CHAP. VIII.] THE VASCULAR SYSTEM. 87 



is over, they are also seen to contract, their walls becoming very 

 tense and hard; the apex is tilted upwards, and the heart 

 twists somewhat on its own axis. During the ventricular 

 systole, the blood in the ventricles is forced through the semi- 

 lunar valves into the arteries, which are seen to elongate and 

 expand as the blood is pumped into them. 



The work of the auricles and ventricles is very unequal. 

 All the auricles have to do is to pump the blood into the ven- 

 tricles, 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 ; and if there 

 were no auriculo-ventricular valves, the blood would meet with 

 less resistance in pushing its way backward into the auricles 

 than in pushing open the semilunar valves and forcing its way 

 into the arteries. 



Hence the necessity, firstly, 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 over- 

 come 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 cer- 

 tain, 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 tlic 

 heart. They weaken the systole, and prolong the diastole. 

 The other set come from the sympathetic nerves, and are accel- 

 erating fibres which, upon stimulation, increase not only the 

 rapidity, but the force of the beat. The diastole is shortened, 

 and the systole strengthened. 



