174 PHYSIOLOGY CHAP. 



In all the vessels a sharp delimitation can be seen under the 

 microscope, in the form of two parallel dark lines, which represent 

 the walls of the vessels. In the most transparent and superficial 

 parts, the structure of the vessel walls can also be made out to a 

 certain extent below the tissues that cover them. 



The movement of the blood within the vessels, which is visible 

 by means of the transported corpuscles, is continuous, and is always 

 in the same direction, save in a few branches of the capillary net- 

 work, in which a temporary block, due to the accumulation of 

 blood -corpuscles or a transitory reversal of the current, can 

 occasionally be detected (Spallanzani). 



In certain vessels the current is centrifugal, that is, it sets 

 from the greater trunks to the lesser ramifications. In others it is 

 centripetal, i.e. from the lesser ramifications to the greater trunks. 

 The former are evidently arterial vessels, the latter venous. 



In the arteries the current is continuous, with rhythmical 

 accelerations ; in the veins it is continuous and uniform ; in the 

 capillary network it is irregular, subject to impediments, arrests, 

 deviations, or accelerations, according as the blood corpuscles are 

 dispersed or accumulated. 



In the medium vessels, arterial as well as venous, a more rapid 

 axial, and a far slower peripheral current can be distinguished 

 (Poiseuille, 1834). The erythrocytes move compactly along the 

 axial stream, and between them and the vessel walls on both sides 

 a thin streak of clear plasma is plainly visible, in which the leuco- 

 cytes move at irregular distances from each other, ten to twelve 

 times slower than the erythrocytes. The diameter of this clear 

 layer, which is occupied by plasma and leucocytes, and its relation 

 to the diameter of the axial current, varies considerably in the 

 different vessels, even in those of equal cross-section. 



K. Wagner described as characteristic of the blood-stream in 

 the lungs and gills, a complete absence or excessive tenuity of the 

 parietal stratum, and lack of separation between leucocytes and 

 the erythrocytes the time required for such separation being cut 

 short, owing to the greater current velocity and less extensive path 

 of the lesser circulation. 



Poiseuille designated the parietal layer as the stratum 

 adhesivum, and considered it to be immobile or capable only of very 

 slow motion. From the theoretical standpoint, however, it is only 

 the thinnest stratum of plasma immediately bathing the internal 

 walls of the vessels that can be termed immobile. There can be 

 no doubt that the separation of the slower peripheral from the more 

 rapid axial current is a phenomenon of adhesion and of internal 

 friction caused by the viscosity of the blood plasma. Hydrodynamic 

 observations on the nature of the movement of fluids in tubes have 

 determined that the velocity of motion is increased for the axial 

 portion of the current, and decreases gradually from the central to 



