4 THE PHILOSOPHY OF BIOLOGY 



consider what are the processes that are involved in 

 these kinds of neuro-muscular activity. 



The term " reflex action " is one that denotes rather 

 a scheme of sensori-motor activity than anything that 

 actually happens in the animal body ; it is a concept 

 that is useful as a means of analysis of complex pheno- 

 mena. In a reflex three things happen, (i) the 

 stimulation of a receptor organ and of the nerve 

 connecting this with the brain, (2) the reflection, or 

 shunting, of the nervous impulse so initiated from the 

 terminus ad quern of the afferent or sensory nerve, to 

 the terminus a quo of the efferent or motor nerve, 

 and (3) the stimulation of some effector organ, say 

 a motor organ or muscle, by the nervous impulse so 

 set up. The simplest case, perhaps, of a reflex is the 

 rapid closure of the eyelids when something, say a few 

 drops of water, is flicked into the face. Stated in the 

 way we have stated it the simple reflex does not exist. 

 In the first place, it is a concept based on the structural 

 analysis of the complex animal where the body is 

 differentiated to form tissues — receptor organs, nerves, 

 muscles, glands, and so on. But a protozoan animal, 

 a Paramcecium for instance, responds to an external 

 stimulus by some kind of bodily activity, and yet it is 

 a homogeneous, or nearly homogeneous, piece of pro- 

 toplasm, and this simple protoplasm acts at the same 

 time as receptor organ, conducting tissue or nerve, 

 and effector organ. In the higher animal certain parts 

 of the integument are differentiated so as to form 

 visual organs, and the threshold of these for light 

 stimuli is raised while it is lowered for other kinds of 

 physical stimuli. Similarly other parts of the in- 

 tegument are modified for the reception of auditory 

 stimuli, becoming more susceptible for these but less 

 susceptible for other kinds of stimuli than the adjacent 



