144 



THE BLOOD AND ITS CIRCULATION. 



These differences in the lateral pressure effected by the 

 blood upon the walls of the vessels through which it passes 



may be more correctly ascer- 

 tained by placing different 

 parts of the circulatory sys- 

 tem in communication with 

 a manometric apparatus, 

 called, when applied to this 

 special use, a hemodynamo- 

 met&r. The first hemodyna- 

 mometer, employed by Hales, 

 consisted of a long tube, 

 which this physiologist in- 

 troduced into a vessel, and 

 in which the blood rose to a 

 height proportioned to the 

 pressure. This instrument 

 has been greatly improved, 

 and a mercurial manometer 

 is now employed, in which, 

 in order to avoid the coagu- 

 lation of the blood, the col- 

 umn of blood is separated 

 from the mercury by a col- 

 umn of some alkaline solu- 

 tion (solution of carbonate 

 of soda), which prevents the 

 too speedy consolidation of 

 the fibrine (Fig. 43). 



The pressure of the at- 

 mosphere for the larger arte- 

 ries has thus been found to 

 be about one-fourth; for those which are farther from the 

 heart, as the humeral artery, one-sixth, and so on. In the 



* This instrument consists of a thick, heavy glass bottle. At T is a tube, 

 open at one end : the other extremity of the tube leaves the bottle, and is bent 

 upwards, receiving at n a graduated glass tube (T). The lower part of the bottle 

 and the beginning of the graduated tube are filled with mercury. 



The upper part of the bottle is closed by a stopper containing a tube (<), which 

 is joined to a metal tube c. The latter passes into the vessel in which the pres- 

 sure is to be measured. 



When the instrument is in action the whole upper part of the apparatus C, c, <, 

 is filled with a solution of bicarbonate of soda, in order to prevent the coagula- 

 tion of the blood. The pressure effected by the blood upon the surface of the 

 mercury is communicated through the opening T to the mercury in the gradu- 

 ated tube, and by this means the tension of the blood is measured. 



This instrument (Magendie's cardiometer) has, over the manometers usually 



Fig. 43. Hempdynamometer (or car- 

 diometer).* 



