FUNCTIONS OF BLOOD AND LYMPH 61 



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 specifically upon the 

 micro-organisms in question. A numerical expression, 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 discussion. 



Diapedesis. The fact that leucocytes can pass out of the blood- 

 vessels into the tissues has a very important bearing on the subject 

 of phagocytosis. The phenomenon is called diapedesis, and is best 

 seen when a transparent part, such as the mesentery of the frog, is 

 irritated. The first effect of irritation 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 pro- 

 duced. 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, 

 the whole forming an inflammatory exudation. Even red blood- 

 corpuscles may pass out of the vessels in small numbers. The 

 exudation may be gradually reabsorbed, or destruction of tissue 

 may ensue, and a collection of pus be formed. The cells of pus are 

 emigrated leucocytes (Practical Exercises, Chap. III., p. 193). 



Their emigration is connected with the defence of the organism 

 against the entrance of certain forms of bacteria at the seat of 

 injury, and with the repair of the injured tissue, but the nature of 

 the summons which gathers them there is not yet clearly under- 

 stood. It is probably some sort of chemical attraction (chemio- 

 taxis) between constituents of the bacteria or decomposition prod- 

 ucts of the injured tissue on the one hand, and constituents of the 

 leucocytes on the other. 



As for the blood-plates, it will suffice to say by way of summary 

 that their important function in the sealing of wounded vessels (p. 46) 



