358 MORBID ANATOMY OF THE SKIN. 



trifling elevation of tlie surface ; it is, however, far smaller and 

 more circumscribed than in the case of the callosity. The 

 swelling which proceeds from the under sm-face of the thick- 

 ened homy layer at this point, is of far gi'eater moment (a). 

 This forms a truncated cone with its axis at right angles to the 

 surface of the skin, into wliich it has penetrated for some 

 distance. Its pressure has flattened the papillary elevations; 

 the cutis itself is beginning to get thinner, and cases not 

 unfrequently occur in which it is actually perforated. On 

 turning our attention to the epidermis itself, we notice a striking 

 departure from the usual plane of its stratification where the 

 corn is thickest. For, coinciding with and exactly parallel to 

 the curvature of the conical plug which is forcing its way into- 



Tertical section throiigli a corn (after Simon). The" papillary 

 body at a is seen to be flattened by the pressure of the 

 central part of the corn. .jV- 



the cutis, all tlie superjacent ,>trata of epidermis are bent with 

 their convexity inwards, so that the centre of the com is in 

 some sort differentiated from the parts around it. This inward 

 curvature is also due to the external ju'essure : which may be 

 said as it were to have manufactured a tool for itself out of the 

 epidermis, with which it can proceed to operate against the 

 deeper parts of the skin. 



§ 303. The keratoses of Lehert form a com2}aratively rare^ 

 but all the more interesting group of thickenings of the horny 

 layer of the cuticle. Monstrous accumulations of homy matter 

 on the surface of the skin are rendered possible in the keratoses^ 



