REFLEX ACTIOX. 



247 



The posterior root fiber, shortly after entering the cord, gives off 

 a branch at right angles (called a collateral), or in its course up 

 the cord it may give off several collaterals, their destination 

 being the grey matter of the cord, in which they terminate by 



Fig. 42. Diagram of section of spinal cord, showing tracts, i After K61- 

 Mker) ; p, posterior median, and b, postero-lateral columns ; p.c., crossed 

 pyramidal, and p.d., direct pyramidal tracts, /, cerebellar tract. (After 

 HowelL) 



synapses around nerve cells. Certain of these may be cells of 

 the anterior horn. These cells give rise to the efferent fibers, 

 which leave the spinal cord by the anterior or motor roots (see 

 Fig. 41). Other collaterals run to intermediary cells, which 

 then communicate with the anterior horn cells (Fig. 43). 



THE NERVE CENTER AND INTERMEDIARY NEURONES. When 

 the entering nerve impulse travels by a collateral to an anterior 

 horn cell, we have the simplest type of reflex action, namely, one 

 involving a receptor, a sensory nerve fiber, the posterior root, a 

 collateral, the anterior horn cell, the anterior root, a motor nerve 

 fiber and an effector organ. But such a simple reflex seldom 



