THE CIRCULATION OF THE BLOOD. IAT 
The student may very simply illustrate this matter for him- 
self. By tapping sharply against a pipe through which a 
stream is flowing slowly and quietly,a wave may be seen to 
arise and pass with considerable velocity along the moving 
water, and with a speed far in excess of the rapidity of the main 
current. When the left ventricle throws its six ounces of blood 
into vessels already full to distention, there must be consider- 
able concussion in consequence of the rapid and forcible nature 
of the cardiac systole, and this gives rise to a wave in the blood 
which, as it passes along its surface, causes each part of every 
artery in succession to respond by an elevation above the gen- 
eral level, and it is this which the finger feels when laid upon 
an artery. ei 
That there is considerable distention of the arterial system 
with each pulse may be realized in various ways, as by watch- 
ing and feeling an artery laid bare in its course, or in very 
thin or very old people, and by noticing the jerking of one leg 
crossed over the other, by which method in fact the pulse-rate 
may be ascertained. And that not only the whole body but 
the entire room in which a person sits is thrown into vibration 
by the heart’s beat, may be learned by the use of a telescope to 
observe objects in the room, which may thus be seen to be in 
motion. 
Features of an Arterial Pulse-Tracing.—In order to judge of the 
nature of arterial tracings, it is important that the circum- 
stances under which they are obtained should be known. 
The movements of the vessel wall in most mammals suit- 
Fic. 217.—Marey’s improved sphygmograph arranged for taking a tracing. A, steel spring ; 
B, first fears Cc, Writing-lever: Crits free wring end ; D, screw for bringing Bin con- 
tact with C; G, slide with smoked paper; H, clock-work; L, screw for increasing the 
pressure ; M, dial indicating the amount of pressure; K, K, straps for fixing the instra- 
ment to the arm, and the latter to the double-inclined plane or support (Byrom Bramwell). 
