370 DEFENCE 



they are attracted towards the bacteria (Positive Chemio- 

 taxis). They attempt to ingest them, but the reproductive 

 and toxic powers of the latter prove too strong for them 

 and they succumb. Phagocytes arriving later upon the 

 scene of action are endowed with stronger properties and 

 succeed in destropng the bacteria. While the capacity for 

 defence is thus being gradually acquired, the infected 

 individual runs through the course of the disease, and it is 

 owing to the development of the mechanism of defence 

 that he recovers. 



The changes which result from infection may be readily 

 followed in a thin vascular tissue. The first change is a 

 dilatation of capillaries, accompanied, however, by retar- 

 dation of the blood-stream. Leucocytes pass by diapedesis 

 through the capillary walls, accompanied by an excessive 

 flow of lymph which distends the intercellular spaces. 

 This reaction is known as inflammation. It was recognised 

 by the ancients by its four signs — rubor, tumor, dolor and 

 calor; rubor, the redness due to the capillary dilatation; 

 tumor, the distension and puffiness of the tissue due to the 

 exudation ; dolor, the pain produced by irritation of the 

 nerve-endings ; calor, the increased warmth of the part due 

 to the dilatation of the vessels. 



The essential features of the inflammatory process are 

 the effusion of lymph whereby the toxins are diluted, and 

 the mobihsation of leucocytes whereby the bacteria are 

 ingested. The infected region becomes a mass of bacteria 

 and cells floating in lymph. Of the cells, some are the 

 proper cells of the part in different stages of degeneration. 

 Some are the leucocytes, those in the centre of the mass 

 being dead, while those situated peripherally are ingesting 

 the bacteria. If the diseased area is small in extent all 

 the dead cells and bacteria will be absorbed by leucocytes. 

 If it is large, absorption will take place only to a hmited 

 degree, there remaining a central dead mass cut off from 

 the supply of blood and surrounded by a capsule of newly 

 formed fibrous tissue laid down by cells known as fibro- 



