98 
OF THE VESSELS 
can look to no other account of its origin or formation tha i 
the intending mind of a Creator. Nor can we without ao- 
miration reflect, that such thin membranes, such weak and 
tender instruments, as these valves are, should be able t» 
hold out for seventy or eighty years. 
Here also we cannot consider but with gratitude, how 
happy it is that our vital motions are involuntary . We 
should have enough to do, if we had to keep our hearts 
beating, and our stomachs at work. Did these things de¬ 
pend, we will not say upon our effort, but upon our bidding, 
our care, or our attention, they would leave us leisure for 
nothing else. We must have been continually upon the 
watch, and continually in fear; nor would this constitution 
have allowed of sleep. 
It might perhaps be expected, that an organ so precious, 
of such central and primary importance as the heart is, 
should be defended by a case. The fact is, that a mem¬ 
branous purse or bag, made of strong, tough materials, is 
provided for it; holding the heart within its cavity; sitting 
loosely and easily about it; guarding its substance, without 
confining its motion; and containing likewise a spoonful 
or two of water, just sufficient to keep the surface of the 
heart in a state of suppleness and moisture. How should 
such a loose covering be generated by the action of the 
heart? Does not the enclosing of it in a sack, answering 
no other purpose but that enclosure, show the care that has 
been taken of its preservation? 
One use of the circulation of the blood probably (amongst 
other uses) is, to distribute nourishment to the different 
parts of the body. How minute and multiplied the ramifi¬ 
cations of the blood-vessels, for that purpose, are; and 
how thickly spread, over at least the superfices of the body, 
is proved by the single observation, that we cannot prick 
the point of a pin into the flesh, without drawing blood, 
i. e. without finding a blood-vessel. Nor, internally, is their 
diffusion less universal. Blood-vessels run along the sur¬ 
face of membranes, pervade the substance of muscles, pen¬ 
etrate the bones. Even into every tooth, we trace, through 
a small hole in the root, an artery to feed the bone, as well 
as a vein to bring back the spare blood from it; both which, 
with the addition of an accompanying nerve, form a thread 
only a little thicker than a horse-hair. 
Wherefore, when the nourishment taken in at the mouth 
has once reached, and mixed itself with the blood, every 
part of the body is in the way of being supplied with it 
And this introduces another grand topic, namely, the man- 
