INFLAMMATION. 109 



other has shown, they are not to be ignored by the serious student of the 

 living body, either in health or disease. 



In the bladder or mesentery of the frog the arteries and veins with 

 their connecting system of capillaries are clearly seen. When the cur- 

 rent is rapid the cells of the blood in the arteries and veins are gathered 

 into a red axial mass in which the separate cells may not be distinguish- 

 able, owing to their crowding and the rapid flow. Outside of this, against 

 the walls, is a clear marginal zone in which a leucocyte is occasionally 

 visible. If for any reason the rate of flow be considerably diminished, 

 the leucocytes, which are specifically lighter than the red cells, gather in 

 the marginal zone. If the current become very slow or be arrested, the 

 marginal zone disappears and the red and white cells intermingle. ' 



Soon after the exposure of the bladder or mesentery the arteries, 

 veins, and capillaries dilate, and the blood, encountering less resistance 

 from the walls, flows more rapidly through them. This increased rapid- 

 ity of the blood current does not, however, last long, although the vessel 

 still remains dilated. After a variable period, owing, it is believed, to 

 changes in the endothelia of the vessels, the blood meets with so much 

 resistance that it flows more slowly than under normal conditions. Tem- 

 porary or even permanent stasis may occur in some of the vessels, but 

 this is not a constant nor a characteristic occurrence. White blood cells 

 leucocytes now begin to accumulate along the inner walls of the veins 

 and to become fixed there, so that after a time the whole inner surface of 

 the veins may be more or less thickly sprinkled, and even closely crowded, 

 with adherent leucocytes. These may either lie firmly against the eiido- 

 thelium or be dragged slowly along by the current of blood sweeping 

 past them. Some are dragged by the blood current into pyriform 

 shapes, showing that they are adherent at a small point only, and thus 

 they may be detached from the wall and rejoin the circulating blood. 

 They may by amreboid movement crawl slowly along the interior of the 

 wall, even against the current. In the capillaries, also, a similar com- 

 portment of the leucocytes may be seen. 



After a time, which varies considerably in the bladder sometimes 

 within an hour after its exposure ; in the mesentery usually later some 

 of the leucocytes commence to make their way slowly through the walls 

 of the veins and capillaries. At first a little shining knob appears on 

 the outside of the wall opposite to the cell which is sticking within, and 

 this outer portion grows larger as the part still within grows smaller, 

 until at length the entire cell is outside of the vessel. The cell now may 

 immediately detach itself and wander off in the tissue spaces, or it may 

 remain for some time attached to the outside of the wall (Fig. 36). This 

 passage of the leucocytes through the walls of the capillaries and veins 

 it does not occur in the arteries is called emigration. The emigrating 



1 This distribution of the blood is almost, if not wholly mechanical, and may be 

 simulated in glass tubes with a fluid in which lifeless particles of different specific 

 gravities are suspended. The particles of the lesser specific gravity assume, when the 

 rlmv is established, the peripheral portion of the stream. 



