CHAP, iv.] THE VASCULAR MECHANISM. 335 



change between the blood and the tissues which is the main fact 

 of the circulation. 



If the web of the frog's foot, or better still if some transparent 

 tissue of a mammal be watched under the microscope, it will be ob- 

 served that, while in the small capillaries the corpuscles are pressed 

 through the channel in single file, one after the other, each corpuscle 

 as it passes occupying the whole bore of the capillary, in the larger 

 capillaries (of the mammal), and especially in the small arteries 

 and veins which permit the passage of more than one corpuscle 

 abreast, the red corpuscles run in the middle of the channel, forming 

 a coloured core, between which and the sides of the vessels all 

 round is a colourless layer, containing no red corpuscles, called 

 the 'plasma tic layer' or 'peripheral zone.' This division into a 

 peripheral zone and an axial stream is due to the fact that in any 

 stream passing through a closed channel the friction is greatest 

 at the sides, and diminishes towards the axis. The corpuscles 

 pass where the friction is least, in the axis. A quite similar axial 

 core is seen when any fine particles are driven with a sufficient 

 velocity in a stream of fluid through a narrow tube. As the 

 velocity is diminished the axial core becomes less marked and 

 disappears. 



In the peripheral zone, especially in that of the veins, are 

 frequently seen white corpuscles, sometimes clinging to the sides 

 of the vessel, sometimes rolling slowly along, and in general moving 

 irregularly, stopping for a while and then suddenly moving on. 

 The greater the velocity of the flow of blood, the fewer the white 

 corpuscles in the peripheral zone, and with a very rapid flow they, 

 as well as the red corpuscles, may be all confined to the axial 

 stream. The presence of the white corpuscles in the peripheral 

 zone has been attributed to their being specifically lighter than 

 the red corpuscles, since when fine particles of two kinds, one lighter 

 than the other, are driven through a narrow tube, the heavier 

 particles flow in the axis and the lighter in the more peripheral 

 portions of the stream. But, besides this, the white corpuscles 

 have a greater tendency to adhere to surfaces than have the red, 

 as is seen by the manner in which the former become fixed to 

 the glass slide and cover-slip when a drop of blood is mounted 

 for microscopical examination. They probably thus adhere by 

 virtue of the amoeboid movements of their protoplasm, so that the 

 adhesion is to be considered not so much a mere physical as a 

 physiological process, and hence may be expected to vary with the 

 varying nutritive conditions of the corpuscles and of the blood 

 vessels. Thus while the appearance of the white corpuscles in the 

 peripheral zone may be due to their lightness, their temporary 

 attachment to the sides of the vessels and characteristic progression 

 is the result of their power to adhere ; and as we shall presently 

 see their amoeboid movements may carry them on beyond mere 

 adhesion. 



