THE HUMAN HEART. 
we said above, that, under extraordinary circumstances, valves 
in the upper great vein would not only be useless, but even 
injurious, since it was necessary that regurgitation should 
occasionally take place. 
The right ventricle then, as it contracts, forces the blood into 
the pulmonary artery, a large vessel which soon divides into two 
branches, one going to each lung. This vessel, like all other 
large arteries, has elastic walls, which yield under the impulse 
of the blood from the heart, and immediately afterwards con- 
tract again, as all elastic tubes would do after dilating ; and thus 
they, as it were, store up the force of the heart’s contraction, and 
transmit it along the whole length of the vessel for the purpose 
of forcing- the blood on throughout. In addition to this, their 
yielding prevents then being torn by the force with which the 
blood is impelled into them by each contraction of the ventricle 
Here, again, we have a beautiful arrangement to prevent the 
blood going back into the ventricle under the force with which 
the elastic artery contracts again. Three semicircular or semi- 
lunar folds of membrane, strengthened by fibrous structure, 
form valves, which are attached by then semicircular edges to 
the walls of the artery, while then straight edges look towards 
the centre. They are so loosely attached that they can be 
pouched out by the blood when driven back against them, and 
so driven out from the sides of the artery against which they 
otherwise he, and be made to stop the opening. But it would 
be impossible to have muscular bands and tendinous cords inside 
the artery to hold these valves from going through into the 
ventricle, for such an arrangement would hinder the blood 
flowing freely along the artery, and accordingly we have other 
contrivances to prevent such an accident in this case. In the 
first place, from the shape of the valves, and their being- 
attached by so much of their margin to the arteiy, there is less 
liability to the occurrence of such an accident; and, secondly, 
as the fleshy mass of the contracted ventricle lies close up under 
these valves, it gives them support for an instant, until the 
blood has passed on, and the artery just beyond the valves is 
once more empty. Since these valves, however, lie so close along 
the walls of the artery, and are attached by so large a margin 
to it, another danger is thus incurred, — namely, that the blood 
which has to shut these valves should altogether fail to get 
between them and the walls of the artery, and so should keep 
them open instead of shutting them. This danger is avoided 
by the elasticity of the artery, for, as the vessel dilates under 
the shock of the blood, it is evident that it will form pouches 
behind the valves, into which the blood must flow, and so act 
on them just as the water in a canal does on the gates of a 
lock, which it can never shut so long as they lie flat against the 
NO. ITT. 2 B 
