870 PHYSIOLOGY OF DIGESTION AND SECRETION. 



work has demonstrated that special nerve fibers are supplied to 

 the glandular epithelium. According to Arnstein, the terminal 

 fibers form a small, branching, varicose ending in contact with the 

 epithelial cells. The sweat gland may be made to secrete in many 

 ways other than by direct artificial excitation of the sweat fibers, 

 for example, by external heat, dyspnea, muscular exercise, strong 

 emotions, and by the action of various drugs, such as pilocarpin, 

 muscarin, strychnin, nicotin, picrotoxin, and physostigmin. In all 

 such cases the effect is supposed to result from an action on the 

 sweat fibers, either directly on their terminations or indirectly upon 

 their cells of origin in the central nervous system. In ordinary 

 life the usual cause of profuse sweating is a high external temper- 

 ature or muscular exercise. With regard to the former it is known 

 that the high temperature does not excite the sweat glands im- 

 mediately, but through the intervention of the central nervous 

 system. If the nerves going to a limb be cut, exposure of that 

 limb to a high temperature does not cause a secretion, showing 

 that the temperature change alone is not sufficient to excite the 

 gland or its terminal nerve fibers. We must suppose, therefore, 

 that the high temperature acts upon the sensory cutaneous nerves, 

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

 This reflex response constitutes a very important means of regulat- 

 ing the body temperature (see p. 968), 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 irritability 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. 



