THE CIRCULATION OF THE BLOOD. 



333 



propagated under normal conditions beyond the arterial system. The 

 height to which it rises is but slight as compared with that in the arterial 

 tube. The pressure in both vessels is thus recorded in millimeters of blood. 

 Strictly speaking the pressure thus obtained does not represent the lateral 

 pressure in the carotid artery but in the vessel from which it arises. The 

 central end of the carotid is, under the circumstances, but a continuation 

 of the cannula and the pressure thus obtained is the lateral pressure of either 

 the innominate artery or the aorta as the case may be. In order to obtain the 

 lateral pressure in the carotid or any other artery it is only necessary to take 

 the end pressure of any one of its branches or what amounts to the same 

 thing, to divide the vessel and insert the horizontal portion of a T-shaped 

 tube into the central and distal ends through which the blood can con- 



B.P.TRACING 



ABSCISSA..- 



FIG. 154. DIAGRAM TO SHOW THE RELATION or THE MERCURIAL MANOMETER TO THE ARTERY, 

 ON ONE HAND, AND TO THE RECORDING CYLINDER, ON THE OTHER HAND, WHEN ARRANGED FOR 

 RECORDING BLOOD-PRESSURE. 



tinue to flow, and to connect the vertical portion with a vertical pressure 

 tube or with a mercurial manometer. The absolute pressure on any given 

 unit of vessel surface e.g., i sq. mm. is obtained by multiplying the height 

 of the column, expressed in millimeters, by the unit of surface, and then 

 determining the weight of this mass of blood. Thus if the height of the 

 column of blood in the carotid artery tube is 2000 mm., then the pressure on 

 i sq. mm. is 2000 mm. of blood. The weight of 2000 c.mm. of blood is equal 

 to 2.1 grams. 



The Arterial Blood-pressure. For accurate and long-continued ob- 

 servation the arterial blood-pressure is more conveniently studied by means 

 of a U-shaped tube (a manometer) partially filled with mercury. One limb 

 of the manometer is connected by means of a tube and a cannula with an 

 artery (Fig. 154). For the purpose of retarding coagulation of the blood and 

 for preventing the escape of a large volume of blood from the vessels, the 

 system is filled with a solution of carbonate of soda of sp. gr. 1060, 55.8 

 grams per 1000 c.c., or a 25 per cent, solution of magnesium sulphate of 



