IRRITABILITY 



131 



Fig. 23. Movements of a precisely similar character may be 

 produced in substances which are certainly not alive, such as 

 Brailsford Robertson's model amoeba made of camphor, benzene 

 and water (Pt. II.). These purely physico-chemical reactions 

 are produced by alterations of the surface tension of the fluids 

 under observation. Macallum has shown (pp. 140 and 151), that 

 alterations in surface tension occur in living tissue during motion. 

 Movement can, therefore, not be considered as a specifically vital 

 phenomenon. Certain parts of the cell, e.g. the vacuoles, show a 

 rhythm in their movements. In polycellular organisms, certain 

 organs, e.g. the heart, pulsate. It is comparatively easy to produce 

 rhythmical movement in material which is not living. A globule 



FIG. 23. Leucocyte of invertebrate, (lledrawn after Goodrich.) 



of mercury more than an inch in diameter may be made to pulsate 

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

 Heart," Pt. II.). 



(b) Irritability is a general property of living matter. When 

 amoeba is touched, it withdraws its pseudopodia (barotaxis). 

 It moves towards and over suitable food and moves away from 

 quinine or from a hypertonic solution of crystalloids (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 



