162 THE HUMAN BODY 



so blocked out as to lead to adaptive 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 impulses, 

 instead of following the usual path, spread 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 impulse 

 streams to pass them, and that stronger impulse streams can 

 overcome enough additional resistance to enter the paths of 

 higher resistance leading to the jumping muscles, while the paths 

 to muscles not concerned in any way in an adaptive response have 

 too high resistance to be passed at all, we can account for reflex 

 actions of very great complexity. 



Simple Reflexes Mediated by the Spinal Cord. The simple re- 

 flexes described in the preceding paragraphs are all of a sort that 

 can be carried on through the lowest part of the central nervous 

 system, the spinal cord. A frog whose brain has been destroyed 

 and which is therefore wholly devoid of feeling and consciousness 

 can still perform highly complicated reflex acts; he will retract a 

 foot which is pinched; he will wipe off a bit of acid-soaked paper 



