THE CIRCULATION OF THE BLOOD. 205 



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

 by the left side. 



(b.) Auricular.' The maximum pressure within the right auricle is 

 equal to about f of an inch (20 mm.) of mercury, and is probably some- 

 what less in the left. It has been found that during diastole the pres- 

 sure within both auricles sinks considerably below that of the atmos- 

 phere; 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 respi- 

 ration in the right auricle, this negative pressure is equal to about 

 10 mm. 



In estimating the work done by any machine it is usual to express 

 it in terms of the unit of work. In England, 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 kilo- 

 gram-metre. The work done by the heart at each contraction can be 

 readily found by multiplying the weight of blood expelled by the ven- 

 tricles by the height to which the blood rises in a tube tied into an 

 artery. This height is probably about 9 ft. 3.21 metres in man. Tak- 

 ing the weight of blood expelled from the left ventricle at each systole 

 at 6 oz., i.e., f Ib., we have 9 X f = 3.375 foot-pounds, or 3.21 X 180 grms. 

 or 578 gram- metres, as the work done by the left ventricle at each sys- 

 tole; and adding to this the work done by the right ventricle (about 

 one-fourth that of the left) we have 3.375 + .822 = 4.19 foot-pounds, or 

 722 gram-metres as the work done by the heart at each contraction. 



Blood Pressure. 



The subject of blood-pressure has been already incidentally men- 

 tioned more than once in the preceding pages, the time has now arrived 

 for it to receive more detailed consideration. 



That the blood exercises pressure upon the walls of the vessels con- 

 taining it, is due to the following facts : 



Firstly, that the heart at each contraction forcibly injects a consid- 

 erable amount of blood, viz., 4 to 6 oz. (120 to 180 grms.) suddenly and 

 quickly into the arteries. 



Secondly, that the arteries are already full of blood at the com- 

 mencement of the ventricular systole, since there is not sufficient time 

 between the heart beats for the blood to pass into the veins. 



Thirdly, that the arteries are highly distensible and stretch to ac- 

 commodate the extra amount of blood forced into them; and 



Fourthly, that there is a distinct resistance interposed to the pas- 

 sage of the blood from the arteries into the veins, from the enormous 

 number of minute vessels, small arteries (arterioles) and capillaries into 



