ENLARGEMENT OF PARTS FROM USE. 17 
bone in man, the patella or knee-cap ; when the patella is 
broken by muscular violence it is rarely repaired by 
bone, but by yielding fibrous tissue. 
In the navicular bone of the horse and the knee-cap of 
‘man, analogous conditions prevail, viz., bones which in 
many mammals are small and insignificant have become 
by excessive use enlarged and of such importance that, 
when damaged, permanent lameness in man, and useless- 
ness in the horse, ensue. 
When parts are enlarged in this way from increased 
use, they are said to be hypertrophied ; in the case of the 
horse’s foot the hypertrophy is said to be functional, in 
that of the big toe figured on page 15, pathological, as it 
arises in consequence of abnormal conditions. 
The most striking examples of hypertrophy may be 
studied in the muscular system and in paired organs. 
For instance, should one kidney from any cause be 
slowly destroyed, the other will gradually enlarge and 
often double its size, thus compensating the animal and 
often preserving it from disaster. Many such cases 
have been reported in man and I have met with kidneys 
enlarged from this cause in horses, sheep, oxen, 
pheasants, and in a hen. Enlargement of a part from 
such causes is said to be compensatory ; conspicuous 
examples of this form of hypertrophy occur in the 
animal kingdom. Variations in the size of a_ part 
according to the amount of work performed by it is 
illustrated by the gizzards of birds. In flesh- or fish- 
eating birds the muscular walls of the gizzard are 
relatively thin; in grain-eaters they are exceedingly 
thick. Hunter fed a sea-gull for a year on barley 
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