248 PHYSIOLOGY CHAP. 



he says, " we learn theoretically that the vessels of the immersed 

 limb are wholly relaxed, and that their walls fluctuate as it were 

 indifferently between the internal pressure of the blood and the 

 external pressure of the water. The pressure of the blood, there- 

 fore, acts as though it were exerted directly upon the manometer." 

 This is as much as to say that we then obtain the true measure of 

 the lateral pressure exerted by the blood upon the arteries of the 

 finger. The arterial walls at that time must be in a state of elastic 

 equilibrium, since the internal force which makes for their dis- 

 tension is completely counterbalanced by the external force which 

 makes for their compression. If this criterion is applied to the 

 results shown in Fig. 94, the average pressure of the arteries of the 

 forearm in man is found equal to 8 cm. Hg, since the pulsations 

 reach their maximal amplitude when a counter-pressure of 8 cm. 

 Hg is put upon them. 



Mosso continued and developed these investigations of Marey. 

 His sphygmomanometer is a modification of that shown in the 

 previous figure. He makes the counter -pressure act on four 

 fingers instead of on one, in order to obtain the total pulsations of a 

 larger number of arteries, and to record the tracings with a Ludwig's 

 mercury manometer. The following tracings, obtained by Mosso 

 and Colombo, are very instructive, the effects of various degrees 

 of compression of the four fingers under normal conditions being 

 compared with the tracings obtained from the same person after 

 a warm bath (Fig. 96). 



As might be expected, the average pressure sinks on account 

 of the relaxation of the vessels due to the warm bath, and falls to 

 20 mm. Hg. At the same time the height of the pulsations after 

 the bath increases considerably, showing that they do not depend 

 on the internal pressure of the blood, but are in inverse ratio with 

 the degree of tonic contraction of the vessels. Evidence for this is 

 found in the fact that when the vessels of the fingers are strongly 

 contracted (as often happens in winter), it is impossible to obtain 

 the slightest sign of pulsation with Mosso's sphygmomanometer. 

 This is a disadvantage which renders the instrument applicable 

 only to a limited number of persons, and which is avoided by 

 returning to the original method of Marey, i.e. that of applying 

 compression to the whole forearm. 



Starting from Marey's original method, Hlirthle (1896) intro- 

 duced some interesting modifications which deserve notice. 



Both in Marey's and in Mosso's apparatus it is necessary in 

 measuring the lateral pressure prevailing in the arteries of the part 

 of the body examined, to increase or lower the external counter- 

 pressure repeatedly, in order to discover at what strength of counter- 

 pressure the maximal pulsations are obtained. With Hiirthle's ap- 

 paratus, on the contrary, the observation is continuous, and there is no 

 need to vary the counter-pressure applied at the outset to the forearm. 



