182 EFFECTS 0F ‘THE POSITION OF A PART 
height of 9 ft. 8 in. above the level of the left ventricle,t whereas the distance 
from the level of the heart to that of the metacarpal artery, where I operated, 
is only about four feet. That is tosay, the column of blood from the level of the 
heart to that of the artery, when the limb was raised, had not half the height 
of that which the force of the heart was able to sustain in one of the main 
arterial trunks. Now we know that the pressure of the blood is not materially 
less in arteries of secondary dimensions, like the metacarpal, than in the main 
trunks.? And therefore the raising of the limb into the vertical position could 
not, as the mere result of the action of gravity upon the blood, diminish the 
pressure of the fluid upon the arterial wall by as much as one half ; or, conversely, 
the putting the limb down again could not do so much as double the pressure 
of the blood upon the vessel. 
Seeing, then, that the doubling of the pressure which results from the 
contraction of the ventricle has no appreciable effect upon the diameter of an 
artery, it appears clear that the great alterations in the size of the metacarpal 
artery of the horse which resulted from varying the positions of the limb cannot 
be explained on mere hydrostatic principles, and that, in order to account for 
them, we must admit contractions and relaxations of the muscular coat of the 
arteries in obedience to nervous action. 
That the force of the heart is amply adequate in the human subject to 
drive the blood freely through the vessels of the distal parts of a limb, in spite 
of the elevated position, provided that the arteries are relaxed, I shall now have 
the means of plainly demonstrating. The constricting band has been on this 
man’s arm for eight minutes, yet, thanks to its efficacy as a tourniquet, the limb 
remains as pale and corpse-like as when the application was made, although he 
has kept his hand down in the interval. I shall now ask him to raise the hand 
again to the utmost degree ; and, while it is so placed, I shall remove the elastic 
bandage. This having been done, you observe that the skin of the hand is begin- 
ning to show patches of redness, and now, a few seconds more having elapsed, 
even the finger-tips, as well as the rest of the limb, are of florid red hue. The 
' See Statical Essays. By Stephen Hales, D.D., F.R.S. 1769. 
* Poiseuille, the inventor of the mercurial haemodynamometer, believed, as the result of his experi- 
ments, that there was no appreciable difference in blood-pressure between vessels of much greater 
divergence in size. Thus, in one of his experiments (see Poiseuille, Recherches sur la Force du Ceur, &c., 
1828, p. 36), he found that the mercury rose to the same height in the haemodynamometer in the 
carotid of a horse as in a muscular branch of the femoral only two milimetres in diameter, whereas 
the metacarpal artery on which I operated had fully two and a half times that dimension. Marey, 
indeed, states (Circulation du Sang, 1863, p. 150) that the mean tension of the arteries decreases 
as the vessel is situated further from the heart, and as its calibre diminishes. But, though this is no 
doubt strictly true, yet it is allowed by physiologists that ‘ the fall’ in pressure ‘is a very gradual one 
until the smallest arteries are reached’ (Michael Foster, A Text-book of Physiology, second edition, 
1878, p. 102). 
