ENLARGEMENT OF PARTS FROM USE. 21 
shown in a striking manner in the foot of the horse. 
When inflamed the hoof will sometimes become much 
longer than usual, and the softer part, known as the frog, 
will enlarge and form a spongy mass, filling up the sole. 
This cannot be attributed to disuse as the remaining 
hoofs are also idle, for the horse is unable to work on 
account of lameness, and we can assure ourselves of the 
existence of inflammation in the affected foot by ob- 
serving its increased heat. 
In the next chapter I shall have occasion to demon- 
strate that overgrowth may occur from diminished use 
alone ; nevertheless, the most striking examples are 
associated with increased blood supply consequent on 
irritation, and as this assists in explaining some interest- 
ing normal conditions it will be advantageous to consider 
some additional instances. It has long been known that 
when the skin is irritated, especially in young people, by 
long continued discharges from wounds, or by the per- 
sistent application of poultices, the hairs of the part 
grow thick and long: when the irritation subsides the 
hairs gradually return to their normal condition. This 
overgrowth of hair may be attributed to a local increase 
in the blood supply, for it is a fundamental principle in 
Pathology that irritation produces redness of the skin; 
the heightened colour is a consequence of additional 
supply of blood to the part. Hunter demonstrated the 
relation between blood supply and overgrowth in an in- 
genious way: he transferred the spurs of cocks to the 
vascular tissue of the comb ; here they took root and, in 
consequence of the extra supply of blood, and in part no 
doubt from disuse, grew inordinately. These specimens, 
