THE CIRCULATION OF THE BLOOD. 



305 



artery of the dog it ranges from 140 to 160 mm.; in the horse, from 

 1 60 to 170 mm.; in the rabbit, from 90 to 100 mm. In two obser- 

 vations made on human beings during amputation of the limb the 

 pressure was found in the brachial artery of one patient to range from 

 no to 120 mm., and in the anterior tibial of the other patient from 

 no to 160 mm. 



The Estimation of the Blood-pressure in Man. The fore- 

 going method of obtaining the blood-pressure is not of general 

 application to human beings for obvious reasons, hence special 

 instruments have been devised by means of which the pressure may 

 be determined at least approximately without any surgical procedure. 

 Such instruments are termed sphygmomanometers. One of the best 

 is that of Mosso, 

 represented in Fig. 

 142. It consists es- 

 sentially of rubber 

 capsules, contained 

 within metallic 

 tubes and into 

 which two fingers 

 of each hand are 

 inserted. This sys- 

 tem is connected, 

 on the one hand, 

 with a pressure 

 apparatus, and, on 

 the other, with a 

 manometer pro- 

 vided with a scale. 

 A float and writing- 

 pen record the 

 movements of the 



mercurial column on a moving blackened surface. In using this 

 apparatus the pressure is raised to the point at which the mercurial 

 column exhibits the greatest oscillations. 



This sphygmomanometer, as well as the interpretation of the 

 results obtained with it, are based on the theory that the greatest 

 oscillations of the arterial walls, and hence the greatest oscillations 

 of the mercurial column, take place when the external pressure is 

 just equal to the mean arterial pressure, the latter being the mean 

 between the maximum pressure during the systole and the minimum 

 pressure during the diastole of the heart. It is only necessary, there- 

 fore, to take the mean of the readings corresponding to the excursions 

 of the mercurial column and determine from them the mean arterial 

 pressure. 



FIG. 142. THE SPHYGMOMANOMETER OF Mosso. 



