CALLOSITY. — CORN. 357 



arranged according as one or other constituent is chiefly in- 

 volved, the middle term corresponding to the equal implication 

 of both. 



§ 301. — The callosity (callositas) is a circumscribed thicken- 

 ing of the horny layer of the epidermis. It forms a flattened 

 superficial elevation of horny transparency, which slopes gra- 

 dually down on every side. Its consistency depends on the 

 amount of moisture present, and varies from the elastic and 

 flexible to the horny and brittle. In its minute characters it 

 differs from the normal structure of the horny layer, only in the 

 number of superimposed strata of flattened and horny epidermic 

 cells. 



If it be the function of the epidermis to protect the surface of 

 the body from external influences by a coating which, however 

 thin, is nevertheless very resistant, we may legitimately regard 

 the callosity as a functional hypertrophy, analogous to the 

 hypertroph}' of muscles from over- work. Observation tells us 

 that callosities are exclusively produced at such points of the 

 cutaneous surface as are exposed to severe or often-repeated 

 pressure ; as e.g. the palm of the hand and sole of the foot. 

 AYe should then ascribe the greater thickness of the horny 

 layer, with which these parts are naturally endowed, to natural 

 selection. The notion that a hyperasmic state of the papillary 

 l)ody, and a consequent increase in the nutrition of the epidermis, 

 are caused by external pressure on those parts, is far simpler ; and 

 80 far from being antagonistic to the former view, it rather 

 serves to explain it. 



§ 302. The corn (claims) is a modified callosity. Here 

 too, external pressure acts as the eflicient cause of the over- 

 growth. The difference in the results depends on a variation 

 in its mode of action ; if the point at which the two opposing 

 forces come into collision coincides accurately with the point 

 of contact between the compressed part and the compressing 

 agent, a callosity is produced ; if on the other hand the two 

 points do not coincide, if the seat of conflict is withdrawn into 

 the interior of the compressed part, in other words if the 

 compressed part is capable of yielding to the pressure, a com 

 results. 



Fig. 113 represents a vertical section through a corn under 

 sx magnifying power of 20 diameters. Here too we have a 



