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TEXT-BOOK OF PHYSIOLOGY 



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- 

 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 the weight of 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 



FIG. 152. DIAGRAM TO SHOW THE RELATION OF THE MERCURIAL MANOMETER TO THE ARTERY 

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

 RECORDING BLOOD-PRESSURE. 



artery (Fig. 152). 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 

 sp. gr. 1060, and under a pressure approximately equal to that in the vessel 

 of the animal as determined in previous experiments. When communication 

 is established between the vessel and the cannula, the mercurial column 

 adjusts itself to the pressure in the artery and at once exhibits the same 

 cardiac oscillations and respiratory undulations as did the column of blood 

 in the previous experiment. 



The height of the mercurial column kept in equilibrium by the pressure 

 of the blood within, and the pressure of air without the vessel is that 

 between the lower level of the mercury in the proximal, and the higher level 

 in the distal limb of the manometer, both of which can be read off on a scale 

 placed between the two limbs. 



