THE SKIN 319 



also at all times traces of sulphocyanides, and it is these, perhaps, in 

 combination with proteid decomposition-products of unknown nature, 

 that make the sweat toxic when ingested. The odor of sweat, especially 

 if old, varies with the volatile fatty acids excreted at that place, for the 

 latter differ somewhat in various localities. The sodium chloride present 

 gives sweat a salty taste. Under the microscope sweat is seen to contain 

 epidermal debris and an occasional waste-product from the tissues in 

 and about the sweat-glands. The urea present at all times more or less 

 may be of great importance during incapacitating disease of the kidneys. 

 Ordinarily it constitutes about one part in a thousand of sweat, but when 

 the excretion of urine is suppressed it may be in quantity sufficient to be 

 seen in crystals on the skin, and to materially aid in the life-saving excre- 

 tion of nitrogen. 



The nervous system superintends the secretion of sweat. Its influ- 

 ences pass out in the neuraxones of the sympathetic by way of the anterior 

 roots of the cord. Centrally the fibers of the sweat-nerves probably pass 

 upward in the cord to a center or centers in the medulla probably, but 

 as yet not exactly located. Function indicates their very close relation 

 both with the vasomotor and with the thermogenic centers. The uni- 

 lateral sweating seen occasionally in nerve-clinics suggests very strongly 

 that there is a center on each side of the medulla. 



Sweat is excreted continually and when in quantities too small to be 

 seen on the skin is termed invisible or "insensible" sweat. Its quantity 

 is continually changing as the regulation of body-heat requires, but may 

 average 1500 c.c. daily. Under certain normal conditions, all of which 

 involve an increase of body-katabolism or at least of body-heat, the sweat 

 is so much augmented that it is no longer evaporated as fast as excreted, 

 and it then collects on the surface. It is then termed "sensible" or 

 visible perspiration. As we have seen, these two sorts may differ some- 

 what in composition, but not much; the more abundant the sweat, 

 however, the larger the proportion of water and the more likely it is to be 

 alkaline instead of acid. Besides the normal conditions mentioned 

 below, sweat may be augmented by pathological causes not involving 

 metabolism, for example certain drugs (as pilocarpine and nicotine), and 

 by some sort of derangement of the sympathetic sweat-nerves or -centers 

 in certain nervous diseases. This is called by dermatologists a condition 

 of hyperidrosis. Its etiology is often obscure, but is usually some 

 derangement of the sympathetic. 



Among the physiological conditions determining an increase of sweat are 

 these six : An increase in the temperature of the blood ; vaso-dilatation in 

 the skin; a high external temperature; an increase in the proportion of 

 water in the blood (hydremia) ; venosity of the blood ; and direct cerebral 

 stimulation of the sweat-centers in emotional conditions, for example 

 fear. Physical and even mental exercise (the former being perhaps the 

 most common cause of the appearance of sweat on the skin) produce the 

 effect by heating the blood thus stimulating the centers and perhaps 

 also the glands directly, at the same time causing peripheral vaso-dila- 



