328 THE URINARY APPARATUS 



clear, colorless liquid of a specific gravity varying 

 from 1.003 to 1.006; slightly acid in reaction and salty 

 to the taste. Except when collected from the soles 

 of the feet and palms of the hands, it is mixed with 

 epithelial cells and sebum secreted by the sebaceous 

 glands. The amount of sweat secreted in twenty-four 

 hours has been estimated at 700 to 1000 grams; however, 

 this is uncertain, owing to the difficulty of collection, 

 and the influence temperature, diet, drink, season of 

 the year, etc., exert upon its secretion. 



The secretion of sweat, though essentially an ex- 

 cretion, is chiefly concerned in the regulation of the 

 body temperature in maintaining heat dissipation, 

 rather than the elimination of waste materials by 

 means of the sweat glands brought to them by the 

 blood and lymph vessels. 



The Part Played by the Nerve System on the Produc- 

 tion of Sweat. The secretion of sweat is practically 

 the result of the activity of the epithelial cells of the 

 sweat glands and is regulated by the nerve system. 

 The fluid contained in the sweat is derived from 

 the materials in the lymph channels, furnished by 

 the blood. 



To produce sweat there must be a glandular activity, 

 and a regulation of the blood-supply. The former is 

 brought about by a set of nerves called the secretor, 

 which stimulate the cells to activity; the latter is 

 regulated by nerves called the vasomotor, that regu- 

 late the blood-pressure of the capillaries and increase 

 or decrease the blood-supply to the glands. Other 

 influences increase the production of sweat, by their 

 related nerve centres sending out impulses in two 

 ways: first, by nerve impulses from the central domin- 

 ating centre, supposed to be located in the medulla, 

 being stimulated by mental emotions, as shock, shame, 

 etc., venosity of the blood, hot drinks, violent muscular 

 exercise, etc.; second, by reflex impulses being con- 

 veyed to the centres in the spinal cord by the sensor 



