I 
THE HUMAN HEAET. 3o3 
the veins because the muscular contraction of their coats 
prevents it, in addition to which there is a most beautiful set 
of valves in the inside of the veins which only open towards 
the heart, so that the blood can pass that way, but not back- 
wards. In the smaller veins of the body a pair of these valves 
can be seen at about every quarter inch, but they do not exist 
in the very small veins, nor in the great vein which comes into 
the right auricle from above ; for, under ordinary circumstances, 
the blood is prevented from flowing backwards in this by the 
force of gravity, and under extraordinary circumstances it is 
sometimes necessary that it should flow back through it, as 
will be explained shortly. The walls of the auricle are very 
smooth, so as to allow the blood to pass freely along them, and 
they are not very strong, as they only have to force the blood 
into the next chamber, which is gradually opening to receive 
it. As soon as this chamber, the right ventricle, is full, it also 
contracts, bnt much more slowly than the auricle, since it has to 
overcome much greater resistance, for it has to force the blood 
along’ the pulmonary artery, and through all the capillary 
vessels of the air-cells of the lungs. And to effect this, its 
walls are very strong, and furnished with several muscular 
bands inside, which contract at the same time that the walls 
are contracting, and so both help the wall, as it were, and 
also by becoming thicker in consequence of their shortening, 
fill up the whole cavity of the ventricle, so that it completely 
empties itself of blood, which the auricle scarcely does. 
There are, however, none of these muscular bands just at 
the mouth of the artery, lest they should cause obstruction to 
( the free passage of the blood. But why does not the contrac- 
tion of the ventricle force back the blood into the auricle which 
is just then dilating again ? Because, inside the ventricle, and 
over the opening between the two chambers, there are three 
thin membranous valves which only open into the ventricle, but 
_ the moment the blood is driven against them by the ventricle 
beginning to contract, they are closed by its impetus, and, fitting 
accurately, completely prevent all regurgitation into the auricle. 
The bases of these valves are connected with the opening all 
round, and their sides are partially connected with each other, so 
as to close the opening completely. But there would be consider- 
able danger that the great force with which the ventricle contracts 
would drive the tips of these valves, which are in the centre 
when they close the opening, completely through into the 
am-icle, and so allow regurgitation to take place notwith- 
standing, and the whole machinery to become hopelessly dis- 
organised. To prevent this terrible accident, which would cause 
instant death, there are a large number of very fine but very 
strong branching tendons attached all over these valves on the 
