326 CIRCULATION OF BLOOD. LECT. XVII. 



its branches, that produces the well-known phenomenon of 

 the pulse, and which we know to be isochronous with the 

 contraction of the ventricle. 



Adopting the numbers given by the experiments of 

 Poiseuille, we have then the pressure supported by the 

 walls of the heart and of the arteries. This pressure is 

 always equal to the weight of a column of mercury, which 

 has for its base the area of the artery or the superficies of 

 the ventricle, and for its height, that obtained by means of 

 the haBmo-dynamometer. From these data, Poiseuille cal- 

 culated that at the moment of the contraction of the heart 

 the blood is propelled into the aorta of a man, twenty-nine 

 years of age,* exercised against the column of liquid which 

 fills it, supposing this to be at rest a pressure equal to 

 1-971779 kilogrammes [= about 5J Ibs. troy.] In the 

 radial artery this pressure is no more than 15-35 grammes 

 [about 137 grains troy.] By knowing exactly the internal 

 surface of the left ventricle, at the moment of contraction, 

 we can easily calculate the pressure exercised upon these 

 walls at this moment. 



Among the most important results of Poiseuille's experi- 

 ments, I would also mention that which shows the constancy 

 of the variations in the height of the column of mercury in 

 the haBmo-dynamometer, during the respiratory movements. 

 This height is invariably greater during expiration, than 

 during inspiration; and this difference is observed in large 

 as well as in small arteries ; though it is more or less con- 

 siderable in different animals. 



It is proper, also, that I should draw your attention to 

 the fact, that the height of the column of the hsemo-dyna- 

 mometer likewise varies according to the position of the 



* It is assumed that the column of mercury raised in the haemo-dyna- 

 mometer, applied to man, should be 160 millimetres, and the diameter of 

 the aorta 34 millimetres. Note by Matteucci. 



