184 ; EPFECTS OF THE: POSITION OF (A PART 
cold inducing reflex arterial contraction, while heat occasions, equally through 
the nervous system, a relaxation. According to this view, then, the necessity 
for circulation, if I may so express myself, is a stimulus to dilatation which, 
when sufficiently urgent, overpowers the stimulus to contraction occasioned 
by the diminution of pressure upon the vessels in the elevated position. And, 
as a matter of fact, we find that the after-blush is greater the longer the time 
during which circulation has been arrested, although without anything to indicate 
nervous paralysis. In complete harmony with this view are the phenomena 
observed in a limb after ligature of its main artery. When the femoral is tied 
for popliteal aneurysm, the first effect upon the foot is pallor and coolness ; but, 
after the lapse of some hours, the converse condition of abnormal redness and 
heat supervenes. Here there is certainly no interference with the nerves, but 
the usual supply of blood is in the first instance notably diminished ; and, as 
a consequence of this, long before there has been any possibility of organic 
increase in the vessels, the anastomosing branches become so much dilated as 
to more than compensate for the obstruction of the principal source of supply. 
This effect can only be brought about through the nervous system ; and the 
most natural explanation seems to be that deficient circulation in a part continued 
for a considerable time comes to operate as a stimulus to arterial relaxation. 
I have now to mention an experiment which any one may easily perform 
upon himself, but which, though extremely simple, is not on that account the 
less instructive. But first let me state the considerations that led me to it. 
If the contraction of the arteries of an elevated limb were really the result of 
an action of a particular part of the nervous system, it might be expected that, 
on the cessation of the stimulus that evoked it, an unusual relaxation would 
ensue, corresponding with a period of repose of the nervous apparatus concerned ; 
and that this would be more marked the greater and more protracted had been 
the effort. Supposing, then, that the hand were raised after the circulation had 
been brought into full activity by brisk exercise, with the heart working power- 
fully and the arteries generally in a state of considerable dilatation, if, in spite 
of these unusual obstacles to arterial contraction, pallor of the limb should 
result from the elevated position, it might be anticipated that, when the hand 
was again lowered, it would not only resume its former redness, but acquire for 
a while a deeper tint than the other, which had been kept dependent throughout. 
My first trial of this kind was made just after I had been walking with great haste 
to catch a train, when my heart was beating with unusual vigour, and my hands 
were of florid colour. Having raised my left hand, I saw it become, within half a 
minute, very pale, and on putting it down after it had been a minute in that 
position, I observed it grow, within a quarter of a minute, much deeper in arteri- 
