THE SKIN 317 



the living world : each part is more or less dependent on every other part. 

 Further than this every animal is at once the product of and dependent 

 on its environment of natural energies and materials. By way of the 

 stomach and the lungs these in great numbers enter more or less 

 into the animal's metabolism or affect it in many ways through its 

 nervous systems. Some of these latter influences enter by way of the 

 sense-organs of the skin. 



There are numerous minute "spots" in the skin sensitive to pressure 

 (touch), pain, heat, and cold, respectively, and it is not difficult to stimu- 

 late these various points in such a way as to produce pure sensations of 

 at least these four kinds. (See below, page 366, where some details are 

 given.) When, for example, a touch-organ is stimulated separately, 

 nothing whatever but a sensation of touch is experienced, no pain or 

 cold or warmth. When a "pain-spot" is properly excited only pain is 

 experienced no suggestion of touch or of cold or of warmth. Besides 

 the definite end-organs, there seem to be everywhere fibrils of nerve- 

 tissue. Especially is this the case among the epithelial cells of the 

 epidermis and of the mucous membranes, and in the cylinders of tissue 

 encasing the lower ends of the almost universally distributed hairs. 

 Mechanical irritation of these fibers or fibrils starts impulses in their 

 afferent nerves, but how these differ in their mode of sensation-making 

 from those set going from the various touch-corpuscles, etc., is at 

 present not understood. 



The considerable variety and immense number of the skin's sense- 

 organs are the facts to be noted here. By them this organ is made 

 practically a perfect coupler between the animal and its physical environ- 

 ment. 



Thermotaxis, the regulation to body-temperature, is one of the im- 

 portant functions of the integument. By the reflex or voluntary action 

 of various and certain elements within the skin's thickness, combined 

 with the nervous system, the temperature is kept within the limits with 

 which we are familiar, namely, one or two degrees above or below 

 37 centigrade. We have already discussed the methods of heat- 

 production arid of heat-loss and have seen that the arrangements for 

 heat-regulation involve nearly all parts of the body. Of the organs thus 

 concerned, none, it will be recalled, is more important than the skin. 

 From it the very large part of the heat lost from the body is radiated or 

 conducted. As we have seen, the proportional area of the skin to the 

 metabolic tissue-mass producing heat within it is a chief factor in the 

 conduct of thermolysis. Again, it is the skin which secretes the sweat, 

 by whose evaporation largely the heat is lost from the body. 



The Sweat. Another function of the human skin, but partly involving 

 the preceding, is the excretion of sweat. This substance is a liquid 

 of very low specific gravity produced by certain closely coiled glands 

 lying in the subcutaneous layer of tissue. Their product, however, 

 is poured out on the skin's surface. Sweat is termed an "excretion" 

 because it is waste matter. It is, however, of great use to the organism 



