CAUSES OF DISEASE. 217 
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 
containing 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 
