JH LIVING MATTEE 73 



metabolism to such a marked degree so as to render them 

 indispensable to the life of given groups of organisms, e.g. light in 

 the case of green plants. These special external conditions are 

 usually known as stimuli, since they exert a direct influence on 

 the excitability of the protoplasm as expressed in the various 

 forms of excitation. 



In the previous chapter we distinguished between automatic 

 and reflex excitation ; the former being determined by internal, 

 the latter by external stimuli. This must not be understood to 

 mean that the excitations which have the character of spontaneity, 

 as, opposed to the reflexes provoked from without, are independent 

 of all external determining factors. The first, like the second, are 

 effected under the constant influence of the general and normal 

 external conditions of life ; but while automatic excitations have 

 for their immediate and determining cause a stimulus or impulse 

 proceeding from the living matter itself, reflex excitations have 

 for their immediate and determining cause either a sudden change 

 in the normal external conditions, or the abrupt and unexpected 

 intervention of other special external agents. 



The external agents that commonly function as stimuli are 

 represented by different chemical actions, by various mechanical 

 shocks, by light, heat, and electricity. 



The changes in metabolism determined by the action of 

 stimulating agents may be predominatingly anabolic or katabolic 

 in character. In the first case there is development of kinetic 

 energy, and the phenomena are those of- excitation properly so- 

 called ; in the second, there is an accumulation of potential energy, 

 and the phenomena are said to be assimilatory. or trophic, or 

 inhibitory, according to the most conspicuous characteristic which 

 they present under observation. 



When the action of the stimuli is too prolonged, or too frequently 

 repeated, or exceeds the physiological limits in its intensity, 

 there may result not an increase but a depression, suspension, or 

 abolition of metabolism, as exhibited in the phenomena of fatigue, 

 paralysis, or death of the protoplasm. 



We must now briefly summarise the most universal and best 

 ascertained conclusions in each of these categories of phenomena. 



VII. Innumerable chemical compounds function as stimuli 

 when brought into relation with living matter, i.e. they provoke 

 phenomena of excitation. The mode in which they act has, how- 

 ever, been experimentally studied only in a very few cases. We 

 must therefore confine ourselves to recording certain typical 

 phenomena which are particularly worthy of attention. 



Max Schultze (1863) and Kiihne (1864) made classical re- 

 searches on the effect of chemical stimuli upon the amoeboid 

 movements of masses of naked protoplasm, such as the Rhizopoda 

 (Amoebae, Myxomycetes, Polythalamidae, etc.). The effect most 



