ANIMAL MECHANICS. 



137 



7. On the Mechanical Work done by the Human Heart. — The 

 heart constitutes so peculiar and so important a muscle, that it 

 is worthy of a distinct consideration. It contracts constantly 

 by means of a self-acting nerve supply, and works day and 

 night while life lasts, without ever experiencing a sense of fa- 

 tigue, or the necessity for rest. Let us examine what is the 

 mechanical equivalent of the work done by an average human 

 heart during the course of a single day and night : — 



Let Q denote the weight of the quantity of blood that is 

 expelled from the left ventricle into the aorta at each stroke 

 of the heart, and let h denote the height of a column of blood 

 that would measure the fluid pressure inside the ventricle at 

 the moment of contraction. It follows, from the hydraulic 

 laws of spouting fluids, that the blood Q, if thrown vertically 

 upwards, would ascend to the height h, so that the work done 

 in a single stroke of the left ventricle is represented by the 

 product 



Q x h = Work done. 



We have now to estimate the values of the quantities Q 

 and h : — 



The estimates given by various anatomists of the capacity 

 of the left ventricle vary from 2 to 5 ounces (water), the 

 former estimate being that of the empty ventricle, which ad- 

 mits of considerable distention, especially during life. It is 

 extremely difficult, if not impossible, to say what is the amount 

 of distention experienced by the left ventricle before it con- 

 tracts upon its contents, and ~^pels them. From some ex- 

 periments that I have made, it seem 0 me probable that the 

 distention does not amount to more than fifty per cent., and 

 that we shall not be far from the truth if we take Q to be 

 3 ounces of blood, neglecting the difference of specific gravity 

 between blood and water. 



