328 CIRCULATION OF BLOOD. LrECT. XVII, 



for a few minutes, on account of the diminution of volume, 

 and of the kind of contraction which the elastic coats of the 

 vessels suffer when the blood ceases to be propelled by the 

 heart. By the aid of the microscope there was seen an im- 

 moveabJe layer of serum, adherent to the coats of the ves- 

 sel ; and the liquid blood thus moves in this tube, formed 

 by its own substance. Poiseuille examined the passage of 

 the same liquid, both in a glass capillary tube, and in a ca- 

 pillary blood-vessel of a living or dead animal, and found 

 that it followed the same laws in all cases. This fact as- 

 suredly proves, that in these various cases the liquid really 

 circulates in a tube always formed of the same matter; that 

 is to say, of an immoveable and adherent liquid layer, and 

 which is the same as the liquid which flows through, what- 

 ever may be the material of the tube. It is curious to ob- 

 serve that the capillary circulation continues uninfluenced 

 either by a vacuum or by a pressure of eight or ten atmo- 

 spheres. 



Having explained to you, as far as the limits of this course 

 will permit me, the most accurate and conclusive experiments 

 upon the various questions relating to the sanguineous cir- 

 culation, we have now all the necessary elements for giving 

 an account of the mechanism of this function. 



I consider it useless to detain you longer for the purpose 

 of experimentally demonstrating, that the contraction of the 

 heart and the elasticity of the coats of the vessels, especially 

 of the arteries, are the principal powers of the circulation. 

 If we tie an artery in a living animal, the vessel almost en- 

 tirely empties itself of blood, and the circulation continues 

 in it only for a short space of time. The contrary happens 

 with the ligature of a vein ; for in that case the blood soon 

 accumulates, and the vein swells below the ligature. I 

 shall confine myself to the relation of a single experiment 

 by Magendie. The crural artery and vein of a dog were 



