SEC. 2. THE HEART. 



The heart is a pump, the motive power of which is supplied 

 by the contraction of its muscular fibres. Its action consequently 

 presents problems which are partly mechanical, and partly vital. 

 Regarded as a pump, its effects are determined by the frequency of 

 the beats, by the force of each beat, by the character of each beat 

 whether, for instance, slow and lingering, or sudden and sharp 

 and by the quantity of fluid ejected at each beat. Hence, with a 

 given frequency, force, and character of beat, and a given quantity 

 ejected at each beat, the problems which have to be dealt with are 

 for the most part mechanical. The vital problems are chiefly con- 

 nected with the causes which determine the frequency, force, and 

 character of the beat. The quantity ejected at each beat is governed 

 more by the state of the rest of the body, than by that of the 

 heart itself. 



TJie Phenomena of the Normal Beat. 



The visible movements. When the chest of a mammal is 

 opened and artificial respiration kept up, a complete beat of the 

 whole heart, or cardiac cycle, may be observed to take place. as 

 follows. 



The great veins, inferior and superior vense cavse and pulmonary- 

 veins, are seen, while full of blood, to contract in the neighbourhood 

 of the heart : the contraction runs in a peristaltic wave towards the 

 auricles, increasing in intensity as it goes. Arrived at the auricles, 

 which are then full of blood, the wave suddenly spreads, at a rate 

 too rapid to be fairly judged by the eye, over the whole of those 

 organs, which accordingly contract with a sudden sharp systole. In 

 the systole, the walls of the auricles press towards the auriculc- 



