INFLAMMATION 371 



blasts. This is an abscess, and the dead material which 

 it contains is known as pus. An abscess usually ruptures 

 on the surface of the body and its contents thus discharged. 

 Its walls collapse, further development of fibrous tissue 

 leading to the formation of a scar. Finally, the rent in 

 the skin heals by new growth of epithehum. 



The cells which are actively engaged in the inflammatory 

 process may be divided into two groups : (1) those normally 

 present in the blood in considerable number; (2) those 

 formed from connective-tissue in general and relatively 

 scanty in the blood. Of the first group the most important 

 are the polymorphonuclear leucocytes. These are the 

 Microphages of MetchnikofE. They are present in great 

 numbers in all acute infections, and are at the same time 

 greatly increased in number in the blood. They are 

 actively amoeboid and digest the bacteria. In certain 

 infections the coarsely granular eosinophiles are increased, 

 but the function of this type of leucocyte is not properly 

 understood. The lymjjJiocytes appear to play no part in 

 acute infections, but they are increased in chronic con- 

 ditions such as tuberculosis. What part they play is not 

 known ; their phagocytic powers are very feeble. 



The cells of the second group are of three kinds : — 



(a) Endothelial (hyahne, mononuclear) cells present in 

 very small numbers in the blood. They arise from the 

 endothehum of serous cavities and of blood-vessels when 

 these are infected. Metchnikoff called them Macrophages, 

 and beheved that they devour especially the microphages 

 which have succumbed to the bacteria. 



(6) Fibroblasts. — These are spindle-shaped cells arising 

 in and forming fibrous tissue. 



(c) Plasma cells. — These are small cells resembhng and 

 probably identical with lymphocytes. 



We have said that bacteria owe their deleterious effects 

 to the chemical action of the toxins which they form. 

 Bacteria may be divided into two classes. In the first class 



