l88 FROM FISH TO PHILOSOPHER 



an important role among the vertebrates in the trans- 

 mission of nerve impulses across the synaptic jimction 

 and also across the junctions between nerves and other 

 cells, such as those of muscles and glands. 



The synaptic junction between nerve and nerve, first 

 appearing in the jellyfish and allied forms, introduces 

 the imique features of one-way conduction and pro- 

 visional or conditional response. Nerve impulses arriving 

 simultaneously at a synapse may have an additive effect, 

 thus producing locally the phenomenon which the 

 neurophysiologist calls 'summation'; or, alternatively, 

 such convergent impulses may prevent transmission from 

 one nerve to another, producing synaptic 'inhibition.' 

 The synaptic junction has been likened to a valve: it is 

 the essential mechanism in the reflex arc which subordi- 

 nates the activity of the arc to events occurring else- 

 where in the body. 



The nervous system acquires its capacity for complex 

 reactions by combining these four basic operations- 

 protoplasmic conduction, neural conduction, humoral 

 excitation or inhibition, and synaptic conduction— into an 

 elaborate network of communication extending to all 

 parts of the body, and co-ordinated by a 'central nerv- 

 ous system.' Li the lower metameric animals (take the 

 centipede again, or the angleworm) the central nervous 

 system consists of a chain of reflexes in which each event 

 in the chain gives rise to impulses eliciting the next reac- 

 tion in the series, the entire series being 'fired' by, or 

 imder the control of, one or more large nerve centers or 

 ganglia in the anterior end of the body. 



In the protovertebrate the 'central nervous system' 

 was probably a simple longitudinal tube of nervous tissue 

 with which the peripheral nerves were connected in a 

 segmental fashion, to meet the needs of the segmentally 

 arranged muscles. This is essentially the pattern of the 

 spinal cord in all vertebrates. But as the animal increased 

 in complexity, and particularly with the progressive evo- 

 lution of the distance receptors— the nose, eyes, and ears 



