Causes of the Motion of the Blood. 345 



axis of the vessel, will acquire a certain velocity ; then the 

 globule will move round its own axis, in order to acquire 

 its normal velocity in following the centre of the vessel. 

 If of two globules, one is placed in advance of the other in 

 the layer, the former will pursue its course, and the latter 

 will be delayed, and the motions described will be presented. 



The labours of M. Girard upon the flow of liquids in 

 tubes of small diameter have established, in most tubes 

 susceptible of being softened by the liquid moving in them, 

 the existence of a similar layer. The author passed through 

 tubes of very small diameter, liquids holding in suspension 

 opaque bodies ; and, having examined this current by a 

 microscope, he found this layer immoveable, and of a thick- 

 ness much smaller than that obtained by the calculations of 

 Girard. 



Hence, the author concludes, that the blood transported 

 by the vessels of the heart to all parts of the body does not 

 impinge against the coats ; that a layer of serum, by its 

 state of rest, guards the coats from any such effects. Be- 

 sides, we can conceive the importance of this immoveable 

 layer of serum lining the coats of the vessels in the act of 

 nutrition, since the recent experiments of Muller of Berlin 

 have demonstrated, that the fibrin is held in solution by 

 the serum. 



Dr. Poiseuille has farther studied the influence of cold 

 and heat upon this layer of serum. The following experi- 

 ment shews the result. At the temperature of 77°, he 

 examined the circulation in the foot of a frog, and, in the 

 vessel where the foot was placed, he deposited pieces of ice. 

 In the large vessels, the transparent part of the serum 

 obviously increased in thickness; the globules in immediate 

 contact with it moved more slowly ; the three orders of 

 vessels, arteries, capillaries and veins, preserved their 

 diameters sensibly ; the velocity in the capillaries was consi- 

 derably diminished, and in some of these vessels it became 

 insensible; during six or eight minutes, for example, the 

 circulation in the capillaries of the other foot of the frog 

 preserved its normal velocity : and it was not till a quarter 

 of an hour after the submersion of the first foot in the iced 

 water that the velocity of the blood in the second foot, 

 placed in the atmosphere, was diminished, in consequence 



