. CA USES OF DISEASE. 2 1 7 



these are free from any dirty particles and are not 

 lodged in a vital organ, the leucocytes at once attack 

 them, but find such intruders unconquerable ; the result 

 is that very large numbers of these cells surround them 

 and gradually become transformed into neutral tissue, 

 thus isolating the intruding bodies from neighbouring 

 parts. The tissue thus formed is known as fibrous-tissue, 

 and the process is termed encystment. Should the 

 intruded body contain particles of dirt offensive to the 

 leucocytes, the action becomes intensified and often 

 disastrous to the cells, for they die in the conflict, and in 

 a few hours the foreign body is surrounded by fluid 

 con-taining the dead cells. This fluid is usually of a 

 yellowish colour and is known technically as pus ; a 

 collection of pus is termed an abscess. As long as an 

 offensive foreign body remains in the organism the 

 abscess enlarges until it finds its way to a free surface 

 and discharges itself: with the evacuation of the pus the 

 cause of the disturbance often escapes. 



In their behaviour to foreign bodies leucocytes remind 

 us of the resentment of bees to insects intruding into 

 their hives. When the offender is small it is quickly 

 stung to death and turned out ; when large, and they 

 succeed in depriving it of life, it may be too heavy to 

 admit of removal, and the bees render the dead organism 

 inocuous by a covering of wax. 



This aggressive behaviour of leucocytes to foreign 

 bodies is extended to such unwelcome guests as 

 pathogenic bacteria. When micro-organisms effect an 

 entrance into an animal the leucocytes attack and 

 attempt to destroy them, and the details of such 



