viii BLOOD-STEEAM : MOVEMENT IN VESSELS 251 



lateral pressure that prevails in the aorta or innominate artery, 

 according as the apparatus is applied to the left or to the 

 right arm. 



Kiva-Eocci's sphygmomanometer is an ingenious modification 

 of the method of v. Basch. The elastic finger-stall is replaced by 

 a hollow rubber ring (made inextensible by a cloth cover) which 

 fits round the arm and is connected with a mercury manometer 

 (Fig. 97). Air is then blown into the hollow ring by an ordinary 

 spray bellows, which becomes inflated and compresses all the 

 vessels of the limb, while the mercury rises in the tube of the 

 manometer. If more air is gradually forced in, so that the 

 mercury rises evenly, there 

 comes a moment at which 

 the radial pulse disappears. 

 The height of the mercury 

 column at that moment 

 represents the total head of 

 pressure supported by the 

 brachial artery during the 

 interruption of the circu- 

 lation in the arm, which 

 value approximates to that 

 of the lateral pressure in 

 the aorta, as demonstrated 

 by experiments with arti- 

 ficial circulation in rubber 

 tubes, or in the brachial 



arteries of 

 body, as well 

 periments On 



P 

 arterieS OI 



rabbits. 



the dead 

 by ex- 



as 



the 

 ClOgS 



Crural FIG. 97. Riva-Rocci's sphygmomanometer. a, Hollow 



and 



rubber ring covered with silk ; m, mercury inano- 

 meter with only one arm ; i, double rubber bellows. 



As the sphygmomanometer of Eiva-Eocci is applied to the 

 upper arm, which has only one central bone, the muscles, when 

 fully relaxed, behave exactly like a fluid, and convey the pressure 

 of the elastic ring perfectly to all the vessels of the upper arm, as 

 was not the case when the apparatus was applied to the forearm. 

 The chief detects of v. Basch's sphygmomanometer thus seem to 

 be excluded. 



In order to obtain utilisable values with the Riva-Rocci apparatus, it is 

 essential that the subject whose pressure is to be measured should be 

 absolutely quiescent. In this way only can the value of the individual minimal 

 pressure be obtained, uninfluenced by the disturbances produced by emotional 

 influences, which vary to a considerable extent in the same person, with the 

 same stimulus. The armlet is fastened preferably to the middle of the right 

 arm by means of the ligature attached from behind to the lower part of the 

 arm, so that it is applied to it like a flat bandage. 



The forearm is bent towards the upper arm, care being taken that all the 



