THE CAPILLARIES. 175 



of the movements of the red blood- corpuscles, which move 

 in the centre of the stream. At the circumference of the 

 stream, in contact with the walls of the vessel, and adhering 

 to them, there is a layer of liquor sanguinis which appears 

 to be motionless. The existence of this still layer, as it is 

 termed, is inferred, both from the general fact that such an 

 one exists in all fine tubes traversed by fluid, and from 

 what can be seen in watching the movements of the blood- 

 corpuscles. The red corpuscles occupy the middle of the 

 stream and move with comparative rapidity ; the colourless 

 lymph- corpuscles run much more slowly by the walls of 

 the vessel ; while next to the wall there is often a trans- 

 parent space in which the fluid appears to be at rest ; for 

 if any of the corpuscles happen to be forced within it, they 

 move more slowly than before, rolling lazily along the side 

 of the vessel, and often adhering to its wall. Part of this 

 slow movement of the pale corpuscles and their occasional 

 stoppage may be due, as E. H. "Weber has suggested, to 

 their having a natural tendency to adhere to the walls of 

 the vessels. Sometimes, indeed, when the motion of the 

 blood is not strong, many of the white corpuscles collect 

 in a capillary vessel, and for a time entirely prevent the 

 passage of the red corpuscles. But there is no doubt that 

 such a still layer of liquor sanguinis exists next the walls 

 of the vessels, and it is between this and the tissues around 

 the vessels that those interchanges of particles take place 

 which ensue in nutrition, secretion and absorption by the 

 blood vessels ; interchanges which are probably facilitated 

 by the tranquillity of the fluids between which they are 

 effected. 



There is no reason for supposing that under ordinary 

 circumstances, in health, either the pale or red corpuscles 

 ever remain permanently fixed to the wall of the vessels 

 and become united with them, or that they pass through 

 the walls to enter into the structure of the tissues. Their 

 office appears to be fulfilled exclusively within the vessels. 



