KIDNEY AND SKIN AS SECRETORY GLANDS. 861 



This reflex response constitutes a very important means of regulat- 

 ing the body temperature (see p. 962), especially when the external 

 temperature is high. Under the last named condition the loss of 

 heat from the body by radiation is greatly reduced, but the 

 secretion of sweat, by virture of the heat absorbed in its vaporiza- 

 tion, serves to augment this loss of heat from the body in propor- 

 tion to the quantity of sweat formed and in inverse relation to the 

 humidity of the surrounding air. Although external temperature 

 does not directly excite the glands, it should be stated that it 

 affects their irritability either by direct action on the gland cells 

 or upon the terminal nerve fibers. At a sufficiently low temperature 

 the cat's paw does not secrete at all, and the irritabihty of the glands 

 is increased by a rise of temperature up to about 45° C. 



Dyspnea, muscular exercise, emotions, and many drugs affect 

 the secretion, probably by action on the nerve centers. Pilocarpin, 

 on the contrary, is supposed to stimulate the endings of the nerve- 

 fibers in the glands, while atropin has the opposite effect, com- 

 pletely paralyzing the secretory fibers. 



Sweat Centers in the Central Nervous System. — The fact that 

 secretion of sweat may be occasioned by stimulation of afferent 

 nerves or by direct action upon the central nervous system, as in 

 the case of dyspnea, implies the existence of physiological centers 

 controlling the secretory fibers. The precise location of the sweat 

 center or centers has not, however, been satisfactorily determined. 

 Histologically and anatomically the arrangement of the sweat 

 fibers resembles that of the vasoconstrictor fibers, and, reasoning 

 from analogy, one might suppose the existence of a general sweat 

 center in the medulla comparable to the vasoconstrictor center, 

 but positive evidence of the existence of such an arrangement is 

 lacking. It has been shown that when the medulla is separated 

 from the cord by a section in the cervical or thoracic region the 

 action of dyspnea, or of various sudorific drugs supposed to act on 

 the central nervous system, may still cause a secretion. On the 

 evidence of results of this character it is assumed that there are spinal 

 sw^eat centers; but whether these are few in number or represent 

 simply the various nuclei of origin of the fibers to different regions 

 is not definitely known. It is possible that in addition to these 

 spinal centers there is a general regulating center in the medulla. 



Sebaceous Secretion. — ^The sebaceous glands are simple or 

 compound alveolar glands found over the cutaneous surface, usually 

 in association with the hairs, although in some cases they occur 

 separately, as, for instance, on the prepuce and glans penis, and 

 on the lips. When they occur with the hairs the short duct opens 

 into the hair follicle, so that the secretion is passed out upon the 

 hair near the point at which it projects from the skin. The alveoli are 

 filled with cuboidal or polygonal epithelial cells, which are arranged 



