THE SPESTAL COED AS A NEEVE-CEXTEE. 299 



produced by direct stimulation of the muscular tissue or of 

 individual motor nerves. 



We must regard the gray matter of the brain and spinal 

 cord as a connected chain of ganglia, capable of receiving 

 impressions through the sensory nerves, and of generating 

 the so-called nerve-force. The great cerebro-spinal axis, 

 taken as a whole, has this general function ; but some parts 

 have separate and distinct properties," and can act indepen- 

 dently of the others. The cord, regarded as a conductor, 

 connects the brain with the parts to which the spinal nerves 

 are distributed. If the cord be separated from the brain in 

 a living animal, it may act as a centre, independently of the 

 brain ; but the encephalon has no communication with the 

 parts supplied with nerves from the cord, and can only act 

 upon the parts which receive nerves from the brain itself. 



It has been pretty clearly shown that when the cord is 

 separated from the encephalon, an impression made upon 

 the general sensory nerves is conveyed to its gray substance, 

 and is transformed, as it were, into a stimulus, which is 

 transmitted to the voluntary muscles, giving rise to certain 

 movements, independently of sensation and volition. This 

 impression is said to be reflected back from the cord through 

 the motor nerves ; and the movements occurring under these 

 conditions are called reflex. As they are movements excited 

 by stimulation of sensory nerves, they are sometimes called 

 excito-motor. 



The term reflex may properly be applied to any genera 

 tion of nerve-force which occurs as a consequence of an im- 

 pression received by a nerve-centre ; and reflex phenomena 

 are by no means confined to the action of the spinal cord. 

 The movements of the iris are reflex, and yet they take place 

 in many instances without the intervention of the cord. 

 The movements of respiration are reflex, and these are pre- 

 sided over by the medulla oblongata. Movements of the 

 intestines and the involuntary muscles generally are reflex, 

 and they involve the action of the sympathetic system of 



