SECRETION OF SWEAT 213 



Practically, in health, the only conditions which increase the 

 flow of perspiration are muscular exercise and a high external 

 temperature. Of these, exercise probably works through the 

 nerve centers; external heat does not stimulate the glands directly, 

 but irritates the cutaneous terminations of afferent fibers which 

 convey impressions to the sweat centers, whence messages are 

 sent out by secretory fibers to the glandular epithelium and 

 their activity begins. In both cases there is accompanying 

 dilatation of the superficial vessels under the influence of the 

 vaso-dilator fibers. 



It is supposed that the chief center is in the medulla oblon- 

 gata and that secondary centers exist in the lumbar region of 

 the cord. 



The amount of CO 2 eliminated by the skin is inconsiderable 

 in the human being. 



