Conditional Reflexes 173 



system of living matter. This system consists 

 of a sensory organ from which ingoing 

 (efferent) nerve fibres pass to end in a central 

 nerve cell; from this cell (afferent) outgoing 

 fibres extend to effective organs such as muscle 

 fibres and secreting cells. Stimuli derived from 

 sensory organs travel to a central cell, and thus 

 release a part of its potential energy, which is 

 conducted along afferent nerve fibres to an 

 effective organ. For instance, a particle of dust 

 falls into our eye, it acts as a local source of 

 irritation, and thus excites corresponding cen- 

 tral nerve cells, from which energy is dis- 

 charged which, passing to the muscles of the 

 eyelids and to its lachrymal gland, sets up 

 spasm of the lids and a flow of tears. 



Professor Pawlow, of Petrograd, has for 

 many years been engaged in studying what he 

 terms acquired or " conditional " reflexes; while 

 movements of the lower classes of animals are 

 controlled by innate, simple, or " uncondi- 

 tional " reflexes ; under the influence of evolu- 

 tion and the adaptability of living matter to its 

 surroundings, the higher orders of beings have 

 acquired the power of forming new reflexes by 



