CHAP, iv.] THE VASCULAR MECHANISM. 157 



distended arteries from the emptied ventricle. Coincidently with 

 this closure, the systole as we have seen probably ends and 

 relaxation begins ; then once more the cavity of the ventricle be- 

 comes unfolded and finally distended by the influx of blood from 

 the auricle. 



During the whole of this time the left side has with still 

 greater energy been executing the same manoeuvre. At the same 

 time that the vense cavas are filling the right auricle, the pulmonary 

 veins are filling the left auricle. At the same time that the right 

 auricle is contracting, the left auricle is contracting too. The 

 systole of the left ventricle is synchronous with that of the right 

 ventricle, but executed with greater force ; and the flow of blood 

 is guided on the left side by the mitral and aortic valves in the 

 same way that it is on the right by the tricuspid valves and those 

 of the pulmonary artery. 



The Work done. 



We can measure with approximative exactness the intraven- 

 tricular pressure, the length of each systole, and the number of 

 times the systole is repeated in a given period, but perhaps the 

 most important factor of all in the determination of the work of 

 the vascular mechanism, the quantity ejected from the ventricle 

 into the aorta at each systole, cannot be accurately determined ; 

 we are obliged to fall back on calculations having many sources 

 of error. The mean result of these calculations gives about 180 

 grms. (6 oz.) as the quantity of blood which is driven from each 

 ventricle at each systole in a full-grown man of average size and 

 weight. It is evident that exactly the same quantity must issue at 

 a beat from each ventricle ; for if the right ventricle at each beat 

 gave out rather less than the left, after a certain number of beats 

 the whole of the blood would be gathered in the systemic circu- 

 lation. Similarly, if the left ventricle gave out less than the right, 

 all the blood would soon be crowded into the lungs. The fact that 

 the pressure in the right ventricle is so much less than that 

 in the left (probably 30 or 40 mm. as compared with 200 mm. 

 of mercury), is due, not to differences in the quantity of blood in 

 the cavities, but to the fact that the peripheral resistance which 

 has to be overcome in the lungs is so much less than that in the 

 rest of the body. 



Various methods have been adopted for calculating the average 

 amount of blood ejected at each ventricular systole. It has been 

 calculated from the capacity of the recently removed and as yet not 

 rigid ventricle, filled with blood under a pressure equal to the calculated 

 average pressure in the ventricle. This method of course presupposes 



