72 ANIMAL MECHANICS 



ject. By the exercise of the tendons (and their 

 exercise is the act of being pulled upon by the 

 muscles, or having a strain made on them), they 

 become firmer and stronger ; but in the failure of 

 muscular activity, they become less capable of 

 resisting the tug made upon them, and if, after 

 a long confinement, a man has some powerful 

 excitement to muscular exertion, then the tendon 

 breaks. An old gentleman, whose habits have 

 been long staid and sedentary, and who is very 

 guarded in his walk, is upon an annual festival 

 tempted to join the young people in a dance; 

 then he breaks his tendo Achillis. Or a sick 

 person, long confined to bed, is, on rising, subject 

 to a rupture or hernia, because the tendinous 

 expansions guarding against protrusion of the 

 internal parts have become weak from disuse. 



Such circumstances remind us that we are 

 speaking of a living body, and that, in estimating 

 the properties of the machinery, we ought not to 

 forget the influence of life, and that the natural 

 exercise of the parts, whether they be active or 

 passive, is the stimulus to the circulation through 

 them 5 and to their growth and perfection. 



