62 CIRCULATION OF THE BLOOD 



empty. Yet the vessel will continue sufficiently distended 

 above that valve, as at O, G. ... If you now apply a 

 finger of the other hand upon the distended part of the 

 vein above the valve O (Plate VII. Fig. 3), and press down- 

 wards, you will find that you cannot force the blood 

 through or beyond the valve. . . . You will only see the 

 portion of vein between the finger and the valve become 

 more distended, while the portion of the vein below the 

 valve (H, O, Plate VII. Fig. 3) still remains empty." 



" It therefore appears that the function of the valves 

 in the veins is the same as that of the three sigmoid valves 

 at the commencement of the aorta and pulmonary artery, 

 namely, to prevent all reflux of the blood that is passing 

 through them." 



" Further, the arm being bound at A, A as before, and 

 the veins full and distended, compress a vein with one 

 finger L (Fig. 4), at a point below a valve or knot. Then 

 with another finger stroke the blood upwards beyond the 

 next valve N. You will now perceive that this portion 

 of the vein L, N still continues empty. . . . But if the 

 finger first applied (H, Fig. 2, L, Fig. 4) is removed, the 

 vein is immediately filled from below, and the arm be- 

 comes again as at D, C (Fig. i)." 



" That the blood in the veins therefore proceeds from 

 more remote to less remote parts and towards the heart, 

 moving always in the vessels in this, and not in the con- 

 trary direction, appears most plain . . . for the veins 

 are free and open conduits of the blood returning to the 

 heart, and are yet effectually prevented from serving as its 

 channels of distribution from the heart." 



(d) SUMMARY AND CONCLUSION 



17. " Now I may give my view of the circulation of the 

 blood and propose it for general adoption." 



" All things, both argument and ocular demonstration, 



