56 HUMAN PHYSIOLOGY 



presence is required, but we do not have to assume any- 

 thing like intelligence on the part of these cells to account 

 for the facts. We are probably to suppose that conditions 

 arise in such regions which arrest the leucocytes brought 

 into the area by the flowing blood. 



A large proportion of the leucocytes which are thus 

 concerned in contending with an infection are found 

 not to be inside but outside the blood-vessels. How 

 they made their escape is a natural question. Direct 

 observation with the microscope of inflamed areas has 

 shown that the leucocytes slip out of the capillaries by 



FIG. 8. A leucocyte is escaping through a cleft between adjoining 

 cells of a capillary wall. Such a passage of the ameboid corpuscles from 

 within the vessels to the tissue spaces outside is called diapedesis. 



exercising their power of ameboid movement. The 

 walls of these, the most slender of the blood-vessels, 

 are exquisitely thin. The cells which compose them 

 are flattened to an extreme degree and where they 

 are joined along their edges there seems to be little 

 resistance to force applied to push them apart. These 

 joints are forced by the leucocytes which gradually 

 transfer themselves/ from the interior to the outside 

 by flowing through the minute gaps thus opened. While 

 the operation is in progress the leucocyte which is being 

 watched consists of two principal masses united by a 

 strand that runs through the crack. One of the masses 



