188 HYDROSTATICS AND HYDRAULICS IN PRACTICAL MEDICINE 
following experiment :—Tying into the external carotid artery of a dog a glass 
tube with an aperture less than one-hundredth of an inch in diameter (so that 
the loss of blood should be insignificant), he directed the fine fountain of blood 
on to a sheet of white paper, which was regularly moved along; in this way 
a tracing was made by the blood itself, which showed that the arterial pressure 
during the systole was double that during diastole. 
From these facts it is evident that we must look to the vaso-motor system 
to explain the cause and cure of a fainting-fit by position of the head, and the 
effects of position upon the blood-flow. When a limb is raised, blood flows 
down by gravity and the veins are relaxed, and we can imagine that an afferent 
stimulus is thus excited which, reflected along the vaso-motor nerves, contracts 
the arteries. If, however, the tissues are kept for a long time ill supplied with 
blood, there is such a demand set up for this fluid that the vessels dilate even in 
spite of the elevated position of the limb, which originally caused anaemia. 
This was illustrated by the well-known experiment of keeping one arm raised 
above the head and the other dependent, the former becoming pale and the 
latter turgid. An elastic band was then rapidly wound round the upper part 
of the elevated arm, and when, after a few minutes, this was removed, the whole 
limb became suffused and redder than its dependent fellow. Illustrations of 
the working of this principle were to be seen in the sequence to the ligature 
of a large artery, the limb at first becoming cold and pale from mechanical 
cutting off of the blood-supply, and then hot and suffused from dilatation of the 
vessels as a result of tissue-starvation. For the same reason, in piles and 
affections of the pelvic viscera raising the lower limbs gave great relief, for the 
contraction of the arteries was not limited to the limbs, but spread to the vessels 
of the pelvis. As a further instance, he mentioned the case of a man who 
suffered extreme pain in his testicles when in an upright position, but was imme- 
diately relieved by sitting down and putting up his feet. Raising the arms 
above the head too was a well-known means of stopping epistaxis, and suc- 
ceeded, because the contraction of the arteries of the arms spread by sympathy 
to those of the Schneiderian mucous membrane. Mr. Lister concluded by stating 
that he hoped that the few illustrations he had been able to give would show 
the value of a knowledge of hydrostatics and hydraulics, especially as indicating 
where physiological effects were produced by other than physical causes. 
