138 CAPILLARY CIRCULATION. 



taken up with the axial current, and the plasma layer occupies 4 on each side of it 

 (fi^. 116). A great many, but not all, of the colourless corpuscles move in this 

 layer. It is much less distinct in the capillaries. Rud. Wagner stated that it is 

 absent in the finest vessels of the lung and gills, [although Gunning was unable to 

 confirm this statement]. The coloured corpuscles move in the smallest capillaries 

 in single nle one after the other ; in the larger vessels, several corpuscles may 

 move abreast, with a gliding motion, and in their course they may turn over and 

 even be twisted if any obstruction is offered to the blood-stream. As a general 

 rule, in these vessels the movement is uniform, but at a sharp bend of the vessel it 

 may partly be retarded and partly accelerated. Where a vessel divides, not 

 unfrequently a corpuscle remains upon the projecting angle of the division, and is 

 doubled over it so that its ends project into the two branches of the tube. There it 

 may remain for a time, until it is dislodged, when it soon regains its original form on 

 account of its elasticity. Not unfrequently we see a red corpuscle becoming bent 

 where two vessels meet, but on all occasions it rapidly regains its original form. 

 This is a good proof of the elasticity of the coloured corpuscles. The motion 

 of the colourless corpuscles is quite different in character ; they roll directly 

 on the vascular wall, moistened on their peripheral zone by the plasma in 

 Poiseuille's space, their other surface being in contact with the thread of coloured 

 corpuscles in the centre of the stream. Schklarewsky (1868) has shown by 

 physical experiments, that the particles of least specific gravity in all capillaries 

 (e.g., of glass) are pressed toward the wall, while those of greater specific gravity 

 remain in the middle of the stream. [Graphite and particles of carmine were sus 

 pended in water, and caused to circulate through capillary tubes placed under a 

 microscope, when the graphite kept the centre of the stream, and the carmine 

 moved in the layer next the wall of the tube.] 



When the colourless corpuscles reach the wall of the vessel, they must roll along, 

 partly on account of their surface being sticky, whereby they readily adhere to the 

 vessel, and partly because one surface is directed towards the axis of the vessel 

 where the movement is most rapid, and where they receive impulses directly from 

 the rapidly moving coloured blood-corpuscles (Danders). The rolling motion is not 

 always uniform, not unfrequently it is retrograde in direction, which seems to be 

 due to an irregular adhesion to the vascular wall. Their slower movement (10 to 

 12 times slower than the red corpuscles) is partly due to their stickiness, and partly 

 to the fact that, as they are placed near the wall, a large part of their surface lies in 

 the peripheral threads of the fluid, which of course move more slowly (in fact the 

 layer of fluid next the wall is passive p. 91). 



[D. J. Hamilton finds that, when a frog's web is examined in a vertical position, by far the 

 greater proportion of leucocytes float on the upper surface, and only a few on the lower surface, 

 of a small blood-vessel. In experiments to determine why the coloured corpuscles float or glide 

 exclusively in the axial stream, while a great many, but not all, of the leucocytes roll in the 

 peripheral layers. Hamilton ascertained that the nearer the suspended body approaches to the 

 siecific gravity of the liquid in which it is immersed, the more it tends to occupy the centre of 

 the stream. He is of opinion that the phenomenon of the separation of the blood-corpuscles in 

 the circulating fluid is due to the colourless corpuscles being specifically lighter, and the coloured 

 either of the same or of very slightly greater specific gravity, than the blood-plasma. Hamilton 

 controverts the statement of Schklarewsky, and he finds that it is the relative specific gravity 

 of a body which ultimately determines its position in a tube. These experiments point to the 

 immense importance of a due relation subsisting between the specific gravity of the blood-plasma 

 and that of the corpuscles.] 



In the vessels first formed in the incubated egg, as well as in young tadpoles, the movement 

 of the blood from the heart occurs in jerks {Spallanzani, 1768). 



The velocity of the blood-stream is influenced by the diameter of the vessels, which 

 undergo periodic changes of calibre. This change occurs not only in vessels pro- 

 vided with muscular fibres, but also in the capillaries, which vary in diameter, 

 owing to the contraction of the cells composing their walls (p. 96). 



