CHAP, iv.] THE VASCULAR MECHANISM. 217 



same course, now become different. While the pressure sinks 

 rapidly in the ventricle, falling it may be below that of the atmos- 

 sphere, and thus becoming a negative pressure, which in some cases 

 may possibly be considerable, that in the aorta does not sink to 

 a corresponding degree ; in fact, as we shall see, it is reinforced to 

 a certain extent in a secondary rise, the so-called dicrotic rise. 



We have reason to believe not only that the quantity of blood 

 ejected at the systole may vary from time to time, but also that 

 at times at all events if not normally, the whole of the blood 

 present in the ventricle at the systole may fail to leave the 

 ventricle during the systole, more or less remaining behind at the 

 close ; the ventricle in such cases does not completely empty itself. 

 . On the other hand, we may perhaps admit that, at least under cer- 

 tain circumstances, when, for instance, the contents of the ventricle 

 are small, and the ventricle vigorous or the systole prolonged, the 

 whole of the contents may be discharged in the earlier part of the 

 systole, the ventricle remaining contracted for some little time after 

 it has emptied itself. 



The Work done. 



120. We have already ( 114) spoken of that most important 

 factor in the determination of the work of the heart, the pulse- 

 volume, or the quantity ejected from the ventricle into the aorta 

 at each systole, and of the various methods by which it may be 

 estimated. We have seen that it probably varies within very 

 considerable limits. 



We may here repeat the remark 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 circulation. 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. 



Not only does the amount ejected vary, but the pressure under 

 which it is ejected also varies within very considerable limits. 

 Moreover, the number of times the systole is repeated within a 

 given period may also vary considerably. The work done, therefore, 

 varies very much. But it may be interesting and instructive to 

 note the results of calculating out a very high estimate. ^Thus 

 if we take 180 grms. as the quantity, in man, ejected 

 at each stroke at a pressure of 250 mm. of mercury, which is 



