Chap. XII] GENERAL CIRCULATION OF BLOOD 217 



tral nervous system. That the contractions of the heart do 

 not depend upon the nervous system is certain, for the heart 

 will continue to beat for some time after its removal from the 

 body. It probably depends i?pon a power inherent in the 

 muscle tissue, and this power is most highly developed in 

 the wall of the right auricle between the openings of the two 

 venae cavse. Contractions seem to start in this area and pass 

 first over the walls of the auricles, and thence over the walls of 

 the ventricles. It is thought that the contractions are stimulated 

 by the chlorides of sodium, potassium, and calcium contained in 

 the blood. 



Imiervation of the heart. — As previously stated, the heart 

 receives inhibitory nerve fibres from the pneumogastric nerve, 

 and accelerator fibres from the sympathetic system. Both the 

 inhibitory and accelerator fibres are in a state of constant, though 

 slight, activity. This means that under normal conditions the 

 heart-beat is controlled by two antagonistic influences, one tending 

 to slow the heart action, and the other to quicken it. This arrange- 

 ment is comparable to the antagonistic action of the flexor and 

 extensor muscles and probably enables the heart to respond more 

 promptly to stimulation. Stimulation of the pneumogastric may 

 cause marked slowing of the heart-beat, and depression of the 

 pneumogastric, or stimulation of the accelerator fibres, may quicken 

 the heart-beat. 



(2) The elasticity and extensibility of the arterial walls. — Each 

 time the ventricles contract they force about six ounces of blood 

 into arteries that are already full. This extra blood finds room for 

 itself partly by distending the arteries and partly by driving 

 forward the blood which is before it. The arteries are capable of 

 distention because they contain a large amount of yellow elastic 

 tissue in their coats. Following the distention the arteries recoil 

 upon the contained blood and cause such a pressure to be exerted, 

 that what would be an intermittent stream is converted into a 

 continuous one. They thus serve not only as conducting tubes 

 but exert a force that assists the heart in driving the blood into 

 the capillaries. 



The elasticity and extensibility of the arteries change with the 

 health and age of the individual. Sometimes as the result of 

 disease, and always as we grow older, the arterial walls grow stiffer 



