90 
OF THE VESSELS. 
firmed. It may be asked, do not the tendons of the human body 
sometimes break? They do; but in circumstances which only 
add to the interest of the subject. By the exercise of the tendons 
(and their exercise is the act of being pulled upon by the mus 
cles, 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 Achilles. 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 ex¬ 
ercise of the parts, whether they be active or passive, is the 
stimulus to the circulation through them, and to their growth and 
perfection. 
CHAPTER X. 
OF THE VESSELS OF ANIMAL BODIES. 
The circulation of the blood, through the bodies of men 
and quadrupeds, and the apparatus by which it is carried 
on, compose a system, and testify a contrivance, perhaps 
the best understood of any part of the animal frame. The 
lymphatic vessels, or the nervous system, may be more sub¬ 
tile and intricate; nay, it is possible that in their structure 
they may be even more artificial than the sanguiferous; 
but we do not know so much about them. 
The utility of the circulation of the blood I assume as 
an acknowledged point. One grand purpose is plainly 
answered by it; the distributing to every part, every ex¬ 
tremity, every nook and corner of the body, the nourish¬ 
ment which is received into it by one aperture. What en¬ 
ters at the mouth finds its way to the fingers’ ends. A more 
difficult mechanical problem could hardly, I think, be pro¬ 
posed, than to discover a method of constantly repairing 
the waste, and of supplying an accession of substance to 
every part of a complicated machine, at the same time. 
This system presents itself under two views: first, the 
