CUTANEOUS FUNCTIONS. 655 



increased blood supply of the part, but to direct action on the nervous 

 system. For if the sciatic nerve be divided in a cat, on exposure to 

 high temperature the part removed from the central nervous system will 

 form no secretion, while the other surfaces of the body will form an 

 abundant secretion of sweat, thus clearly showing that heat acts mainly 

 as a stimulant to the secretion of sweat by calling into play the activity 

 of the central nervous system. 



Sweat may also be produced by stimulating the central end of the 

 divided sciatic nerve, where, of course, its production is clearly of a 

 reflex nature, and is attributed to the stimulation of the so-called sweat 

 centres located in the spinal cord. The sweat secretion may also be 

 called forth by various drugs, especially by pilocarpine, the alkaloid of 

 jaborandi, which appears to act as a local stimulant to the sweat-glands, 

 although it may also have some stimulating action on the sweat centres. 

 As in the case of the secretion of saliva, it maj' be antagonized by atro- 

 pine. The function of the sweat-glands in the formation of sweat is 

 mainly that of an excretory organ, while, added to this, by the evapora- 

 tion of the perspiration from the external surfaces of the body it exerts 

 a considerable influence as a regulator of the temperature of the body. 

 As a consequence, therefore, when, as in warm weather, the secretion of 

 the skin is increased, the corresponding increase in evaporation tends to 

 prevent the overheating of the body. 



2. The Sebaceous Secretion of the Skin. — In the derm are found 

 a large number of racemose glands whose excretor}- ducts, as a rule, 

 open into the hair-follicles. The excretory ducts are lined with pave- 

 ment epithelium, which in the deeper portions gives place to true secretory 

 cells in which a nucleus is present, although only capable of being 

 detected with difficulty, its presence being usually obscured by the large 

 number of fatty globules surrounding it. Such sebaceous glands are 

 not uniformly distributed over the animal body, but are especially 

 developed in points most abundantly supplied with hair. During foetal 

 life the external body surface is covered with a thick Lnyer of sebaceous 

 matter, the so-called vernix caseosa, which protects it from maceration 

 in the amniotic fluid. The secretion formed by these glands is a soft, 

 crumbling, fatty mass, suspended in a tolerably small amount of water, 

 and contains a small amount of albuminous matter and considerable 

 amounts of potassium salts and of cholesterin. The secretion is formed 

 by the activity of the protoplasmic secretory cells, which remove certain 

 substances from the transudations from the blood-vessels, and whose 

 protoplasm itself undergoes fatty degeneration. The secretion, there- 

 fore, consists mainly of the breaking clown of the cell-contents. It is 

 composed of about 31 per cent, water, 61 per cent, of albuminous matter 

 and epithelium, 5 per cent, of neutral fat and soaps, and 1 per cent, of 



