REFLEX ACTION 



77 



matter what modifications the latter may undergo, though this 

 principle is not now considered so absolutely rigid as it was formerly 

 thought to be. 



Reflex Action 



It is difficult to determine what portion of a muscular con- 

 traction, even of one which is considered to be purely voluntary, 

 results from an impulse actually originating in the central nervous 

 system. The living body, however, affords many examples of 

 muscular actions as direct responses to immediately previous in- 



FiG. 41. Camera lucida tracings of transverse sections of the spinal cord 

 of a rabbit to show the variations in the shape and in the proportions of grey 

 matter and of white matter in representative segments: 2nd cervical, 7th 

 cervical, 5th thoracic, 4th lumbar, 1st sacral, and 1st caudal. 



