186 PHAGOCYTOSIS 



intestinal digestion becomes extracellular. However, certain meso- 

 dermal cells, even in the highest mammals, including man, continue in 

 the possession of ameboid movements and the exercise of intracellular 

 digestion. The burden of digestion for the animal body as a whole 

 rests on certain glands, but as a further protection against invasion by 

 foreign proteins, the white blood cells retain active movements and 

 the ability to engulf and destroy micro-organisms. 



Metchnikoff has shown that the metamorphic change through 

 which many insects and other animals pass is brought about by phago- 

 cytic action. The tail of the tadpole is eaten away by the white blood 

 corpuscles. Some of the tissue changes both normal and abnormal in, 

 man are due to the same agents. The involution of the uterus after 

 childbirth is due to the absorption and intracellular digestion by phago- 

 cytes of the unnecessary tissue. The whitening of the hair with 

 advancing age results from the consumption of the pigment by phago- 

 cytes. The general deterioration of the brain with advancing senility 

 is due to the activity of the neuronophages. In short, the man who 

 lives to die physiologically is slowly but certainly devoured by his own 

 phagocytes. Sometimes, however, they seemingly are not content to 

 let us live out the allotted time, and they fall on and devour some part 

 of our anatomy prematurely. According to Metchnikoff, progressive 

 muscular atrophy is an illustration of this apparently undue greed on 

 the part of our wandering cells. This must not lead us to a hasty and 

 rash condemnation of our eating cells, for if they did not feed on the 

 micro-organisms which are frequently entering our bodies, our lives 

 would be even shorter and more precarious than they are. 



If some of its own blood, or that of another individual of the same 

 species, be injected into the peritoneal cavity of an animal, it is readily 

 absorbed and does no harm. If it be from another species the phago- 

 cytes come from all parts of the body and devour the foreign material. 

 This is true not only of blood but of other foreign cells. Indeed, cells 

 whose normal habitat is limited to one organ or tissue become foreign 

 substances when introduced into another part. Spermatic cells 

 injected into the abdominal cavity of the animal from which they are 

 taken become the prey of the white blood cells. When foreign material 

 is injected into the abdominal cavity, at first the white blood cells, 

 already there, seem to be frightened and run away, but soon they 



