74 PROFESSOR ALISON ON THE NERVES OF THE EYEBALL. 



power at the origins of these nerves cannot he very strong, and, therefore, that 

 proximity of origin can afford hut a very imperfect explanation of the very 

 strong tendency to consentience remarked in almost all the motions of the trunk 

 of the body. Consciousness informs us that, although it is very difficult to act 

 at the same moment on dissimilar portions of the same pair of nerves, yet there 

 is in general no difficulty in refraining from acting at the same moment on the cor- 

 responding portions ; and in no case any difficulty in acting, at the same mo- 

 ment, on dissimilar and distant nerves. And there are facts observed in the eye, 

 which have quite the value of the experimentum crucis, as shewing, that the chief 

 cause of consentience of movement in our muscular organs is very different from 

 the connection of nerves, at their roots or in their course. These facts are, that 

 while those corresponding portions of the 3d nerve, which elevate and depress the 

 eyeball, i. e. those which go to the superior and inferior recti, always act simul- 

 taneously ; those which go the rectus internus and inferior oblique do not usually 

 act together in the two eyes. Again, the 4th and 6th nerves never act together on 

 the two sides of the body, but each is uniformly combined in its movement with 

 a portion of the 3d on the other side. The reason obviously is, that the Sensations 

 which result from the action of the 4th and 6th nerves of the one eye, cannot be 

 identified with those which result from the action of the nerves of the same pair 

 in the other eye, and cannot be separated from those which result from the action 

 of that portion < f the 3d pair in the other eye. There is no other circumstance, 

 but the identity of the resulting and guiding sensation, which can be pointed out 

 as existing where the consentience is observed, and not existing where it is not 

 observed. 



From these facts, therefore, we learn that the main cause of Consentience of 

 muscular movement is simply Identity of the Guiding Sensations. Whether it is 

 by an original instinct, or by repeated trials and acquired experience, that the 

 acts of volition are directed to the nerves in each eye, which so turn the eyeballs 

 as to keep the optic axes parallel, and so produce the single sensations, is a dif- 

 ferent question ; but what has been stated seems to me quite enough to shew, that 

 it is because the single sensations result, that these nerves are consentient. 



I have no doubt that this principle, deduced from the movements of the eye- 

 ball, is strictly applicable to all the cases of consentient movement excited by 

 the nerves of the same pairs on the face, fauces, thorax, abdomen, and pelvis, in 

 the different actions which have been already mentioned. The movements which 

 these nerves excite, are always followed by certain sensations, generally grateful, 

 influenced by the degree in which the actions are performed ; and by these sen- 

 sations, the extent to which the actions are carried, and the energy with which 

 they are performed, are felt to be habitually regulated. These resulting and guid- 

 ing sensations are felt to be affected exactly alike by the movement which is 



