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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 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. 65. 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. 66, or they may make 

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

 nected with one or more motor neurons (Fig. 67). According 



Fig. 65. — 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, on the other hand, 

 several sensory paths may make connections with one efferent 

 path. A fact which must be borne in mind in studying some of 

 the well-known reflex activities of the cord and medulla is that 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 coflaterals, connect with motor nuclei in the cord 

 or medulla and thus at the same time give origin to involuntary and 

 even unconscious reflexes. Painful stimulation of the skin, for ex- 

 ample, may give us a conscious sensation of pain and at the same 

 time reflexly stimulate the vasomotor center and cause a constric- 

 tion of the small arteries. The fact that in this case two distinct 

 events occur does not necessitate the assumption that the impulses 



