REFLEX ACTIONS. 



141 



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

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

 body lies in the sensory ganglia of the posterior roots or of 

 the cranial nerves, 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. 66. 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 is represented in Fig. 67, or they may make 

 connections with intermediate cells which, in turn, are con- 

 nected with one or more motor neurons (Fig. 68). According 



Fig. 66. 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 

 skin are carried to the cord by two different varieties of fibers. 



