80 THE MECHANISM OF THE CIRCULATION. 



beyond the seat of compression. The reading of the manometer at this point 

 is taken to be equal to that of the arterial pressure. The fallacies in the use 

 of such an instrument are very considerable. It is impossible to avoid com- 

 pressing the vense comites radialis, the skin and surrounding tissues, together 

 Avith the artery, and it is difficult to note the exact moment when the pulse 

 ceases to be perceptible. Experimentally, it has been found that the errors 

 may be as great as 30 to 70 mm. Hg. 



Recently a sphygmomanometer has been invented, which possesses con- 

 siderable accuracy. This instrument consists of a broad armlet, which is 

 strapped round the upper arm. The armlet is formed of a watch spring or 

 stiff leather band, on the inside of which is a bag of thin indiarubber. The 

 rubber bag is connected by a T-tube with an air-pump and a spring manometer. 

 The pressure within the bag is raised until the pulsation in the manometer 

 becomes of maximal excursion. At this point the pressure indicated by the 

 manometer is read, and this pressure is the mean arterial pressure. 1 



The instrument has been tested thus : It was placed round the neck of a 

 dog (excluding the trachea), and simultaneous readings were then taken, both 

 of the pressure in the femoral artery, as indicated by a mercurial manometer, 

 and of the pressure in the carotid arteries, as indicated by the instrument. 

 The maximal excursion of the pulse gave a sharp and accurate index of the 

 mean arterial pressure. 



In the case of healthy young men the pressure in the brachial 

 artery is from 110 to 130 mm. Hg. The pressure in a normal healthy 

 individual is maximal in the vertical and minimal in the horizontal 

 position. 2 A hot bath diminishes, and a cold bath increases, the pressure. 

 Violent exercise raises the tension by 20 mm. Hg ; this rise lasts for 

 some fifteen minutes, and is followed by a fall. Alcohol lowers the 

 arterial tension. During chloroform anaesthesia the tension falls 20 to 

 40 mm. Hg. During deep sleep the pressure may be 20 to 30 mm. Hg 

 lower than in the active waking state. This lowered pressure is not, 

 however, peculiar to the condition of sleep, for just as low a pressure 

 may obtain if the body be kept at rest, and this is so even although the 

 mind be engaged in quiet work. 



It must be remembered, in using such an instrument for the purpose of 

 obtaining a series of accurate comparative results, that certain principles 

 of experiment must be complied with. Thus, the subject of the experiment 

 must be placed uniformly in one position, and the arm which is under 

 investigation must be put at a uniform level in relation to the heart. If 

 this be not done, large errors will arise in the readings, owing to variations in 

 the hydrostatic effect of gravity and the efforts of the vasomotor mechanism 

 to compensate for the same. 



Faivre has directly measured the arterial pressure in man during 

 the amputation of a limb. His results gave a pressure equal to 110 

 to 120 mm. Hg. The arterial pressure is considerably higher in warm- 

 than in cold-blooded animals ; in the former it is at least three times 

 greater. The pressure is independent of the size of the animal, and 

 thus may be as great in the cat as in the horse. 3 In the sheep's 

 embryo (in utcro) the pressure has been found to be from 43 to 84 mm. 

 Hg. 4 The total work of the heart, however, is greater in large than 



1 Maximal pulsation was first used as an index of mean tension in an instrument in- 

 vented by Roy and Adanu (Practitioner, London, 1890). Mosso has recently employed 

 the same index (Arch. ital. dc biol., Turin, 1895, vol. xxiii. p. 196). 



2 In conditions of asthenia the opposite obtains. 



3 Volkmann, " Die Hamodynamik," Leipzig, 1850, S. 177. 



4 Cohnstein and Zuntz, Arch. f. d. ges. Physiol., Bonn. 1884, Bd. xxxiw S. 215. 



