THE CIRCULATING LIQUIDS OF THE BODY 53 



rate in the case of the higher animals and man, although the 

 hrilliant biological observations on which it was originally built 

 retain all their value. He supposed that in the immunizing 

 process the leucocytes underwent certain changes, acquired, so 

 to speak, a sort of ' education ' that enabled them to cope with 

 bacteria against which they were previously powerless. It seems 

 more probable that in the presence of the substances that confer 

 immunity, not only the leucocytes, but other cells, are stimulated 

 to produce bodies which cut short the life, or inhibit the growth, 

 of the bacteria (alexins), or prepare them for being taken up by 

 the phagocytes (opsonins). It has been shown that bacteria 

 which have been in contact with serum containing the appro- 

 priate opsonins are taken up readily by leucocytes washed free 

 from serum constituents by physiological salt solution, whereas 

 the washed leucocytes either do not ingest bacteria which have 

 not been acted on by serum, or take them up in much smaller 

 numbers. There is some evidence that in certain bacterial 

 infections for example, chronic furunculosis, a condition in 

 which crops of boils continue to appear the grip of the 

 bacteria on the body is perpetuated by a deficiency in the 

 amount or in the activity of opsonins capable of acting specifi- 

 cally upon the micro-organisms in question. A numerical ex- 

 pression, which in certain cases, perhaps, gives a measure of the 

 patient's resistance to the infection, has been worked out by 

 Wright under the name ' opsonic index.' This index is the ratio 

 between the average number of bacteria taken up, under certain 

 fixed conditions, by each polymorphonuclear leucocyte in an 

 emulsion made with the patient's serum, and the average number 

 taken up by similar leucocytes in an emulsion made with normal 

 serum. The significance of this index, and even the practicability 

 of the methods used to ascertain it, are still the subject of 

 lively discussion. 



Diapedesis. The fact that leucocytes can pass out of the 

 bloodvessels into the tissues has a very important bearing on 

 the subject of phagocytosis. The phenomenon is called diape- 

 desis, and is best seen when a transparent part, such as the 

 mesentery of the frog, is irritated. The first effect of initation 

 is an increase in the flow of blood through the affected region. 

 If the irritation continues, or if it was originally severe, the 

 current soon begins to slacken, the corpuscles stagnate in the 

 vessels, and inflammatory stasis is produced. The leucocytes 

 adhere in large numbers to the walls of the capillaries, and 

 particularly of the small veins, and then begin to pass slowly 

 through them by amoeboid movements, the passage taking place 

 at the junctions between, or it may be through the substance of, 

 the endothelial cells. Plasma is also poured out into the tissues. 



