300 NEKVOUS SYSTEM. 



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 ; 

 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 impressions made 

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

 to dream of certain things, as a consequence of hearing pe- 

 culiar noises. Examples of this kind of mental reflex action 

 are sufficiently numerous and well authenticated. 1 



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 movements, occurring without con- 

 sciousness, as the result of impressions made upon the affe- 

 rent nerves, and involving the independent action of the 

 spinal cord. 



Reflex Action of the Spinal Cord. In 1832 and 1833, 

 Marshall Hall described minutely the movements which take 

 place in decapitated animals as a consequence of stimulation 

 of the sensory nerves, and formularized these phenomena 

 under the head of " the reflex function of the medulla ob- 

 longata and medulla spinalis." a Since this publication, a 

 new interest has been attached to the writings of some of 

 the older physiologists, in which reflex action, as it is now 



1 For numerous instances of peculiar dreams referable to external impres- 

 sions received during sleep, see, HAMMOND, Sleep and its Derangements, Philadel- 

 phia, 1869, p. 12Y, et seq. 



2 MARSHALL HALL, On the Reflex Function of the Medulla Oblongata and Me' 

 dulla Spinalis, London, 1833. 



