THE STORY OF THE BACTERIA. 65 



in the smaller spaces in the tissues. Exactly 

 what they do under these circumstances we do 

 not know. There is some reason for think- 

 ing that they act to some extent as scaven- 

 gers, and when they come across a particle of 

 worn-out or foreign material in the tissues, take 

 it into themselves, just as amceba does its food 

 in water, and either digest it or carry it back 

 to those parts of the body in which waste ma- 

 terial is systematically disposed of. 



But let an injury such as an open wound 

 occur, and the whole attitude of these leuco- 

 cytes changes. They get out of the blood- 

 vessels with all speed, in greater or less num- 

 bers as the occasion may demand, and gather 

 about the edges of the wound, and after a 

 time they, together with some other cells of 

 the injured tissue, change their shape and 

 character, and actually form, with the aid of 

 the blood-vessels near-by, a mass of new tis- 

 sues, which replaces that which was lost by 

 the injury, and so permanently binds the edges 

 of the wound together. Sometimes these white 

 blood-cells gather in much greater quantities 

 about the wound than is necessary, and then 



