ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. 



(571 



Reflex Action. A motor reflex act is the transmission of an 

 itation by the neuraxon of a sensory neuron to the dendrons of a 

 lotor neuron and by its neuraxon in turn to the muscle. 



The functions of the gray substance of the nervous centers can 

 known only through reflex movements; so that, to study reflex 

 iction is to study the nervous centers. 



From a knowledge of the principles of a reflex action it will be 

 jen that three stages must be considered: 1. The external excita- 



Fig. 271. Elementary Reflex Arc. Course of Sensory Impressions 

 and a Motor Impulse, Passing Through the same Level of the Spinal 

 Cord. ( MORAT. ) 



1, Skin. 2, Sensory nerve. 3, Posterior root ganglion. 4, Anterior root. 

 5, Motor nerve. 6, Muscle. 



ion which goes to excite the nervous centers through the sensitive 

 jrves as a medium. 2. The excitation of the nervous centers which 

 iive the irritation and then transform and modify it; through 

 e medium of the motor nerves it is communicated to the muscles. 

 The contraction of the muscle thus innervated. 



OTHER SEATS. It is not only in the spinal cord properly so 

 died that there are reflex acts. There are some in the medulla 

 rtongata, in the pons, and in the gray parts of the brain. 



The physiological study of strychnine shows what intimate con- 



