250 PHYSIOLOGY CHAP. 



extremity, as far as half the forearm, into a glass cylinder, connected 

 on one side with a pressure bottle, on the other with his spring 

 manometer. An ingenious apparatus that is absolutely air-tight 

 fixes the end of the cylinder to the forearm. After the cylinder 

 has been filled with water and the connection with the pressure 

 bottle closed, the ligature is taken off; the blood from the artery 

 then flows into the limb and drives some of the water against the 

 manometer, which records a pressure equivalent to that exerted by 

 the blood streaming into the artery. Since the spring manometer 

 permits only small excursions, the quantity of blood entering the 

 artery of that part of the arm which is enclosed in the cylinder 

 will also be small. Hiirthle takes it to be not more than 10 c.c., 

 which is certainly not sufficient to restore circulation in the vessels 

 of the limb. He therefore concludes that the values recorded by 

 his apparatus represent^ not the simple lateral pressure, but the 

 total arterial pressure (total head) which would obtain if the large 

 artery of the forearm were opened, and directly connected with 

 the manometer. 



Hiirthle has not, up to the present, published any control 

 experiments that justify his conclusions, and there are good 

 reasons for doubting whether he has really succeeded in completely 

 obliterating all the vessels of the forearm, so as to interrupt 

 the circulation in the vessels of the interosseous space. The 

 external pressure is not readily transmitted to this space, since the 

 two bones are connected by strong aponeuroses, which make this 

 cavity a box with rigid walls that yield little to pressure greater 

 even than that of the largest arteries. It is therefore probable, 

 in consequence of the incomplete stoppage of the circulation, that 

 Hlirthle's apparatus does not register the total head, but merely a 

 pressure head which is not of the same value as the lateral pressure 

 of the blood normally circulating in the vessels of the forearm, but 

 increases in proportion with the sudden restriction of current-bed 

 in the greater part of the lirnb that is under investigation. 



From the clinical standpoint these methods of Marey, Mosso, 

 and Hiirthle involve too complicated an apparatus, requiring no 

 little skill on the part of operator as well as patient, to ensure 

 success. Moreover, they only determine the lateral pressure in 

 arteries too small and too remote from the heart to give the 

 physician any adequate expression of the energy with which the 

 heart is acting under various morbid conditions. 



Eiva-Kocci (1896) accordingly invented a simple and easily 

 applied sphygmomanometer, which measures by the manometer 

 the external counter-pressure required to block the progress of the 

 pulse-wave in one of the larger branches of the aorta, e.g. in the 

 brachial artery. The measurements obtained with this instrument, 

 which express the total pressure head (i.e. the lateral pressure plus 

 the velocity head) in the brachial artery, express the values of the 



