of afferent nerves 

 such as the crural and peroneal. 



Nervous diseases are often accompanied with disordered sweat- 

 ing ; thus unilateral perspiration is seen in some cases of 

 hemiplegia ; degeneration of the anterior nerve-cells of the cord 

 may cause stoppage of the secretion. It is sometimes increased 

 m paralysed limbs. 



The changes that occur in the secreting cells have been 

 investigated by Renaut in the horse. When charged they are 

 clear and swollen, the nucleus being situated near their attached 

 ends ; when discharged they are smaller, granular, and their 

 nucleus is more central. 



The sweat, like the urine, must be regarded as an excretion, 

 the secreting cells eliminating substances formed elsewhere. 



Composition of the Sweat. Sweat may be obtained in 

 abundant quantities by placing the animal or man in a closed 

 hot-air bath, or from a limb by enclosing it in a vessel made aii 1 - 

 tight with an elastic bandage. Thus obtained it is mixed with 

 epidermal scales and a small quantity of fatty matter from the 

 sebaceous glands. The continual shedding of epidermal scales is 

 in reality an excretion. Keratin, of which they are chiefly 

 composed, is rich in sulphur, and, consequently, this is one means 

 by which sulphur is removed from the body. 



The reaction of sweat is acid, and the acidity, as in the urine, is 

 due to acid sodium phosphate. In profuse sweating, however, 

 the secretion usually becomes alkaline or neutral. It has a 

 peculiar and characteristic odour, which varies in different parts 

 of the body, and is due to volatile fatty acids ; its taste is saltish, 

 its specific gravity about 1005. 



In round numbers the percentage of solids is i'2, of which o'8 

 is inorganic matter. The following table is a compilation from 

 several analyses : 



