PRESSURE MEASUREMENTS IN MAN 213 



a recording apparatus. This instrument obviates the errors which might 

 be caused by the inertia of the mercury in the mercurial manometer; it also 

 shows in more detail the variations of the blood pressure in the vessel during 

 and after each individual beat of the heart. 



As regards the actual amount of blood pressure, from observations which 

 have been made by means of the mercurial manometer, it has been found 

 that the pressure of blood in the carotid of a rabbit is capable of supporting 

 a column of 90 to 120 mm. of mercury; in the dog 100 to 175 mm.; in the horse 

 152 to 200 mm.; and in man the pressure is estimated to be about the same 

 as in the dog. To measure the absolute amount of this pressure in any 

 artery multiply the area of its transverse section by the height of the column 

 of mercury which is already known to be supported by the blood pressure 

 in any part of the arterial system. The weight of a column of mercury thus 

 found will represent the absolute pressure of the blood. Calculated in this 

 way, the blood pressure in the human aorta is equal to i .93 kilogrammeters; 

 that in the aorta of the horse being 5 . 2 kilogrammeters; and that in the radial 

 artery at the human wrist only o . 08 kilogrammeter. Supposing the muscu- 

 lar power of the right ventricle to be one-fourth that of the left, the blood 

 pressure in the pulmonary artery will be only 0.5 kilogrammeter. The 

 amounts above stated represent the arterial tension at the time of the 

 ventricular contraction. 



The arterial pressure is greatest at the beginning of the aorta, and de- 

 creases toward the capillaries. It is greatest in the arteries at the period of 

 the ventricular systole and least during the diastole. The blood pressure 

 gradually lessens as we proceed from the arteries near the heart to those 

 more remote, and again from these to the capillaries, as it does also from 

 the capillaries along the veins to the right auricle. 



Arterial Blood Pressure Measurements in Man. A number of 

 instruments have been devised for estimating blood pressure in man for 

 clinical purposes. Some of these, though excellent in principle, are too com- 

 plicated for general use. The first simple and approximately accurate form 

 of apparatus was that devised by Riva-Rocci in 1896. This has been modi- 

 fied and improved in minor points since, but the principles of the original 

 instrument remain practically the same. 



In brief, the apparatus, figure 188, consists of an elastic tube ending in a 

 rubber bag which can be adjusted about the arm or forearm, and a mercury 

 manometer connected with this tube and also with some form of air pump 

 used for inflating the tube about the arm and thus exerting pressure upon 

 its blood vessels. The elastic tube is covered by some inelastic tissue, usu- 

 ally a leather cuff, in order that the inflation of the bag may cause the full in- 

 crease of pressure to be exerted upon the encased arm. By inflating the bag 

 until the pulse at the wrist just disappears, and reading the height of the 

 column of mercury in the manometer, the maximum or systolic pressure is 



