204 INNERVATION. [CHAP, vni. 



fear, joy, disgust, horror, are each accompanied with sensations so 

 peculiar, as to leave an indelible impression on the minds of those 

 who have once experienced them. 



There are many actions of the living frame, however, in which 

 the play of the nervous system is unconnected with mental change, 

 which are therefore wholly physical, in origin, as well as in nature. 

 The movement of the oesophagus in propelling food onwards to the 

 stomach is dependent mainly, if not solely, upon the physical 

 stimulus of the food acting upon the nerves of the organ, which in 

 their turn provoke its muscular fibres to contract. The slightest 

 touch, even of a feather, to the mucous membrane of the fauces 

 causes the muscles of deglutition to contract forcibly, as in the act 

 of swallowing; nor can the will controul or prevent their action. 

 When the edge of the eyelid is touched, the prbicular muscle con- 

 tracts forcibly, and in immediate response to the stimulus applied. 

 When light is suddenly admitted to the eye, the pupil may be 

 observed to contract, to a degree proportionate to the intensity of 

 the stimulus. Of this action of the iris the individual is quite 

 unconscious, although perfectly sensible of the admission of light 

 to the eye : nor can he, by any direct influence of volition, modify 

 or oppose it. 



We remark, in reference to these actions, that the mind has no 

 share in their production. In some of them, indeed, it is conscious of 

 the application of the stimulus, as well as of the muscular act which 

 follows. But no effort of the will, however great, could interrupt 

 the uniform and natural sequence of the phenomena. And it is 

 well known to medical men that actions of this kind may take place 

 in coma, when all mental manifestations are completely in 'abeyance. 



These facts afford abundant evidence of a class of nervous 

 actions, which in respect of their exciting cause, as well as in their 

 intrinsic nature, are independent of mental influence, and which 

 ought on this account to be distinguished from those of volition, 

 sensation, and emotion. Their mechanism is more complex than 

 that of the mental nervous actions; for, while in the latter the 

 change in the nerves is propagated in only one direction, in the 

 former it passes first to some central part of the nervous system, 

 and thence it travels in an opposite course to the motor organs. 

 Hence two nerves are necessary for such actions; the one as an 

 excitor, the other as a motor nerve; and, on this account, Dr. 

 Marshall Hall has distinguished these actions by the name of 

 excito-motory . 



That a physical change may excite nervous action quite indepen- 



