268 MR. MAYHEW ON PHYSIOLOGY OF THE HEART. 
that the presence of blood in the auricles is not sufficient to 
prevent regurgitation, unless the auric ulo-ventricular opening 
be filled by the valves. The fact that the mitral valves are 
stronger than the tricuspid, appears to support this opinion ; 
for, on the left side of the heart, there is never any regurgi- 
tation of blood into the auricle in health ; while, on the right 
side, regurgitation is not uncommon in cases of obstruction 
to the pulmonary circulation. The stronger the valves, the 
more able are they to fulfil their natural functions. Lastly, 
the very occurrence of the first sound of the heart appears to 
depend, in part, upon the “backward impulse of the blood” 
against the valves, ( Carpenter ). 
As to the action of the semilunar valves, Mr. Mayhew ap- 
pears to have a very vague and indistinct idea ; for he takes 
occasion to censure “ the learned editors” of Guy’s Hospital 
Reports, for not having been able to perceive “ that the 
semilunar valves, when there is blood on both sides of them, 
become merely floating membranes.” Now r , as I understand 
the subject, the semilunar valves are three fibro-serous flaps 
found at the ends of the aorta and pulmonary artery, one 
of their edges being attached to the w r all of the vessel, and 
the other projecting into its cavity. Each of these flaps is 
more than sufficiently large to extend horizontally into the 
centre of the cavity of the aorta, or pulmonary artery ; but 
they do not overlap ; for the surplus part of each is doubled up 
vertically, and meets its fellows back to back, their free edges 
coming together in the middle, and shutting up the cavity 
completely when the ventricles are dilated. The parts of the 
valves which are not turned up, are thicker than the turned 
up portions, and during the dilatation of the ventricles, pre- 
sent a slightly convex appearance when looked at from 
below. When the ventricles contract, the blood, unable to 
escape by the auriculo-ventricular opening, forces up the 
semilunar valves, and enters the pulmonary artery and aorta. 
The valves are, however, prevented from being driven flat 
up against the walls of the vessels by the small amount of 
blood still contained in the sinuses of Valsalva behind them. 
After the evacuation of the ventricles, the blood, from the 
contraction of the arterial w alls, and from its ow T n weight, 
has a tendency to be driven back again towards the valves, 
and, by pressure upon them, to force them back into their 
place. This action is further promoted by the mere weight 
of the valves themselves, and by the pressure of the blood in 
the sinuses of Valsalva. From this it will be seen, that the 
valves, like self-shutting doors, move freely by pressure of 
blood only in one direction, that is, up into the cavity of the 
