POPULAR SCIENCE REVIEW. 
356 
banks, but shuts immediately if they are pushed out from the 
banks. Just in the middle of the unattached portion of the 
margin of each valve there is a httle projecting fibrous particle 
which has been considered to be useful in effecting the same 
object, for, as it will touch the wall of the artery first when the 
valve opens back, it is evident that it will keep the rest of the 
valve out a httle from the artery, and so always leave a passage 
behind it for the blood. These three little particles also, one 
on each valve, have been considered to be of use in another 
way when the valves are shut ; namely, by filling up the very 
centre of the opening, which might otherwise not be completely 
closed, although the margins elsewhere overlap a good deal. 
As the blood comes back from the lungs after being 
oxygenated, it is poured by four veins into the left auricle of 
the heart, which is the third chamber ; and this, just like the 
right auricle, pumps it into the left ventricle through an opening 
a little smaller than that on the right side of the heart, and 
guarded similarly by a valve having only two leaves or flaps, 
instead of three, but provided with the same arrangement of 
cords and muscles, only that here there is no safety-valve 
arrangement as on the right side, — since, in the first place, the 
left ventricle may always, and can always, empty itself of the 
blood as fast as it is filled with it, for it drives the blood through 
the body, the structure of whose capillaries is much stronger 
than in the capillaries of the lungs, and consequently in no 
danger of giving way ; and, secondly, if there were a safety- 
valve action, it would only OA r erload the lungs, for there are no 
valves in the pulmonary veins to prevent it going back as there 
are in other veins ; and thus the very mischief would be pro- 
duced, to avoid which the safety-valve arrangement is provided 
on the right side of the heart. 
The left ventricle, or fourth chamber of the heart, is the 
strongest of all, since it has to drive the blood through the 
whole body, and it also drives a small quantity through the 
muscular substance of the heart itself. The great artery 
through which it sends the blood is called the aorta ; it after- 
wards gives off branches, which again ramify until the subdi- 
visions become innumerable and supply all parts of the body. 
Its opening is closed by three semilunar valves precisely similar 
to those closing the opening into the pulmonary artery ; and as 
both these large arteries proceed upwards from the heart, the 
-force of gravity aids the blood in shutting the valves. Some- 
times, however, any of these valves may become diseased, and 
not act perfectly, and then death is sure to ensue shortly, and 
may be very sudden. We remember having a patient under 
our care, who suffered much, and died suddenly thus ; and we 
found, afterwards, that every one of the three valves which 
