LECT. XVII. PRESSURE ON THE BLOOD. 327 



animal. I have always remarked, that when this instru- 

 ment is introduced into the carotid, the column of mercury 

 rises several millimetres when we raise the animal by its 

 hind part, and falls when we place it in an opposite posi- 

 tion. The cause of this difference is evident. We must, 

 therefore, presume, that in all Poiseuille's experiments, in 

 which he compared the pressure of the blood in different 

 vessels, the animal was kept in the same position. 



I cannot pass over in silence the dilatation of arteries at 

 every pulsation. We are indebted to Poiseuille for proving 

 beyond doubt the existence of this phenomenon, which has 

 so great an influence upon the circulation. He laid bare a 

 certain portion of the carotid artery of a living horse, and 

 enclosed it in a metallic tube filled with water. This tube 

 had an opening closed by cork, in the centre of which was 

 fixed a small glass tube. At every contraction of the left 

 ventricle the liquid rose in the tube, and fell again when the 

 contraction ceased. Thus, then, after the dilatation of the 

 artery, occasioned by the impulse of the blood, the arterial 

 coats return to their former state in virtue of their elasticity. 

 Poiseuille endeavoured to measure this elastic force dis- 

 played by the arterial coats; and without adopting his con- 

 clusion, that the force of contraction of the coat exceeds that 

 of the dilating force, it is certain that, when the contraction 

 of the ventricle, the principal force which puts the blood in 

 motion, ceases, the elasticity of thea rterial coats, which 

 recover themselves, also propels the blood, and thus adds 

 to the force of the heart. 



Lastly, I must say a few words respecting the researches 

 which this physiologist, already so frequently quoted, has 

 made on the movement of the blood in the capillaries. 

 Poiseuille observed, in a great number of experiments, that 

 the motion of the blood in the vessels ceases when the heart 

 is raised or bound, and that this movement continued only 



