TISSUES OF THE BODY. 383 



rally in proportion to the strength of the lens employed. In the minute 

 arteries the characteristic pulsation may be recognised, while in the capil- 

 laries a sluggish and more even flow is observed. The blood in the veins 

 is seen also to move along steadily, but again slightly increased in pace. 

 In the large tubes the oval blood-cells are driven along end foremost, 

 sometimes side by side, or one over 

 the other, and in the more consider- 

 able arterial twigs are frequently 

 seen twisting and whirling in rapid 

 motion. The internal surface, how- 

 ever, of such a vessel, of some what con- 

 siderable calibre (a), is not touched 

 by the rapidly- moving red cor- 

 puscles. In contact with it we ob- 

 serve a clear colourless layer, in 

 which, in the case of veins scattered, 

 white corpuscles may be discovered, 

 which advance much more slowly 

 and lazily than their hurrying com- 

 panions, and sometimes even adhere 



to the Wall of the Vessel for a COn- Fig 383 Stream of blood in the web of a frog's 



siderable time. In the arteries, on the ^%ff J^^SSt *> the epi ' 

 other hand, this colourless stratum 



of fluid consists, according to Conheim, of liquor sanguinis, almost entirely 

 free of cells. Thus, a distinction may be drawn between a rapid axial and 

 lazy parietal stream. In the finest vessels and capillaries this peripheral 

 layer disappears on account of the narrowness of the tube, and instead of 

 the helter-skelter which goes on in the arteries, a more quiet, measured pro- 

 gression commences. At last the coloured and colourless blood-corpuscles 

 glide along singly one after the other, sometimes closely packed, sometimes 

 separated by considerable intervals. The former, which are smooth and 

 pliant, as well as endowed with a high degree of elasticity and extensibility, 

 are driven through the finer canals with greater ease than the latter, which 

 are not unfrequently arrested in their progress owing to their roughness 

 and adhesiveness. As soon, then, as the compression exercised upon it is 

 removed, the red corpuscle returns to its primary form again. In certain 

 cases some of the capillaries appear completely devoid of cells for the 

 time being, and transmit plasma alone. It seems almost superfluous to 

 remark here that, in the normal condition, a continuous transition of the 

 arteries into the veins takes place through the capillaries. In this exqui- 

 site spectacle a great number of subordinate variations may be observed 

 beside those already mentioned. But the moving coloured cells of the 

 mammal are liable to even greater changes, according to Rolletfs interest- 

 ing observations. They assume continually (of course passively) the most 

 diverse fprms, and only appear exceptionally in their normal shape. This 

 they assume again on the blood coming to a state of rest. 



The rapidity of the capillary circulation can only be estimated approxi- 

 mately. The red cell of the frog traverses about the fourth or fifth of a 

 line in a second, while the lymph-corpuscle requires for the same distance 

 about ten or fifteen times as long. It is only the great shortness of the 

 capillaries, of which we have already spoken, which renders possible the 

 rapid circulation of the whole mass of the blood through the body. 



