THE CIRCULATION OF THE BLOOD. 929 
main, fed from an elevated reservoir, bursts, or a hydrant is 
opened, is itself a proof of the existence of blood-pressure, and 
is a crude measure of the amount of the pressure. 
One of the simplest and most impressive ways of demon- 
strating blood-pressure is to connect the carotid, femoral, or 
other large artery of an animal by means of a small glass tube 
(drawn out in a peculiar manner to favor insertion and reten- 
tion by ligature in the vessel), known as a cannula, by rubber 
tubing, with a long glass rod of bore approaching that of the 
artery opened, into which the blood is allowed to flow through 
the above-mentioned connections, while it is maintained in a 
vertical position. 
To prevent the rapid coagulation of the blood in such ex- 
periments, it is customary to fill the cannula and other tubes 
to a certain extent, at least, with a solution of some salt that 
tends to retard coagulation, such as sodium carbonate or bicar- 
bonate, magnesium sulphate, etc. If other connections are 
made in a similar way with smaller arteries and veins, it may 
be seen that the height of the respective columns, representing 
the blood-pressure, varies in each and in accordance with ex- 
pectations, 
While all the essential facts of blood-pressure and many 
others may be illustrated by the above simple methods, it is 
inadequate when exact measurements are to be made or the 
results to be recorded for permanent preservation; hence appa- 
ratus of a somewhat elaborate kind has been devised to accom- 
plish these purposes. 
The graphic methods are substantially those already ex- 
plained in connection with the physiology of muscle; but, 
since it is often desirable to maintain blood-pressure experi- 
ments for a considerable time, instead of a single cylinder, a 
series so connected as to provide a practically endless roll of 
paper (Fig. 208) is employed. 
When, in the sort of experiments referred to above, the 
height of the fluid used in the glass tube to prevent coagula- 
tion just suffices to prevent outflow from the artery into the 
connections, we have, of course, in this a measure of the blood- 
pressure; however, it is convenient in most instances to use 
mercury, contained in a glass tube bent in the form of a U, for 
a measure, as shown in the subjoined illustration. It is also 
desirable, in order to prevent outflow of the blood into the 
apparatus, to get up a pressure in the U-tube or manometer as 
near as may be equal to that of the animal to be employed in 
