ENLARGEMENT OF PARTS FROM USE. 19 



Enlargement of parts may arise from increased blood 

 supply due to irritation ; thus a bone, the tibia, has been 

 known to increase in length when long inflamed, to the 

 extent of an inch and a half as compared with its 

 fellow. 



Skin supplies many curious and instructive instances 

 under the names of corns and callosities. Those 

 troublesome thickenings of the skin covering the toes, 

 caused by ill-fitting boots, known as corns, or on the 

 palms of the hands due to the use of tools in particular 

 occupations, consist anatomically of a raised hard patch 

 of thick epidermis ; beneath it is a small sac containing 

 fluid, termed a bursa. When a thickened patch of 

 skin exists without a bursa it is usually called a 

 callosity. Corns, as most are aware, occur most 

 frequently on the toes, whilst callosities form on the 

 sole of the foot and in the neighbourhood of the heel. 

 Callosities are inherited, as is shown by the fact that the 

 skin on the sole of the foot of a peasant's infant is 

 thicker than that on the foot of the parson's offspring 

 at the moment of birth. We may not unreasonably 

 attribute the readiness with which a badly fitting boot 

 will produce corns to a tendency we inherit from our 

 parents and grandparents. In the same way the 

 callosities on the breasts of camels, on the knuckles 

 of the gorilla's fingers, and the ischial callosities of 

 baboons, may be regarded as inherited local cutaneous 

 thickenings, induced by the intermittent pressure to 

 which the skin of these parts is subject ; in the case 

 of the camel when lying down, the gorilla when walk- 

 ing, and the baboon when sitting on its haunches. 



