OF ANIMAL BODIES. 
91 
disposition of the blood-vessels, i. e. the laying of the pipes; 
and secondly, the construction of the engine at the centre, 
viz. the heart, for driving the blood through them. 
I. The disposition of the blood-vessels, as far as regards 
the supply of the body, is like that of the water pipes in a 
city, viz. large and main trunks branching off by smaller 
pipes (and these again by still narrower tubes) in every 
direction, and towards every part in which the fluid, which 
they convey, can be wanted. So far the water pipes, 
which serve a town, may represent the vessels which carry 
the blood from the heart. But there is another thing 
necessary to the blood, which is not wanted for the water; 
and that is, the carrying of it back again to its source. 
For this office, a reversed system of vessels is prepared, 
which, uniting at their extremities with the extremities of 
the first system, collects the divided and subdivided stream¬ 
lets, first by capillary ramifications into larger branches; 
secondly, by these branches into trunks; and thus returns 
the blood (almost exactly inverting the order in which it 
went out) to the fountain whence its motion proceeded. All 
which is evident mechanism. 
The body, therefore, contains two systems of blood-ves¬ 
sels, arteries and veins. Between the constitution of the 
systems there are also two differences, suited to the func¬ 
tions which the systems have to execute. The blood, in 
going out, passing always from wider into narrower tubes; 
and, in coming back, from narrower into wider; it is evi¬ 
dent, that the impulse and pressure upon the sides of the 
blood-vessel, will be much greater in one case than the 
other. Accordingly, the arteries which carry out the blood, 
are formed with much tougher and stronger coats, than the 
veins which bring it back. That is one difference: the 
other is still more artificial, or, if I may so speak, indicates, 
still more clearly, the care and anxiety of the artificer. 
Forasmuch as in the arteries, by reason of the great force 
with which the blood is urged along them, a wound or rup¬ 
ture would be more dangerous than in the veins; these 
vessels are defended from injury, not only by their texture, 
but by their situation; and by every advantage of situation 
which can be given to them. They are buried in sinuses, 
or they creep along grooves, made for them in the bones; 
for instance, the under edge of the ribs is sloped and fur¬ 
rowed solely for the passage of these vessels. Sometimes 
they proceed in channels, protected by stout parapets on 
each side; which last description is remarkable in the 
bones of the fingers, these being hollowed out, on the 
