332 ACTION OF THE HEAET. 



which, in structure and function, and all things else, is an 

 artery; the left ventricle sending its charge into the aorta, 

 and through this by the arteries to the body at large. 



" These two motions, one of the ventricles, another of the 

 auricles, take place consecutively, but in such a manner that 

 there is a kind of harmony or rhythm preserved between 

 them, the two concurring in such wise that but one motion 

 is apparent, especially in the warmer-blooded animals, in 

 which the movements in question are rapid. Nor is this for 

 any other reason than it is in a piece of machinery, in which, 

 though one wheel gives motion to another, yet all the wheels 

 seem to move simultaneously ; or in that mechanical contriv- 

 ance which is adapted to fire-arms, where the trigger being- 

 touched, down comes the flint, strikes against the steel, elicits 

 a spark, which falling among the powder, it is ignited, upon 

 which the flame extends, enters the barrel, causes the explo- 

 sion, propels the ball, and the mark is attained ; all of which 

 incidents, by reason of the celerity with which they happen, 

 seem to take place in the twinkling of an eye." 



When the ventricle contracts, the blood strikes against the 

 auriculo-ventricular valves, which close upon the volume of 

 blood which is then entering the auricles. This is associated 

 with a sound the first heart sound distinguished by its 

 longer duration, and softness, from a sharp second sound 

 which occurs when the blood regurgitates in the pulmonary 

 artery and aorta, so as forcibly to close the semilunar valves. 

 Dr Leared has published a thesis on this subject, in which he 

 declares that all sounds formed in connection with the circu- 

 lation are produced by and in the blood itself, and their 

 mechanism is the same. The first sound coincides with the 

 contraction of the ventricles, and is caused as follows: 

 ' Bloo.d having been forcibly driven from the ventricles into 

 the aorta and pulmonary artery, comes into forcible contact 



