THE BLOOD. 43 



white corpuscles frequently adhere to the inner wall of the 

 capillaries, which the red rarely do ; and while the red glo- 

 bules, in circulating, occupy the centre of each vessel, the 

 white corpuscles are placed between this and the walls of the 

 vessel. 



A difference may also be observed in the relative speed 

 with which the two kinds of corpuscles circulate, the red 

 flowing onwards with greater rapidity than the white. The 

 forces which determine the circulation in the vessels would 

 appear to act only on the red corpuscles, the motion of the 

 white globules being entirely of a secondary and indirect 

 character, it being communicated to them by the edge of the 

 current in the axis of which the red corpuscles move, in the 

 same way as the stones at the bottom of a stream are rolled 

 over and borne onwards by the superincumbent water. 



The cause of the slower motion of the white corpuscles in 

 the capillaries may be thus explained. A greatly retarded 

 motion of the fluid circulating in any vessel or channel is 

 always observed towards the periphral border of the current. 

 This retardation would appear to arise from the resistance 

 which the circulating fluid encounters by coming in contact 

 with the walls of the vessel or sides of the channel through 

 which it flows. 



In what way, however, is the difference in the position in 

 the vessels occupied by the red and white corpuscles to be 

 explained ? why do the former always circulate in the 

 axis of the vessel while the latter are constantly placed 

 outside this ? and what is the inference to be deduced from 

 this difference in their situation ? 



The red corpuscles, as we know, are flattened discs, con- 

 stituted of an elastic and yielding material ; and the white, on 

 the contrary, are globular bodies of a more dense composition 

 and of but little elasticity : now it is very probable that the 

 peculiar form and properties possessed by the coloured cor- 

 puscles of the blood may result in such an adaptation and 

 arrangement of them, the one with the other, that a physical 

 impossibility is presented to their indiscriminate admixture and 

 circulation in the same vessel with the white corpuscles. 



E 3 



