864 PHYSIOLOGY OF DIGESTION AND SECRETION. 



possibly the heat fibers, and reflexly stimulates the sweat fibers. 

 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 irritability of the glands is increased by a rise of temper- 

 ature 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 

 sweat 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 

 in several layers. Those nearest the lumen of the gland are filled 

 with fatty material. These cells are supposed to be cast off bodily, 

 their detritus going to form the secretion. New cells are formed 

 from the layer nearest the basement membrane, and thus the glands 

 continue to produce a slow but continuous secretion. The sebaceous 



