THE CIRCULATION OF THE BLOOD. 229 



Piu. 197.— Apparatus used in making a blood-pressure experiment (after Foster), p. ft, 

 pressure-oottle, elevated so as to raise the pressure several inches of mercury, as 

 seen in the manometer (m) below. It contains a saturated solution of sodium car- 

 bonate; T. t, rubber tube connecting the jib with the leaden tube; /. t, tube made 

 of lead, so as to be pliable, yet have rigid walls; s. e, a stop-cock, the top of which 

 is removable, to allow escape of bubbles of air; jy, the pen, writing ou the roll of 

 paper, r. The former floats on the mercury; m, the manometer, the shaded por- 

 tion of the bent tube denoting the mercury, the rest is filled with a fluid unfavor- 

 able to the coagnlation of theT)lood, and derived from the pressure-bottle; ca, the 

 carotid, in which is placed the cannula, 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. 198. 



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 inad- 

 equate when exact measurements are to be made or the results 

 to be recorded for permanent preservation ; hence apparatus of 

 a somewhat elaborate kind has been devised to accomplish 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 experiments 

 for a considerable time, instead of a single cylinder, a series so 

 connected as to provide a practically endless roll of paper (Fig. 

 198) is employed. 



When, in the sort of experiments referred to above, the 

 height of the fluid used in the glass tube to prevent coagular 

 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 



