566 Chapter XVII 



tissue, or when they approach one another and come together in order 

 to form a vascular loop. 



The phenomena of the organism which bear the sharpest impress 

 of their physical and chemical nature, also come under the influence 

 of cellular "sensations." Thus, in gastro-intestinal digestion, the 

 secretion of the active juice is subordinated to the control of the nerve 

 centres and even of the psychic centres. The sight of various kinds 

 of food stimulates, unconsciously, by reflex action the activity of 

 different digestive glands. In the same way the contraction of the 

 contents of the cells of a plant subjected to plasmolysis, brings about 

 the secretion of acid in order to augment the osmotic pressure. 



Susceptibility, whose part is so great in the phenomena of 

 immunity, taken as a whole, is a general property of living beings, 

 regulated by a common law. Thus, in the chemiotaxis of the lowest 

 unicellular organisms, as in the movements and the osmotic reaction 

 of plants, there is manifested the same psycho-physical law of Weber- 

 Fechner which regulates our own sensations. 



All cells are able, by modifying their function under the direction 

 of susceptibility, to adapt themselves to changes in the surrounding 

 conditions. All living elements are able, therefore, to acquire a 

 certain degree of immunity. But, amongst all the cells of the animal 

 body, the elements which have retained most independence the 

 phagocytes most easily and first acquire immunity to infective 

 diseases. These are the cells which betake themselves to situations 

 where micro-organisms and their poisons make their appearance, and 

 which manifest a reaction against them. The phagocytes of the 

 immune organism ingest and destroy micro-organisms and absorb 

 toxins and other poisons. The final act of the reaction of the 

 phagocytes is constituted by the chemical or chemico-physical pro- 

 cesses concerned in the digestion of the micro-organisms, with the 

 help of cytases, assisted by the fixatives ; in the defence offered 

 against poisons the phagocytes must also exert a chemical action. 

 [591] Before these phenomena come into play, however, the phagocytes 

 manifest phenomena which are purely biological, such as the per- 

 ception of chemiotactic and other sensations, the migration towards 

 menaced situations, the ingestion of micro-organisms and the absorp- 

 tion of toxins, and finally the secretion of substances to be utilised in 

 intracellular digestion. 



The immunity in infective diseases presents itself, therefore, as a 

 section of cellular physiology, and especially as a phenomenon con- 



