92 
OF THE VESSELS 
under side, like a scoop, and with such a concavity that the 
finger may be cut across to the bone, without hurting the 
artery which runs along it. At other times, the arteries 
pass in canals wrought in the substance, and in the very 
middle of the substance of the bone; this takes place in 
the lower jaw; and is found where there would, otherwise, 
be danger of compression by sudden curvature. All this 
care is wonderful, yet not more than what the importance 
of the case required. To those who venture their lives 
in a ship, it has been often said, that there is only an inch 
board between them and death; but in the body itself, es¬ 
pecially in the arterial system, there is, in many parts, only 
a membrane, a skin, a thread. For which reason, this sys¬ 
tem lies deep under the integuments; whereas the veins, in 
which the mischief that ensues from injuring the coats is 
much less, lie in general above the arteries; come nearer 
to the surface; are more exposed. 
It may be farther observed concerning the two systems 
taken together, that though the arterial, with its trunks 
and branches and small twigs, may be imagined to issue or 
proceed, in other words, to grow from the heart, like a plant 
from its root, or the fibres of a leaf from its foot-stalk, (which, 
however, were it so, would be only to resolve one mechanism 
into another,) yet the venal, the returning system, can never 
be formed in this manner. The arteries might go on shoot¬ 
ing out from their extremities, i. e. lengthening and sub¬ 
dividing indefinitely; but an inverted system, continually 
uniting its streams, instead of dividing, and thus carrying 
back what the other system carried out, could not be refer¬ 
red to the same process. 
II. The next thing to be considered is the engine which 
works this machinery, viz. the heart. [PI. XVII. fig. 1.] 
For our purpose it is unnecessary to ascertain the principle 
upon which the heart acts. Whether it be irritation excited 
by the contact of the blood, by the influx of the nervous 
fluid, or whatever else be the cause of its motion, it is some¬ 
thing which is capable of producing, in a living muscular 
fibre, reciprocal contraction and relaxation. This is the 
power we have to work with; and the inquiry is, how this 
power is applied in the instance before us. There is pro¬ 
vided, in the central part of the body, a hollow muscle, in¬ 
vested with spiral fibres, running in both directions, the 
layers intersecting one another; in some animals, however, 
appearing to be semicircular rather than spiral. By the 
contraction of these fibres, the sides of the muscular cavities 
are necessarily squeezed together, so as to force out from 
