CIRCULATION OP THE BLOOD 127 



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 sufficiently 

 distended above the 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 por- 

 tion of vein that is between the finger and the valve become 

 more distended, that portion of the vein which is below the 

 valve remaining 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 pul- 

 monary artery, viz., to prevent all reflux of the blood that 

 is passing over them. 



[NOTE. Woodcuts of the veins of the arm to which these 

 letters and figures refer appear here in the original. 

 C. N. B. C] 



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 

 another 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 retro- 

 grade, 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. I. That the 

 blood in the veins therefore proceeds from inferior or more 

 remote parts, and towards the heart, moving in these ves- 

 sels in this and not in the contrary direction, appears most 

 obviously. And although in some places the valves, by not 

 acting with such perfect accuracy, or where there is but a 



