SEMILUNAB 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 181bs. : 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 Ij- 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 Ibs. 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 Ibs. 



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 



