110 ANATOMY FOR, NURSES. [CHAP. IX. 



which at the time are nearly empty cavities with relaxed and 

 flaccid walls. The ventricles, on the contrary, have to pump the 

 blood into tubes which are already full; arid if there were no 

 auriculo- ventricular valves, the blood would meet with less resist- 

 ance in pushing its way backward into the auricles than in push- 

 ing open the semilunar valves and forcing its way into the arteries. 



Hence the necessity, first, of the tricuspid and mitral valves; 

 and, secondly, of the superior thickness and strength of the 

 walls of the ventricles, as compared with those of the auricles ; 

 and since the left side of the heart has a larger system of blood- 

 vessels to supply, and more resistance to overcome, than the 

 right side, it follows that the left ventricle needs a thicker 

 muscular wall than the right. 



The beat of the heart is caused by the rhythmical contractions 

 of its muscular fibres. Whether these contractions are auto- 

 matic or dependent upon the ganglia lodged in the cardiac 

 muscular tissue, is uncertain. That the contractions of the 

 heart do not depend upon the general nervous system is certain, 

 for the heart will continue to beat for some little time after its 

 removal from the body. It probably depends upon complex 

 metabolic changes, not yet clearly understood. 



The character of the beat, however, is governed and regulated 

 by two sets of nerves. The first set come from the cerebro-spinal 

 centre, and are supplied by the pneumogastric nerves. They are 

 the inhibitory fibres; that is to say, they slow and, with a strong 

 stimulation, will stop for a short time the action of the heart. 

 They weaken the systole, and prolong the diastole. The other 

 set come from the sympathetic nerves, and are accelerating fibres 

 which, upon stimulation, increase not only the rapidity, but the 

 force of the beat. The diastole is shortened, and the systole 

 strengthened. 



The sounds of the heart. If the ear be applied over the heart, 

 certain sounds are heard, which recur with great regularity. 

 The first sound is a comparatively long, booming sound; the 

 second, a short, sharp, sudden one. Between the first and 

 second sounds, the interval of time is very short, too short to 

 be measurable; but, between the second and the succeeding first 

 sounds there is a distinct pause. The first sound is generally 

 supposed to be caused by the contraction of the ventricular 

 walls; the second sound is undoubtedly caused by the sudden 

 closure of the semilunar valves. 



