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, 
