THE HEART. 157 



that sufficient remains to drive the blood, even without the help of the aux- 

 iliary agents which are generally in action, from the small veins right back 

 to the auricle. Lastly we have seen that while the above is the cause of the 

 flow from ventricle to auricle, the changing rate of the flow, the diminish- 

 ing swiftness in the arteries, the sluggish crawl through the capillaries, the 

 increasing quickness through the veins are determined by the changing width 

 of the vascular " bed." 



Before we proceed to consider any further details as to the phenomena 

 of the flow through the vessels, we must turn aside to study the heart. 



THE HEART. 



115, The heart is a valvular pump which works on mechanical princi- 

 ples, 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 connected 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. 



The Phenomena of the Normal Beat. 



The visible movements. When the chest of a mammal is opened and arti- 

 ficial 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 vense cavse, and pulmonary veins 

 are seen, while full of blood, to contract in the neighborhood of the heart ; 

 the contraction runs in a peristaltic wave toward 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 toward the 

 auriculo-ventricular orifices, and the auricular appendages are drawn inward, 

 becoming smaller and paler. During the auricular systole, the ventricles 

 may be seen to become turgid. Then follows, as it were immediately, the 

 ventricular systole, during which the ventricles become more conical. Held 

 between the fingers they are felt to become tense and hard. As the systole 

 progresses, the aorta and pulmonary arteries expand and elongate, the apex 

 is tilted slightly upward, and the heart twists somewhat on its long axis, 

 moving from the left and behind toward the front and right so that more of 

 the left ventricle becomes displayed. As the systole gives way to the suc- 

 ceeding diastole, the ventricles resume their previous form and position, the 

 aorta and pulmonary artery shrink and shorten, the heart turns back toward 

 the left, and thus the cycle is completed. 



In the normal beat, the two ventricles are perfectly synchronous in ac- 

 tion, they contract at the same time and relax at the same time, and the 



