134 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



cording to the neuron theory, therefore, the simplest reflex arc 

 must consist of two neurons: the sensory neuron, whose cell body 

 lies in one of the posterior root or cranial nerve ganglia, and a 

 motor neuron, whose nerve cell lies in the anterior horn of gray 

 matter of the cord or in the motor nucleus of a cranial nerve. The 

 reflex arc for the spinal cord is represented in Fig. 61. The arc 

 may, however, be more complex. The sensory fibers entering 

 through the posterior roots may pass upward through the entire 

 length of the cord to end in the medulla and on the way give off 

 a number of collaterals as represented in Fig. 62, or they 

 may make connections with intermediate cells which, in turn, are 

 connected with one or more motor neurons (Fig. 63). According 



Fig. 61. Schema to show the connection between the neuron of the posterior root and the 

 neuron of the anterior root, the reflex arc. 



to these schemata, one sensory fiber may establish reflex connections 

 with a number of different motor fibers, or, a fact which must be 

 borne in mind in studying some of the well-known reflex activities 

 of the cord and medulla especially, a sensory fiber carrying an 

 impulse which eventually reaches the cortex of the cerebrum and 

 gives rise to a conscious sensation may, by means of its collaterals, 

 connect with motor nuclei in the cord or medulla and thus at the 

 same time give origin to involuntary and even unconscious re- 

 flexes. Painful stimulation of the skin, for example, may give 

 us a conscious sensation of pain and at the same time reflexly 

 stimulate the vasomotor center and cause a constriction of the 

 small arteries. The fact that in this case two distinct events occur 

 does not necessitate the assumption that the impulses from the 



