96 
OF THE VESSELS 
of its action must depend upon these; for when any one of 
its cavities contracts, the necessary tendency of the force 
will be to drive the enclosed blood, not only into the mouth 
of the artery where it ought to go, but also back again in¬ 
to the mouth of the vein from which it flowed. In like 
manner, when by the relaxation of the fibres the same cav¬ 
ity is dilated, the blood would not only run into it from the 
vein, which was the course intended, but back from the ar¬ 
tery, through which it ought to be moving forward. The 
way of preventing a reflux of the fluid, in both these cases, 
is to fix valves, which, like flood-gates, may open away to 
the stream in one direction, and shut up the passage against 
it in another. [PI. XVII. fig. 2, 3, 4.] The heart, constitut¬ 
ed as it is, can no more work without valves than a pump 
can. When the piston descends in a pump, if it were not 
for the stoppage by the valve beneath, the motion would 
only thrust down the water which it had before drawn up. 
A similar consequence would frustrate the action of the 
heart. Valves, therefore, properly disposed, i. e. properly 
with respect to the course of the blood which it is neces¬ 
sary to promote, are essential to the contrivance. And 
valves so disposed, are accordingly provided. A valve is 
placed in the communication between each auricle and its 
ventricle, lest when the ventricle contracts, part of the blood 
should get back again into the auricle, instead of the whole 
entering, as it ought to do, the mouth of the artery. A valve 
is also fixed at the mouth of each of the great arteries which 
take the blood from the heart; leaving the passage free, so 
long as the blood holds its proper course forward; closing 
it, whenever the blood, in consequence of the relaxation of 
the ventricle, would attempt to flow back. There is some 
variety in the construction of these valves, though all the 
valves of the body act nearly upon the same principle, and 
are destined to the same use. In general they consist of 
a thin membrane, lying close to the side of the vessel, and 
consequently allowing an open passage whilst the stream 
runs one way, but thrust out from the side by the fluid get¬ 
ting behind it, and opposing the passage of the blood, when 
it would flow the other way.* Where more than one mem¬ 
brane is employed, the different membranes only compose 
* The veins and absorbent vessels present in their cavities folds of a 
parabolic form, called valves, like the semilunar valve; the one edge 
adheres to the sides of the vein, the other is loose; the first is farthest 
from the heart, the other nearer. The number of valves is greatest where 
the blood flows contrary to the force of its own weight. See Fig. 7. 
Paxton 
