318 CIRCULATORY SYSTEM. 



will beat for hours, provided it is supplied with defibrinated blood 

 through its nutrient artery (Porter). This second theory is rapidly 

 gaining favor among physiologists. 



Cardiac Nerves. Whatever may be the cause of the heart-beat, 

 its regulation is brought about by the central nervous system. The 

 cardiac nerves are derived from two sources, the vagus and the 

 sympathetic, through the cardiac plexuses, of which there are four : 

 1, the superficial, situated in the concavity of the arch of the 

 aorta ; 2, the deep, or great ; 3, the right coronary, and, 4, the left 

 coronary ; the last two being derived from the superficial and the 

 deep. The impulses which reach the heart through the vagus are 

 of an inhibitory nature, while those passing through the sympa- 

 thetic are accelerating or augmenting (pp. 487 and 523). 



Circulation in the Arteries. Each time that the ven- 

 tricles contract they send into the arteries about 140 c.c. of 

 blood, each ventricle expelling 70 c.c. (p. 314). The arterial 

 system is always overdistended that is, even when the heart 

 is at rest the amount of blood in the arteries is sufficient to stretch 

 their walls a little. When an additional amount of blood is forced 

 into them by the muscular contraction of the heart, these vessels 

 are distended still more, for the blood already in them cannot at 

 once flow on in an amount equal to that which comes from the 

 heart. If an artery at this time should be felt with the finger, it 

 would beat against the latter, this beat being called the pulse. As 

 soon as the systole ceases the elastic coats of the arteries squeeze 

 the blood that is within them, and this blood tends to flow away 

 from the point of pressure in two directions back toward the heart 

 and onward toward the capillaries. Its backward flow at once 

 closes the pulmonary and aortic valves, and in this direction, 

 therefore, its progress is barred. The blood then can go only 

 forward. Before the onward flow of the blood has ceased another 

 systole occurs, and again the ventricles are emptied into the 

 arteries, and thus this action continues during the life of the 

 individual. If a cannula is inserted into the cavity of the ventri- 

 cle, it will be seen that at each systole the blood spurts out 

 in a jet, which ceases at the end of the systole that is, the 

 flow from the heart is intermittent. If the cannula is inserted 

 into the aorta, the blood will jet out at each systole of the heart, 

 but, instead of ceasing to flow during diastole, it will not 

 entirely cease, but will continue to flow a little under the influence 

 of the elastic force of the aorta. If the cannula is inserted into 

 successive portions of the arterial system farther and farther from 

 the heart, the blood will come out in jets as before under the influ- 

 ence of the heart's contraction, but it will continue to flow in the 

 intervals, the difference between the jet and the continuous flow 

 being less and less marked the greater is the distance of the inser- 

 tion of the cannula from the heart. In the capillaries the flow is 

 regular and continuous, unaffected by the action of the heart. 



