THE SKIN 



313 



FIG. 173 



the nails and hairs. If it were reasonable to say of animal functions 

 altogether composing one harmonious whole that "this function is 

 more important than that one," we might consider the order given above 

 as perhaps that of their relative consequence to the organism. (See the 

 diagram, Fig. 172.) 



Protection is well afforded by the skin through its structure, composi- 

 tion, and relations to the bodily parts it covers. It protects from physical 

 and chemical stimuli which might injure the organism in two opposed 

 directions from mild stimuli by its sense- 

 organs which warn the nervous system of 

 their presence, and from severer stimuli 

 by the horny layer which it exposes to 

 them. It is with the latter only that we 

 are just now concerned. The structure 

 of the stratum corneum is peculiarly well 

 adapted to its functions, for it consists 

 of very many layers of dead or half dead 

 squames or scales piled up in multitudes 

 on each other but firmly connected so 

 that they form a membrane, flexible, ex- 

 tensible, and soft. 



Because of the absence of nerves from 

 the epidermis it has no sensibility and 

 the individual is not irritated by contact 

 with the air, water, and solid bodies, as 

 he would be did the nerves ramify in the 

 surface ; of course life under the latter cir- 

 cumstances is almost inconceivable. The 

 epidermal scarf-skin on this account is 

 an ideal bounding surface of an animal 

 body. 



The epidermis because of its chief 

 chemical component, keratin (an albumi- 

 noid), as also because of its scaly struc- 

 ture, is a fine non-conductor of heat. It 

 thus shields the body from the sun to an 

 extent and from excessive artificial heat. 

 When pigment is developed in the skin 

 (as in freckles, in "tanning" from sun- and wind-burn, and in the Negro 

 race), it is biologically supposed to be deposited as a still further protec- 

 tion against excessive light and heat. The dead keratin of the skin's 

 outer layers when dry is also a non-conductor of electricity, and this is 

 a matter of some importance because of the electrical developments of 

 recent years. 



This same scaly structure makes the skin a relatively poor absorber 

 of everything, as good protection requires. As we shall see before long, 

 only fats and oils enter it readily. No substance passes through it, how- 



Vertical section of the epiderm of 

 the hand's palm. X 100 diameters: 1, 

 very thick, horny layer, composed of 

 superimposed cells without their nu- 

 clei; 2, mucous layer of nucleated 

 cells; 3, upper part of this layer sep- 

 arated from the rest by a wavy line; 

 4, interpapillary depressions; 5, pap- 

 illary recesses. (Sappey.) 



