118 THE CAPILLARY CIRCULATION. [BOOK i. 



capillary narrower than itself, or when it becomes temporarily lodged 

 at the angle between two diverging channels. The small mam- 

 malian corpuscles rotate largely as they are driven along. 



In the web of the frog's foot the average velocity with which the 

 corpuscles move may be put down as about half a millimetre in a 

 second. In the human retina, the velocity of the capillary flow 

 has, by indirect methods, been estimated at "75 mm. per sec. The 

 movement of the blood in the capillaries is very slow, compared 

 with that in the arteries or even in the veins. 



In the larger capillaries, and especially in the small arteries and 

 veins which permit the passage of several corpuscles abreast, it is 

 observed that the red corpuscles run in the middle of the channel, 

 forming a coloured core, between which and the sides of the 

 vessel all round is a layer, which has been called the ' inert layer,' 

 or better the 'plasmatic layer,' containing no red corpuscles. This 

 division into a plasmatic layer and an axial stream is due to the 

 fact that in any stream passing through a closed channel the 

 friction is greatest at the immediate 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 plasmatic layer, 

 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, and often in jerks. 

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

 corpuscles in the plasmatic layer, 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 plasmatic 

 layer has been attributed to their being specifically lighter than 

 the red corpuscles, it being affirmed that 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. This however has been disputed, 

 and the phenomenon explained by the white corpuscles being dis- 

 tinctly more adhesive than the red, as is seen by the manner in 

 which they become fixed to the glass slide and cover-slip when 

 a drop of blood is mounted for microscopical examination. By 

 reason of this adhesiveness which possibly may vary with the 

 varying nutritive conditions of the corpuscles and of the blood- 

 vessels, the white corpuscles, it is urged, become temporarily 

 attached to the walls of the vessel, and consequently appear in 

 the plasmatic layer. 



The resistance to the flow of blood thus caused by the friction 

 generated in so many minute passages, is one of the most important 

 physical facts in the circulation. In the large arteries the friction 

 is small; it increases as they divide, and receives a very great 



