302 LECTURE XIV. 



explanation of the phenomena of Fatigue. It has been 

 found, namely, that if a motile organ, that of the leaf 

 of the Sensitive Plant for example, be repeatedly stimu- 

 lated at short intervals, it will soon cease to respond to 

 the stimuli, it will lose its irritability; after a short period 

 of rest it again becomes irritable. The loss of irritability 

 we may ascribe to the consumption, in consequence of the 

 repeated stimulation, of the material by the decomposition 

 of which the necessary energy for the performance of a 

 movement is evolved ; possibly the accumulation of the 

 waste-products of decomposition may contribute to this result. 

 The regaining of irritability we may ascribe to the accumu- 

 lation of a fresh store of decomposable material and perhaps, 

 in some degree, to the removal of the waste-products of 

 previous decompositions. Finally, we have to answer the 

 question, what is this decomposable material ? We have 

 seen throughout our consideration of this subject that move- 

 ment is dependent upon the same general conditions as 

 destructive metabolism. Of the processes of destructive me- 

 tabolism the most conspicuous is the self-decomposition of 

 the protoplasm. We have no reason to suppose that the 

 metabolism of a motile cell differs in kind from that of a 

 non-motile cell. If, then, the evolution of energy in a non- 

 motile cell is mainly due to the self-decomposition of the 

 protoplasm, we may infer that this is also the case in a 

 motile cell. 



Heat. An evolution of energy in the form of heat is the 

 inseparable accompaniment of the processes of destructive 

 metabolism ; in fact, an organism can only be considered 

 to be living so long as it is setting free energy in the form 

 of heat. Pfliiger goes so far as to say that the " intra- 

 molecular heat of an organism is its life," that is, that it 

 is only in consequence of the continued evolution of heat 

 in the protoplasm-molecule that the intramolecular vibra- 

 tory movement is maintained (p. 160). The living proto- 

 plasm-molecule is ever undergoing active self-decomposition 

 and evolving heat, the dead protoplasm-molecule has ceased, 

 probably in consequence of some change of molecular 



