GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 139 



state of our knowledge, but we have a fairly good general idea of 

 the way in which nerve impulses are probably guided. A sensory 

 neuron has several, collaterals, each with its synaptic connection 

 with another neuron. If we suppose these synapses are not all 

 alike, but that certain ones transmit impulses more readily than 

 do the others, an impulse spreading over the sensory neuron will 

 pass most easily to that connecting neuron whose synapse offers 

 least resistance* to its passage. Thus we may imagine an impulse 

 spreading from neuron to neuron following always the path of 

 least resistance until it finally terminates in a muscle which it 

 arouses to activity. In the central nervous system the various* 

 paths of least resistance are so blocked out as to lead to adapt-' 

 ive motions; a prick on the finger causes retraction of the hurt 

 hand; irritation in the nose causes the convulsive movements of 

 the respiratory muscles which constitute a sneeze; in each case the 

 motions are calculated to get rid of the source of irritation. 



That adaptive reflexes are due to paths of least resistance 

 blocked out from an infinite number of possible paths is strik- 

 ingly illustrated by the effects of strychnine poisoning. This 

 drug acts on the central nervous system in such a way as to 

 abolish differences of synaptic resistance. When one suffering 

 from the drug receives a stimulus by way of any sensory nerve 

 the impulse, instead of following the usual path, spreads over the 

 whole central nervous system; all the muscles are stimulated 

 simultaneously and the well-known strychnine convulsion results. 



The Orderly Spreading of Reflexes. The conception of graded 

 synaptic resistances explains also in a very satisfactory way the 

 phenomenon of the orderly spreading of reflexes. A feeble stim- 

 ulus produces reflex movement in those muscles only which are 

 immediately concerned in the adaptive response; stronger stimuli 

 involve more muscles, but only such as by their movement make 

 the response more effective. For example, if a frog's hind leg is 

 touched gently it will be drawn away from the source of irrita- 

 tion; a stronger stimulus is likely to cause contractions of such 

 additional muscles as are required for jumping away from the 

 point of danger. If we assume that the reflex paths to the first 

 set of muscles have such low resistances as to allow feeble impulses 

 to pass them, and that stronger impulses can overcome enough 

 additional resistance to enter the paths of higher resistance lead- 



