204 THE BLOOD 



For this reason, they are frequently spoken of as devouring cells or 

 phagocytes (to eat-cell), and are further characterized as the "police- 

 men of the blood." In illustration of their function the following 

 phenomenon may be cited: As the larva of the fly changes into the 

 mature insect — a metamorphosis which occurs rather rapidly — such 

 structures as the creeping muscles become superfluous and undergo 

 degenerative changes. The substances which are formed during this 

 catabolic process, exert a strong chemotactic influence upon the 

 leukocytes with the result that this tissue soon becomes overcrowded 

 with them. The ensuing phagocytosis soon leads to the removal of 

 these now useless parts. The absorption of the tail of the tadpole is 

 accomplished in a similar manner. It is also known that they invade 

 injured tissues and help in the removal of the superfluous cellular 

 material, but whether they actually take part in the process of recon- 

 struction, is still doubtful. To be sure, Metchnikoff ^ has expressed 



^%- 



Fig. 110. — Leukocytes Engolfinq Particles of India Ink. 



the idea that the emigrated leukocytes undergo certain changes which 

 enable them to become connective-tissue cells, but most authors 

 believe that this regeneration is accomplished solely by the plasma cells 

 of the tissues. 



Of even greater importance and interest are the phagocytic quali- 

 ties displayed by the leukocytes when brought in relation with patho- 

 genic bacteria. But while capable of ridding the body of different 

 dead and living germs, the leukocytes are not capable of destroying 

 all varieties of them. They seem to be attracted especially by the 

 ordinary pus microbes or by the products of their metabolism, which 

 fact is well proven by an experiment described by Pfeffer. A capillary 

 tube, closed at one end, is filled with a culture of staphylococcus pyo- 

 genes albus or aureus. It is then placed under the skin or into the 

 abdominal cavity of a rabbit. On removing it 10 to 12 hours later, 

 the microscopical examination reveals great numbers of leukocytes in 

 the culture, actively engaged in ingesting the bacteria. The fact that 

 the bacteria, and not the culture, are responsible for the migration of 

 the leukocytes into the tube, can readily be proven by employing a 



1 PathoL compar. de F inflammation, Paris, 1892. 



