286 CIRCULATION. 



the tube to a certain distance, called capillary attraction, be- 

 comes an obstacle to the passage of fluid in obedience to 

 pressure. Of course, as the diameter of the tube is reduced, 

 this force becomes relatively increased, for a larger propor- 

 tion of the liquid contents is brought in contact with it. 

 When we come to the smallest arteries and veins, and still 

 more the capillaries, the capillary attraction is sufficient to 

 produce the immovable layer, called the " still layer " by 

 many physiologists, and the liquid only moves in the central 

 portion. The plasma occupies the position next the walls of 

 the vessels, for it is this portion of the blood which is capable 

 of wetting the tubes. The transparent layer was observed 

 by Halpighi, Haller, and all who have described the capillary 

 circulation. Poiseuille recognized its true relation to the 

 blood -current, and explained the phenomenon of the still 

 layer by physical laws, which had been previously established 

 with regard to the flow of liquids in tubes of the diameter of 

 from ^ T to -J of an inch, but which he had succeeded in apply- 

 ing to tubes of the diameter of the capillaries. 1 



A red corpuscle occasionally becomes involved in the still 

 layer, when it moves slowly, turning over and over, or even 

 remains stationary for a time, until it is taken up again and 

 carried along with the central current. A few white corpus- 

 cles are constantly seen in this layer. They move along 

 slowly, and apparently have a tendency to adhere to the 

 walls of the vessel. This is due to the adhesive character of 

 the surface of the white corpuscles as compared with the red, 

 which can easily be observed in examining a drop of blood 

 between glass surfaces, the red corpuscles moving about with 

 great facility, while the white have a tendency to adhere. 



Great differences exist in the character of the flow of 

 blood in the three varieties of vessels which are under obser- 

 vation. In the arterioles, which may be distinguished from 

 the capillaries by their size and the presence of the muscular 



1 POISEUILLE, Recherches mr les Causes du Mouvement du Sang dans les Vais- 

 seaux Capillaires, p. 144 



