206 



THE BLOOD 



serve as an adequate protection if the leukocytes are inferior in num- 

 ber or power. The best results can only be obtained if these two 

 factors are properly balanced. The opsonin content may be deter- 

 mined experimentally, the result being the so-called opsonin-index of 

 the body. By treating an animal in a specific way, the number of its 

 leukocytes and opsonin-content may be increased so that its power of 

 resistance becomes much greater than ever before. 



Diapedesis. This term was originally applied to the passage of 

 the blood or of its formed elements, chiefly the red cells, through the 

 wall of a blood-vessel. Cohnheim, however, has shown in 1869 that 

 this power of migration into the neighboring tissues is a distinct 

 characteristic of the leukocytes. In contradistinction to the passive 

 behavior of the red corpuscles, the latter are aided 

 j ^(gL^ . in their escape from the vascular system by their 



ameboid properties. A delicate pseudopodium is 

 first protruded through a perforation in the vessel 

 wall, after which the principal mass of the cell is 

 slowly drawn through the opening until entirely 

 outside the vascular channel. An assemblage of 

 great numbers of these corpuscles outside the main 

 circulatory system results whenever a tissue has 

 been injured or has become the seat of an infective 

 process. 1 Under these circumstances, their migra- 

 tion is greatly facilitated by certain changes in the 

 flow of the blood, namely: 



(a) A relaxation of the capillaries in the area affected 

 so that the size of the blood-bed becomes larger; (b) an 

 accumulation of a larger quantity of blood in this par- 

 ticular region which tends to produce a local rise in tem- 

 -DIAPEDE- perature; and (c) a diminution in the velocity of the blood 

 flow which enables the white corpuscles to assemble in 

 numbers and to attach themselves more securely to the 

 vessel wall. These dynamical changes indicating an inflammatory reaction, 

 may be studied under the microscope in such tissues as the mesentery, tongue, 

 lung or web of the frog, if they are first moistened with normal saline to which a 

 few drops of alcohol or a small amount of mustard has been added. 



Having invaded the tissue, the leukocytes immediately display their 

 phagocytic properties. Supposing that the inflammatory reaction has 

 been produced by bacteria, the outcome of this interaction depends 

 upon the relative strengths of the leukocytes and invading cell. If 

 the latter is the more powerful factor, the infection will gradually 

 extend to neighboring areas of the tissue, while if the former is the 

 stronger, the bacteria will eventually be encircled and eliminated. 

 But, in either case, large numbers of leukocytes will be destroyed in 

 the course of this process, their remnants appearing in the extrava- 

 sation in the form of pus-corpuscles. The foregoing discussion 



1 Adami, Inflammation, Macmillan, New York, 1909. 



FIG. 112.- 

 si8 OF LEUKOCYTES. 



