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, 



