MUSCLE 253 



stimuli ; in the eye it is exposed to the dissociating action of 

 light. 



Protoplasm is responsive to external force. It conducts the 

 impulses to which stimulation gives rise. Eventually the im- 

 pulses, which travel along strands of tissue highly specialized 

 for the purpose of conduction nerves reach collections of 

 protoplasm which are so disposed that when they dissociate 

 energy is set free. A comprehensive term is much needed for 

 the connotation of this third essential property of protoplasm, 

 the capacity of liberating energy which characterizes " effec- 

 tors." An external force, so small in intensity as to be negli- 

 gible when we are dealing with the body's accounts, acts upon 

 the protoplasm of a receptor. A change in state results. 

 The change is conducted to an energy-liberating organ. This 

 organ is supplied with blood which brings it food. Food is 

 its store of energy, the raw material from which it manufact- 

 ures its ammunition. When an impulse reaches an energy- 

 liberating organ its protoplasm dissociates. But here the 

 protoplasm is so disposed the cells which contain it have such 

 a form that when it dissociates a change in the cell follows ; 

 it alters in shape, or it discharges into its environment heat, or 

 electricity, or light. The dissociation and reassociation of the 

 protoplasm of an effector involves chemical change. Mole- 

 cules of water and of carbonic acid are cast off. The energy 

 sacrificed in letting matter fall into these very stable forms is 

 the energy made visible, as it were, in lifting a weight or 

 dispersing heat. It must be replaced if the organ is to retain 

 its power of acting when next an impulse reaches it. To 

 replace it, protoplasm takes up food and oxygen from the 

 blood. 



The liberation of energy which occurs when a muscle con- 

 tracts is not a special phenomenon something which does not 

 occur when the muscle is at rest. It is an intensification of a 

 process which is always taking place. The substance of muscle, 

 like that of nerve and every other tissue, is always combining 

 with oxygen and giving off water and carbonic acid. When 

 we are auditing the body's accounts, we enter so and so much 

 food and oxygen on the debit side, we credit it with the 

 same weight of water and carbonic acid ; or we debit it with 

 the energy potential in the food, and enter to its credit the 



