STRUCTURAL CHANGES DUE TO DAMAGE. 309 



enescence affects the tissues of the periosteum and endosteum, as 

 well as the surrounding connective tissue of the fibrous type. In 

 the subsequent differentiation of the granulation-tissue, which in 

 this case is called the " callus/' those cells which have been derived 

 from the periosteum and endosteum produce bone, which becomes 

 continuous with the osseous tissue of the fragments and restores the 

 continuity of the broken bone. It is evident that in this case the re- 

 juvenescence of the bone- form ing cells has not caused a reversion to 

 an entirely unspecialized type of connective-tissue cell. It is equally 

 evident that in the production of cicatricial tissue the cells of fibrous 

 tissue retain their special formative powers after rejuvenescence. 



2. Suppuration. This is occasioned by the persistent action of a 

 damaging cause which is accompanied by the presence of substances 

 exerting a " positive chemotactic influence " upon leucocytes (i. e., 

 attracts those cells) and at the same time effecting solution of the 

 tissue-elements. In clinical experience nearly all cases of suppu- 

 ration are due to infection with bacteria ; but purulent inflamma- 

 tions of very limited extent may be caused experimentally by chem- 

 ical substances free from micro-organisms. 



Suppuration does not, however, always follow infection, even by 

 pyogenic bacteria. Sometimes the virulence of the bacteria is too 

 slight for the production of chemotactic substances in sufficient 

 quantity to attract large numbers of leucocytes. Sometimes it is so 

 great that the chemotactic influence becomes " negative " (i. e., repels 

 leucocytes), or the leucocytes are killed before they can collect in 

 sufficient numbers to form pus. The relations between the leuco- 

 cytes and the chemotactic substances are quantitative : if the sub- 

 stances be present in too great dilution, they fail to attract leuco- 

 cytes ; if in too great concentration, they repel them. Nor are bac- 

 teria and their products the only substances that attract leucocytes. 

 Bits of dead tissue may do the same, a fact which would promote 

 their absorption through the agency of the leucocytes. 



These points will be made clearer if illustrated by an example, 

 for which purpose an infection of the kidney through the vascular 

 system may be selected. If a section be made through the organ so 

 as to include a focus of infection, the bacteria will be found in the 

 bloodvessels. The appearance of the tissues surrounding the ves- 

 sel will depend upon a number of circumstances ; among others, the 

 length of time that has elapsed since the bacteria were brought to 

 the part. In one case the walls of the obliterated vessel and the 



