THE ACTION OF THE HEART 311 



meters. The total work output of the heart per beat is, therefore, 

 roughly 283 grammeters, equivalent in the English scale to about 

 2 foot-pounds. When the heart is beating at the rate of 70 per 

 minute it does 140 foot-pounds per minute, making it a 240th 

 horse-power engine. If it maintained this rate throughout the 

 entire twenty-four hours of the day it would do in that time 

 200,000 foot-pounds of work, an amount equivalent to that done 

 by the leg muscles of a man weighing 150 pounds in climbing a 

 mountain 1,300 feet high. 



That the heart is able to do this amount of work daily without 

 fatigue, and keep it up day in and day out for seventy or more 

 years, is due to its ability to recover quickly from the effects of its 

 activity, coupled with the fact that in a whole day its resting time 

 considerably outweighs the time during which it is active. The 

 heart-beat is ordinarily much slower during sleep than during 

 bodily activity; as the result the heart enjoys an " eight hour day " 

 if only its actual contraction time be counted. 



Relations of Nerve and Muscle Elements within the Heart. 

 The heart-muscle consists, as previously stated, of muscle-cells of 

 small size, intimately communicating with one another through 

 their branches, and showing signs of cross-striation. At the junc- 

 tion of the great veins with the heart, a region, as we shall see, of 

 great importance in the heart's activity, these muscular elements 

 form thin sheets; in the auricles the heart-muscle is somewhat 

 heavier and thicker; but it attains its greatest development in the 

 ventricles, where the muscular walls are exceedingly heavy, and 

 very stout. In mammals the only pulsating heart structures are 

 the auricles and ventricles; in lower vertebrates, such as the frog, 

 the great veins near the heart are differentiated into a pulsating 

 structure, the sinus venosus, and the outlet from the ventricle, the 

 bulbus arteriosus, also pulsates. Although in mammals these 

 structures no longer pulsate, the region of the great veins which 

 corresponds to the sinus venosus still seems to preserve to some 

 degree the physiological properties it has in lower animals, and 

 observations made upon frogs' hearts are interpreted for mammals' 

 hearts upon that basis. 



Embedded within the tissue of the heart are numerous nerve- 

 cells. These are most numerous in the region of the sinus venosus 

 and auricles; the base of the ventricles contains some of them, but 



