SEC. 3. THE HEART. 



126. The heart is a valvular pump which works on me- 

 chanical principles, but 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 not only by the action of the heart itself but also and 

 indeed more so by what is going on in the rest of the body. 



The Phenomena of the Normal Beat. 



The visible movements. When the chest of a mammal is 

 opened and artificial respiration kept up the heart may be 

 watched beating. Owing to the removal of the chest-wall, what 

 is seen is not absolutely identical with what takes place within 

 the intact chest, but the main events are the same in both cases. 

 A complete beat of the whole heart, or cardiac cycle, may be 

 observed to take place as follows. 



The great veins, inferior and superior venae 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 



