70 PROFESSOR ALISON ON THE NERVES OF THE EYEBALL. 



2. Conversely, in various instances, different impressions made on the same 

 parts of the body, and therefore on the same sensitive nerve, exert different sen- 

 sations, in which case they are not followed by the same reflex actions. Thus 

 certain impressions on the nostrils and face, followed by the sensation of cold or 

 of tickling, excite the act of inspiration, but other impressions on the same parts, 

 fully as strongly felt, but exciting different sensations, as in cutting or bruising, 

 have no such effect ; and the same is remarkably observed as to different im- 

 pressions on the fauces and on the stomach, some of which excite nausea and 

 then retching, while many others have no such effect. These facts plainly in- 

 dicate that, in the natural state, the reflex actions, characterized as above stated, 

 follow not the impressions on particular nerves, but the excitement of particular 

 sensations. And it is easy to shew that many phenomena seen during sleep, or 

 in decapitated animals (when the medulla oblongata has been left in connexion 

 with the cord), and which have been thought indications even of well regulated 

 reflex movements, independent of sensation, may be reconciled to the same doc- 

 trine, if we remember that sensations may be quite distinct, but momentary, and 

 so leave no trace on the recollection. 



Then it is to be remembered, that several of these reflex actions are abso- 

 lutely identical with those which are excited by emotions and passions, i. e. by 

 changes which are peculiar to the mental part of our constitution, as in the cases 

 of sighing, Aveeping, laughing, even retching and vomiting ; and again, that they 

 are observed to be remarkably obedient to well known laws of mind. Thus they 

 are, like the strictly voluntary actions, obedient to the law of habit, which, as 

 applied to the mental changes preceding muscular contractions, is merely the law 

 of association of ideas ; and they are so effectually controlled by the occurrence 

 of any very engrossing mental act, sensation, emotion, or voluntary effort, as 

 plainly to imply, that they are not only attended by the consciousness, but mo- 

 dified by the agency, of the mental part of our constitution. 



I stated and illustrated these facts, chiefly by commenting on the writings of 

 WHYTT and MONRO, before the offices of the brain and the cerebellum, in animal 

 motion, had been clearly distinguished from those of the spinal cord ;* and it does 

 not appear to me that their force is in the least impaired by the facts which have 

 been since ascertained, touching the portions of the nervous matter with which 

 sensation, or recollection, or any other mental act, is especially connected.' 



The case now before us, however, is one in which we see exemplified, not 

 merely the power of sensations, directly, or through the intervention of other 

 mental acts resulting from them, to excite muscular motion, but more especially 

 their office in guiding and regulating those muscular actions Avhich are excited 

 through the nerves. The difference between the muscles of the eyeball and other 



* See Edinburgh Medico-Chirurgical Trans, vol. ii. 



