214 GENERAL BIOLOGY OF MICRO-ORGANISMS 



Weigert, serves to explain tissue hyperplasia and repair following 

 exercise or local destruction of tissue. Examples of these phe- 

 nomena will occur to the reader. 



Phagocytosis and Encapsulation. — The mere physical mass of 

 a parasite within the tissue acts as a foreign body and it becomes 

 surrounded by tissue elements. If it is minute, certain cells of 

 the body (phagocytes) flow around and ingest it, as was first 

 observed by Metchinkoff. If it is larger, the connective tissue 

 cells proliferate and surround it, and eventually contract into a 

 firm capsule. Further, the tissues produce enzymes capable 

 of dissolving many foreign substances introduced in this way 

 (parenteral digestion). If the foreign body is insoluble, it will 

 remain encapsulated, or, if sufficiently minute, it may be trans- 

 ported considerable distances inside wandering cells and eventu- 

 ally be deposited in a lymph gland. The wholly passive parasite 

 or the dead body of a micro-organism is therefore either digested 

 and dissolved, ingested by cells, or encapsulated in fibrous tissue. 

 Most infectious agents are not passive in this way, as .we have seen, 

 but tend actively to grow and multiply, to absorb and utilize food 

 material, and, most important of all, to produce various substances 

 which stimulate or poison the cells of the host. Against these 

 the physical measures of ingestion (phagoctyosis) and encapsula- 

 tion are often inadequate defenses and may be entirely useless. 



Chemical Constitution of the Cell.^ — Ehrlich has compared 

 the living body cell to a complex chemical molecule; in fact it 

 may be said that he regards the living cell as an enormous mole- 

 cule, a chemical unit of great complexity. Certain atom groups 

 within this molecule are pictured as relatively very stable and 

 they constitute the chemical nucleus (not to be confused with the 

 anatomic nucleus). Grouped about this chemically stable center 

 are very many, more labile atom groups which readily enter into 

 chemical reaction with substances in the surrounding medium. 

 The conception is derived directly from well-known facts in organic 

 chemistry. For example when benzoic acid, CeHsCOOH, reacts 

 with other chemicals the reaction takes place at the reactive group. 



