CHAPTER II
THE LAW OF POISEUILLE
Experimental Verification.—Prior to 1842 it had not been
established as a fact that the movement of the blood through the
capillaries has its origin solely in the contractions of the heart.
There were theories current that the capillaries themselves
caused the flow of blood or that the corpuscles were instrumental
in producing it. Poiseuille reasoned that if the lengths and
diameters of the capillaries are different in the various warm-
blooded animals and if the pressure and temperature of the blood
vary in different parts of the body, light might be thrown upon
the problem by investigating the effects upon the rate of flow
in capillary tubes of changes in (1) pressure, (2) length of capil-
lary, (3) diameter of capillary, and (4) temperature.
The results of Poiseuille's experiments were of a more funda-
mental character than he anticipated for they proved that the
conditions of capillary flow are much simpler than those in the
wide tubes which had previously been employed, and by his
experiments the laws of viscous flow became established. Not
only did Poiseuille perform experiments which resulted in the
law which bears his name, and therefore have affected all subse-
quent work, but he measured the efflux times of water by the
absolute method taking elaborate precautions to insure accuracy,
and using capillaries of various lengths and diameters which are
equivalent to separate instruments—in all over forty in number.
Thus one is justified in studying his work in considerable detail,
not only for its historic interest, but on account of its bearing
upon questions which will arise later. In the Appendix his
measurements are reproduced in full.
In Fig. 1 is shown the most essential part of the apparatus of
Poiseuille. It consists of a horizontal glass capillary d joined
to the bulb, whose volume between the marks c and e was accu-
rately determined. The bulb is connected above with a tube
which leads to (1) a 60-1 reservoir for keeping the pressure of the
air within the apparatus constant, (2) a manometer, filled with