REFLEX ACTION 249 



the cord, but by common consent reflex action is not limited 

 to the cord and its connections. 



If reflex action be defined as any involuntary manifestation 

 of nerve force consequent upon the reception of an impres- 

 sion (general or special) by a nerve center, the term must be 

 made to include such phenomena as intestinal peristalsis, 

 contraction and dilatation of the pupil, certain mental op- 

 erations, etc. In reality most reflex acts are of a complex 

 nature, involving associated action on the part of several 

 neurons and being manifested frequently at several points. 

 For example, a foreign body in the larynx causes reflexly 

 not only closure of the glottis, but also the convulsive mus- 

 cular contractions incident to coughing. The realm of reflex 

 action is obviously a wide one. 



It may be said that ordinary reflexes are usually under the 

 direction of the cord, but this does not imply that the brain 

 may not be concerned. Pricking the sole of the foot of a 

 sleeping person will cause him to draw up his leg without 

 the intervention of consciousness. Probably were he awake 

 the withdrawal would still be a reflex but he would certainly 

 be conscious of the pain, though after the act of withdrawal 

 zvas accomplished. Nor is reflex action by any means lim- 

 ited to the cerebro-spinal system. Either of the two sys- 

 tems, or both, may be concerned. 



Now in order for reflex movements to occur, there must 

 be a transference of impressions received by sensory cells to 

 cells capable of giving origin to motor impulses. The cells 

 communicate by their collaterals, which may be short or 

 long, depending on the distance between the cells concerned. 

 Cells in the gray matter of the cord are "connected" by such 

 fibers, and they run largely in the white matter of the cord 

 joining cells on different planes. They constitute the larger 

 part of the anterior fundamental fasciculi, the anterior radic- 

 ular zones, and the mixed lateral tracts, and it is these paths 

 which are mainly concerned in reftex action of the cord, 



4. Augmentation. Sensory fibers, on reaching the cord, 



