124 HAND-BOOK OF PHYSIOLOGY. 



left, so that the whole of the work of conducting the pulmonary circula- 

 tion does not fall upon the right side of the heart, but is assisted by the 

 left side. 



The Force of the Auricular Systole and Diastole. The 

 maximum pressure within the right auricle is about \ of an inch (20 mm.) 

 of mercury, and is probably somewhat less in the left. It has been found 

 that during diastole the pressure within both auricles sinks considerably 

 below that of the atmosphere; and as some fall in pressure takes place, 

 even when the thorax of the animal operated upon has been opened, a 

 certain proportion of the fall must be due to active auricular dilatation 

 independent of respiration. In the right auricle, this negative pressure 

 is about 10 mm. 



Work Done by the Heart. In estimating the work done by any 

 machine it is usual to express it in terms of the "unit of work." The unit 

 of work is defined to be the energy expended in raising a unit of weight 

 (1 Ib.) through a unit of height (1 ft.). In England, the unit of work 

 is the "foot-pound," in France, the "kilogrammetre." 



The work done by the heart at each contraction can be readily found 

 by multiplying the weight of blood expelled by the ventricles by the 

 height to which the blood rises in a tube tied into an artery. This height 

 was found to be about 9 ft. in the horse, and the estimate is nearly correct 

 for a large artery in man. Taking the weight of blood expelled from the 

 left ventricle at each systole as 6 oz., i.e., f Ib., we have 9 x f = 3 -375 

 foot-pounds as the work done by the left ventricle at each systole; and 

 adding to this the work done by the right ventricle (about one-third that 

 of the left) we have 3*375 X 1*125 = 4*5 foot-pounds as the work done 

 by the heart at each eontraction. Other estimates give \ kilogrammetre, 

 or about 3-J- foot-pounds. Haughton estimates the total work of the heart 

 in 24 hours as about 124 foot-tons. 



Influence of the Nervous System on the Action of the 

 Heart. The hearts of warm-blooded animals cease to beat almost if not 

 quite immediately after removal from the body, and are, therefore, un- 

 favorable for the study of the nervous mechanism which regulates their 

 action. Observations have hitherto, therefore, been principally directed 

 to the heart of cold-blooded animals, e.g., the frog, tortoise, and snake, 

 which will continue to beat under favorable conditions for many hours 

 after removal from the body. Of these animals, the frog is the one mostly 

 employed, and, indeed, until recently, it was from the study of the frog's 

 heart that the chief part of our information was obtained. If removed 

 from the body entire, the frog's heart will continue to beat for many hours 

 and even days, and the beat has no apparent difference from the beat of 

 the heart before removal from the body; it will take place without the 

 presence of blood or other fluid within its chambers. If the beats have 

 "become infrequent, an additional beat may be induced by stimulating 



