1 86 ANATOMY AND PHYSIOLOGY 



function of the nerves being to accurately regulate the rate at which con- 

 tractions. occur. This is the myogenic theory of the cause of the heart beat. 

 Another theory, not so generally held, is the neurogenic, which assumes a 

 special nerve system in the myocardium, acting automatically and regulated 

 or modified by accelerator and inhibitor nerves as above described. 



The velocity of the normal blood current is greatest in the larger 

 arteries and least in the capillaries. It increases in the larger 

 veins but never equals that in the arteries. The time required for 

 the entire circuit from heart to heart again, is about twenty-eight 

 seconds, or approximately, in the time of twenty-six to twenty- 

 eight heart-beats. 



The onward flow of the current, as it is expelled from the heart, 

 is assisted by i, the elastic recoil of the arterial walls (after the 

 stretching caused by the blood which is pumped into them with 

 each systole), 2, the pressure of contracting muscles on the veins, 

 forcing the blood toward the heart, 3, the intake of breath (or 

 aspiration of the thorax) when the auricles are opened to receive 

 blood, and 4, the valves of the veins which allow the blood to flow 

 onward but not backward. 



The act of aspiration is very closely associated with circulation; 

 deep breathing alone will promptly quicken the action of the heart 

 and, consequently, the velocity of the stream. 



Variations due to emotion, excitement, etc., have been al- 

 luded to. Blood pressure will be considered in Chapter XII. 



The total quantity of blood is estimated at about one-twelfth 

 of the body weight Roughly speaking, it is distributed (in a 

 state of rest) as follows: one-quarter in the muscles, one-quarter 

 in the liver, one-quarter in the heart, lungs, arteries and veins, one- 

 quarter in other organs. Of course, during the activity of any spe- 

 cial part of the body, as for example, in the digestive system the 

 proportions are changed, as the most active organs require most 

 blood, leaving less for the others at that time. 



