THE CIRCULATION OF THE BLOOD. 931 
bottle ; ca, the carotid, in which is placed the canula, and below the latter a forceps, which 
may be removed when the blood-pressure is to be actually measured. The registration of 
the height, variation, etc., of blood-pressure, is best made on a continuous roll of paper, as 
seen in Fig 208. 
the experiment. This may be effected in a variety of ways, 
one of the most convenient of which is by means of a vessel 
containing some saturated sodium carbonate or similar solu- 
tion in connection with the manometer. 
It is important that the pressure should express itself as 
directly and truthfully on the mercury of the manometer as 
possible, hence the employment of a tube with rigid walls, yet 
capable of being bent readily in different directions for the sake 
of convenience. 
Mercury, on account of its inertia, is not free from objec- 
tion ; and when very delicate variations in the blood-pressure— 
e. g., feeble pulse-beats—are to be indicated, it fails to expregs 
them, in which case other fluids may be employed. 
Fie. 208.—Large kymograph, with continuous roll of paper (Foster). The clock-work ma- 
chinery unrolls the paper from the roll C, carries it smoothly over the cylinder B, and then 
winds it up into the roll A. Two electro-magnetic markers are seen in position recording 
intervals of time on the moving roll of paper. A manometer may be fixed in any con- 
venient position. 
It will be noted that when an ordinary cannula is used, in- 
serted as it is lengthwise into the blood-vessel, the pressure 
recorded is not that on the side of the vessel into which it is 
inserted as when a -- piece is used, but of the vessel, of which 
the one in question is a branch. The blood-pressure, in the 
main arterial trunk for example, must depend largely on the 
force of the heart-beat ; consequently it would be expected, and it 
