KEFLEX ACTION OF THE SPINAL COED. 683 



Action of the Spinal Cord as a Nerve- Centre. 



It has long been known that decapitation of animals does not immediately arrest mus- 

 cular action ; and the movements observed after this mutilation present a certain degree 

 of regularity, and, of late years, have been shown to be in accordance with well-defined 

 laws. Under these conditions, the regulation of such movements is effected through the 

 spinal cord and the nerves connected with it. If an animal be decapitated, leaving only 

 the cord and its nerves, there is no sensation, for the parts capable of appreciating sensa- 

 tion are absent ; nor are there any true voluntary movements, as the organ of the will is 

 destroyed. Still, in decapitated animals, the sensory nerves are for a time capable of 

 conducting impressions, and the motor nerves can transmit a stimulus to the muscles ; 

 but the only part capable of receiving an impression or of generating a motor stimulus is 

 the gray matter of the cord. If, in addition to the removal of all of the encephalic 

 ganglia, the cord itself be destroyed, all movements of voluntary muscles are abolished, 

 except as they may be produced by direct stimulation of the muscular tissue or of indi- 

 vidual 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 gen- 

 eral function ; but some parts have separate and distinct properties and can act inde- 

 pendently 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 

 it 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 generation of nerve-force which 

 occurs as a consequence of an impression received by a nerve-centre ; and reflex phe- 

 nomena 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 presided 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 nerves. Impressions 

 made upon the nerves of special sense, as those of smell, sight, hearing, etc., give rise to 

 certain trains of thought. These involve the action of the brain, but still they are reflex. 

 In this last example of reflex action, it is sometimes difficult to connect the operations of 

 the mind with external impressions as an exciting cause ; but it is evident, from a little 

 reflection, that this is often the case. This fact is illustrated by operations of the brain 

 which take place, as it were, without consciousness, as in dreams. It has been clearly 

 shown that a particular direction may be given to the thoughts during sleep, by impres- 

 sions made upon the sense of hearing. A person sleeping may be made to dream of 

 certain things, as a consequence of hearing peculiar noises. Examples of this kind of 

 mental reflex action are sufficiently numerous and well-authenticated. 



From the above considerations, it is evident that the term reflex may be properly used 

 in connection with many phenomena involving the action of the sympathetic system and 

 of the brain ; but it is generally understood as applying especially to involuntary move- 



