of Edinburgh, Session 1881-82. 
871 
the peripheral stream, but on the upper surface of the vessel. From 
a number of observations of blood-vessels in the frog’s web, 
examined in this position, it was found that for every 13 leucocytes 
which are seen running along the lower surface, there is an 
average of 92 on the upper. In fact, the only time, apparently, 
in which a leucocyte gets to the lower surface, is in passing round 
a curved capillary, where, in changing its position, the stream of 
coloured corpuscles prevents it from gaining the upper surface of 
the vessel for some distance. If followed along the stream, suffi- 
ciently far, such a leucocyte is eventually found to make its way 
through the stream of coloured blood-corpuscles and to gain the 
upper surface of the vessel. 
If the freshly-shed blood of a triton he passed through a 
capillary glass tube, in the horizontal position, or examined with 
the microscope inclined as in the above observation, the leucocytes 
are found, almost without exception, to run on the upper surface of 
the tube, the hsemocytes in the middle of the stream ; and, when 
the circulation in the tube ceases, the leucocytes remain on the 
upper surface, while the coloured blood-corpuscles have a slight 
tendency to sink to the bottom of the tube. 
If a capillary vessel, capable of admitting only a single file of 
blood-corpuscles, be examined in the upright position, the colourless 
corpuscles are still found to be pressed against the upper surface of 
the vessel and to rotate, while the coloured pass along without 
pressing against either the upper or lower surface, and have, even 
here, a gliding motion. 
From these facts the conclusion is drawn that the leucocytes are 
considerably lighter than the coloured blood-corpuscles in blood 
circulating through blood-vessels or in freshly -drawn blood. The 
coloured blood-corpuscles must also be in circulating blood , (as 
proved by the following experiments) of almost exactly the same 
specific gravity as the blood-plasma. If any divergence between 
the two exists the coloured blood- corpuscles are slightly the heavier. 
The colourless, on the other hand, must be specifically consider- 
ably lighter than the blood-plasma in the living condition. 
The reason of the colourless corpuscles not all coming to the 
upper surface when blood is freshly drawn is (as shown experi- 
mentally), that the coloured blood-corpuscles are in so much greater 
VOL. xi. 3 B 
