128 HANDBOOK OF PHYSIOLOGY. 



fall in pressure, and possibly the greater part, is to be referred to the 

 influence of respiration ; but without this the negative pressure of the 

 left ventricle caused by its active dilatation is about equal to f of an 

 inch (23 mm.) of mercury. 



The right ventricle is undoubtedly aided by this suction power of the 

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

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

 the left side. 



The Force of the Auricular Contractions. The maximum 

 pressure within the right auricle is about of an inch (20 mm. ) of mer- 

 cury, and is probably somewhat less in the left. It has been found that 

 during diastole the pressure within both auricles sinks considerably be- 

 low 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 work/' In Eng- 

 land, the unit of work is the "foot-pound," and is defined to be the 

 energy expended in raising a unit of weight (1 Ib.) through a unit of 

 height (1 ft.) : in France, the " kilogram-metre." 



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

 by multiplying the weight of the 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 this estimate is 

 nearly correct for a large artery in man. Taking the weight of blood 

 expelled from the left ventricle at each systole at 6 oz., i. e., f Ib., we 

 have 9 X |=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 contraction. Other estimates give 

 J kilogram-metre, or about 3 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 very soon after 

 removal from the body, and are, therefore, unfavorable for the study of 

 the nervous mechanism which regulates their action. The hearts of 

 cold-blooded animals, therefore, e. g., the frog, tortoise, and snake, 

 which will continue to beat under favorable conditions for many hours 

 after removal from the body, are generally employed, as more conveni- 

 ent for the purpose. Of these animals, the frog is the one most fre- 



