IRRITABILITY 



151 



of nicrc'iiry iiiort- than an iiicli in diameter may be made to pidsate 

 with perfect regularity for hours. (See Ostwald's " Physical 

 Heart," Part II.). 



[b) Irritability is a general property of Uving matter. When 

 amoeba is touched, it withdraws its pseudopodia (barotaxis) 

 (Chap. XXXIII.). It moves towards and over suitable food and 

 moves away from quinine or from a hypertonic solution of crystal- 

 loids (negative chemiotaxis). Hydrogen ions if not too concentrated 

 exert positive chemiotaxis, while hydroxyl ions have a repellent 

 effect. This may explain galvanotaxis. Strong light repels, while 

 a moderate illumination attracts many lower organisms. Further, 

 the more refrangible rays of light exert a negative phototaxis, 

 while the less refrangible rays are positively attractive. If the 

 swarm spores of certain algae 



are placed in a tank with a 

 cover, half of which is blue 

 glass and half is red, and 

 exposed to light, they will 

 stream away from the blue 

 and towards the red end of 

 the box. Ultra-violet rays 

 have a marked effect on 

 living organisms, for ex- 

 ample, the tubercle bacillus 

 is killed by ultra-violet light 

 and lupus is cured by pro- 

 jection of the Finsen arc on 

 the growth. Change of tem 

 perature may exert either a positive or a negative effect, the 

 animalcule avoiding the abnormal. That is, too high or too low 

 a temperature exerts negative thermotaxis. Non-living matter 

 shows irritability. We have seen how sensitive colloids are to 

 slight alterations in their environment. They exhibit chemiotaxis 

 and galvanotaxis very markedly. Even inorganic matter may 

 respond to stimulation. Lillie has demonstrated this in the case 

 of iron. A piano wire which has been dipped in concentrated 

 nitric acid and then suspended in dilute nitric acid will show 

 changes if " stimulated " mechanically, chemically, or electri- 

 cally. The irritability of living matter is, according to Verworn, 

 of a specific type and is thus indicative of life. 



(c) Ingestion and excretion are phenomena exhibited by all 

 living cells. Nutrient material is taken from the environment, 

 prepared, used, and the non-utilisable rest is forced out. Amoeba 

 engulfs food and forms a vacuole in which will be found food and 



f'OL YMOftPHONUCLE^R. 

 LEUCOCVTa. 



Fig. 37. — Large mono-nuclear cell (macrophage) from 

 the peritoneal fluid of a rabbit suffering from peritonitis 

 inrluced by inoculation with L5. coli, showing phagocytosis 

 of (1) a polymorphonuclear leucocyte, which has itself 

 ingested some bacilli, and (2) an eosinophil leucocyte. 



