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 perfbrmed 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 o 



! 



H 



