344 TEXT-BOOK OF PHYSIOLOGY. 



an elastic pressure applied to the outside of an artery is just sufficient to 

 equalize the diastolic pressure within. Inasmuch as these pulsations can 

 be transmitted to, taken up and reproduced by a mercurial column in connec- 

 tion with the pressure appliances, it becomes possible, when the maximum 

 oscillation of the mercurial column is attained, to read off the diastolic 

 pressure. 



With either form of apparatus it becomes necessary to devise a suitable 

 elastic sac or tube enclosed by non-elastic or rigid walls and capable of 

 being made to encircle a finger or an arm, which can in turn be connected 

 with a pressure apparatus, and with a manometer by which any given 

 pressure can be registered. 



One of the best known of the sphygmomanometers is that of Mosso 

 represented in Fig. 160. It consists essentially of rubber capsules, which are 

 contained within metallic tubes and into which two fingers of each hand can 

 be inserted. This system is connected, on the one hand, with a pressure 

 apparatus, and, on the other, with a manometer provided with a scale. A 



FIG. 160. THE SPHYGMOMANOMETER OF Mosso. 



float and writing-pen record the movements of the mercurial column on a 

 moving blackened surface. In using this apparatus the pressure is adjusted 

 to the point at which the mercurial column exhibits the greatest oscillations. 



Mosso's interpretation of the results obtained with the apparatus was 

 that when the greatest oscillations of the mercurial column were taking place, 

 the external pressure was 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 was necessary, there- 

 fore, only to take the readings corresponding to the excursions of the mer- 

 curial column and to determine from them the mean arterial pressure. 



It has been experimentally demonstrated, however, by Howell and Brush 

 that this interpretation, either for this or any similar form of apparatus, is 

 not correct, but that the maximum oscillations take place when the pressure 

 applied to the exterior of the artery is just equal to the pressure within the 



