SEMILUNAR VALVES. 
235 
whilst that on the left side has only two, so as to bear some 
resemblance to a bishop’s mitre, whence it is called the mitral 
valve. 
273. The aorta and pulmonary artery are in like manner 
furnished with valves, which prevent the blood that has been 
forced into them by the contraction of the ventricles, from 
returning into those cavities when they begin to dilate again. 
These valves, however, are formed upon a different plan, and 
more resemble those of the veins, which will be presently 
described. They consist of three little pocket-shaped folds of 
the lining membrane of these arteries (similar to those at b b , 
fig. 128), which are pressed flat against the walls of those 
tubes when the blood is forced into them; but as soon as they 
are filled, and the ventricles begin to dilate, so that the blood 
has a tendency to return, it presses upon the upper side of 
these pockets, and fills them out against one another, in such 
a manner as completely to close the entrance into the ven¬ 
tricle. The three little pocket-shaped folds, however, would 
not close the centre of the aperture, were it not that each of 
them has a little projection from its most prominent part, 
which meets with those of the others, and effects the requi¬ 
site end. The situation of these valves (which are termed 
semilunar from their half-moon shape) is seen at g , fig. 127, 
/ being the pulmonary artery. 
274. The amount of blood sent-out from either ventricle 
at each contraction, in a middle-sized man, seldom exceeds 
3 ounces; but the whole quantity of blood contained in the 
body is not less than 18 lbs. : hence, it would require 96 
contractions of the heart to propel the whole of this blood 
through the body, and these (at the ordinary rapidity) would 
occupy about 1-J minute. It has been calculated, from recent 
experiments, that the usual force of the heart in man would 
sustain a column of blood about 7 feet 2 inches high, the 
weight of which would be about 4 lbs. 3 oz. on every square 
inch. The backward pressure of this column upon the walls 
of the heart, or in other words, the force which they have to 
overcome in propelling the blood into the aorta, is estimated 
at about 13 lbs. 
275. From the mode in which the blood is forced into the 
arterial system by a series of interrupted impulses, it might 
be supposed that its course would be a succession of distinct 
