230 



COMPARATIVE PHYSIOLOGY. 



near as may be equal to tliat of the animal to be employed in 

 the experiment. This may be ejBfected in a variety of ways, one 

 of the most convenient of which is by means of a vessel con- 

 taining some saturated sodium carbonate or similar solution 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 difiPerent 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 — 



Fi». 198.— Large kymograph, with continuous roll of j)ai)er (Foster). The clock-work 

 machinery 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 convenient position. 



e. g., feeble pulse-beats — are to be indicated, it fails to express 

 tbem, in which case other fluids may be employed. 



It will be noted that when an ordinary cannula is used, 

 inserted 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 an- 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 



