66 MOTION OF THE 



vations (B, c, D, E, F,) will be perceived, and this not only at 

 the places where a branch is received (E, F), but also where 

 none enters (c, D) : these knots or risings are all formed by 

 valves, which thus show themselves externally. And now if 

 you press the blood from the space above one of the valves, from 

 H to o, (fig. 2,) and keep the point of a finger upon the vein 

 inferiorly, you will see no influx of blood from above ; the 

 portion of the vein between the point of the finger and the 

 valve o will be obliterated ; yet will the vessel continue suf- 

 ficiently distended above that valve (o, G). The blood being 

 thus pressed out, and the vein emptied, if you now apply a finger 

 of the other hand upon the distended part of the vein above the 

 valve o, (fig. 3,) and press downwards, you will find that you 

 cannot force the blood through or beyond the valve ; but the 

 greater effort you use, you will only see the portion of vein 

 that is between the finger and the valve become more dis- 

 tended, that portion of the vein which is below the valve re- 

 maining all the while empty (H, o, fig. 3). 



It would therefore appear that the function of the valves in 

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

 we find at the commencement of the aorta and pulmonary ar- 

 tery, viz., to prevent all reflux of the blood that is passing over 

 them. 



Farther, the arm being bound as before, and the veins look- 

 ing full and distended, if you press at one part in the course of 

 a vein with the point of a finger (L, fig. 4), and then with an- 

 other finger streak the blood upwards beyond the next valve 

 (N), you will perceive that this portion of the vein continues 

 empty (L N), and that the blood cannot retrograde, precisely 

 as we have already seen the case to be in fig. 2 ; but the finger 

 first applied (H, fig. 2, L, fig. 4), being removed, immediately the 

 vein is filled from below, and the arm becomes as it appears at 

 D c, fig. 1. That the blood in the veins therefore proceeds from 

 inferior or more remote to superior parts, and towards the heart, 

 moving in these vessels in this and not in the contrary di- 

 rection, appears most obviously. And although in some places 

 the valves, by not acting with such perfect accuracy, or where 

 there is but a single valve, do not seem totally to prevent the 

 passage of the blood from the centre, still the greater number 

 of them plainly do so ; and then, where things appear contrived 



