THE PULSE l8l 



sending blood to a distance, the greater thickness of the left being 

 made necessary by the far greater work required of it. 



The activity of the heart is unlike that of any other organ in its 

 periods of working and resting. The nerve-muscle structure is so 

 arranged that a series of rhythmic contractions at short intervals, 

 goes on continuously day and night from the beginning of life to 

 its close. 



The systole of the ventricles corresponds to the "heart-beat." 

 It occurs at perfectly regular intervals in health, the rate being 



FIG. 123. 



The right ventricle filled, contracts and expels blood through semilunar valves 

 (tricuspid valve closed}. (Dalton in Brubaker.) 



from sixty to seventy per minute in men, and from seventy to 

 eighty in women. The heart's action is more rapid in the upright 

 position than in sitting or lying, and is increased by any exercise, 

 however gentle. Excitement or emotion will quicken it at once, 

 and it is always faster in children, being about one hundred and 

 forty in the newly born and reaching an average rate of ninety 

 to one hundred at the age of three years; ninety in youth, sev- 

 enty in adults, and eighty in old age. The innervation of the 

 heart is described on pages 185, 186. 



The Pulse. The effect of the heart-beat upon the current of 

 the blood may be felt in the arteries, which are distended for an 

 instant by the blood forced into them as the ventricles contract. 



