20 E VOL UTION AND DISEASE. 



Increase in the size of a part may arise from diminished 

 use combined with irritation. For instance, nails, hoofs, 

 and claws grow throughout life, and the wear and tear 

 consequent on continued use is thus compensated. 

 Should such parts be used less than usual, growth 

 continues at the normal rate and the nails or hoofs 

 become abnormally large and inconvenient. This form 

 of enlargement is termed overgrowth, and is more liable 



to occur when diminished 

 use is accompanied by 

 irritation, as illustrated by 

 the following specimen. 

 It was a goat, confined for 

 many weeks in a muddy 

 paddock ; on examining 

 its feet I found them fur- 

 nished with hoofs of great 

 length, one measured 

 thirty-six centimetres fol- 



FiG. 10. An overgrown hoof in a lowing the Curve, and its 

 goat, which had lived many weeks _ ,, 



in a muddy paddock. It measures fellow twenty - five centl- 

 thirty-six centimetres following the metres (fig. IO). The hoofs 

 curve. ft 1 i 



of cows, horses, sheep, and 



deer become similarly overgrown when enclosed on 

 marshy ground, dirty paddocks, or damp sheds. Similar 

 conditions are not rare in our own species, for old, 

 bedridden persons often have long toe-nails, some of 

 them two or three inches long, thick and twisted like a 

 ram's horn. 



The relation between increase in the growth of nail 

 or hoof in consequence of additional blood supply, is 



