DESCRIPTION. 171 



branches of that artery, and whence it is brought back by the pul- 

 monary veins, forming ultimately four large trunks, which discharge 

 themselves into the heart's left auricle, by the contraction of which 

 the blood is next forced into the left ventricle, to the place from 

 which we began. 



"We have sketched out the circulation before treating more mi- 

 nutely of the structure of the heart, in order that we may have the 

 blood for a guide, and proceed from the uses of the organ to the 

 anatomical structure. "We shall however again recur to the circu- 

 lation in greater detail. 



The left ventricle of the heart, a powerful contractile cavity, has 

 the task of projecting the florid or arterial blood which has just tra- 

 versed the lungs, through the aorta over the whole body ; upwards, 

 to the head, downwards to the feet ; this office it discharges by a 

 quick act of contraction, or as it is termed, systole, which propels 

 the blood into the aorta. The aorta, like the other large arteries, 

 is elastic and muscular, and tends constantly to assume its smallest 

 calibre, in consequence of which it moves the blood forwards where- 

 ever a free space is found. But when the heart again expands or 

 performs diastole, the blood would regurgitate from the aortic tube 

 into the ventricle, were there not a provision against this in the pre- 

 sence of three semilunar valves, little crescents of the lining mem- 

 brane of the aorta, which swell out into pouches when the blood re- 

 gurgitates, and close the passage. Their function in this respect is 

 constantly called for, and the sudden stop of the blood by the valve 

 gives rise to a click which may be heard among the other sounds of 

 the heart. 



The volume of blood we are considering is now fairly in the arte- 

 ry, and like a slippery ball it eludes the successive pressure of the 

 vessels, and flies onwards in its course. The opening of the artery 

 by the injected wave, and its contraction upon the same, ensues 

 like a rapid undulation along the whole line, and constitutes the 

 pulse. Eighty times per minute the quiver of this stroke permeates 

 the life tree of the body and its infinite ramifications. The station, 

 the tram and the passengers are all locomotives on this railway. 

 First, the heart closes, and its out-thrown blood opens the artery; 

 then the artery itself successively closes and opens down the entire 



