84 CIRCULATION OF THE BLOOD IN THE VESSELS. 



that the form of the corpuscles can not be distinguished. Only a few of 

 the white corpuscles occupy the still layer, the others being carried on in the 

 central current. 



The circulation in the true capillaries is sui generis. Here the blood is 

 distributed in every direction, in vessels of nearly uniform diameter. The 

 vessels are generally so small as to admit but a single row T of corpuscles. In 

 a single vessel, a line of corpuscles may be seen moving in one direction at 

 one moment, a few moments after, taking a directly opposite course. When 

 the circulation is normal, the movement in the capillaries is always quite 

 slow as compared with the movement in the arterioles, and is continuous. 

 Here, at last, the intermittent impulse of the heart is lost. The corpuscles 

 do not necessarily circulate in all the capillaries that are in the field of view. 

 Certain vessels may not receive a corpuscle for some time, but afterward, 

 one or two corpuscles become engaged in them and a current is estab- 

 lished. A corpuscle is sometimes seen caught at the angle where a vessel 

 divides into two, remaining fixed for a time, distorted and bent by the force 

 of the current. It soon becomes released, and as it enters the vessel, it 

 regains its original form. In some of the vessels of smallest size, the cor- 

 puscles are slightly de- 

 formed as they pass 

 through. The scene is 

 changed with every dif- 

 ferent part which is ex- 

 amined. In the tongue, 

 in addition to the arte- 

 rioles and venules with 

 the rich net- work of cap- 

 illaries, dark - bordered 

 nerve - fibres, striated 

 muscular fibres, and epi- 

 thelium can be distin- 

 guished. In the lungs 

 large, polygonal air-cells 

 are observed, bounded 

 by capillary vessels, in 

 which the corpuscles 

 move with great rapidi- 

 ty. It has been observed, 

 also, that the larger ves- 

 sels in the lungs are 

 crowded to their utmost 



FIG. 36. Portion of the lung of a live triton, drawn under the mi- 

 croscope and magnified 150 diameters (Wagner). 



capacity with corpuscles, leaving no still layer next the walls, such as is seen 

 in the circulation in other situations. 



Pressure of Blood in the Capillaries. There is, apparently, no way of 

 directly estimating the pressure of blood in the capillaries. If, however, a 

 glass plate be placed upon a part in which the capillary circulation is active 



