PROFESSOR ALISON ON THE NERVES OF THE EYEBALL. 73 



the muscles of respiration, in consequence of a sensation excited in one of the 

 nostrils, or one of the bronchise. 



3. Another fact as to these consentient movements, is satisfactorily observed 

 only in the eye, but is no doubt extensively applicable in many parts, viz. that the 

 stimulus of this consentient movement of voluntary muscles passes through the 

 ganglia, and thereby aifects muscles of strictly involuntary motion ; the iris being 

 distinctly observed to contract whenever the eyeball is voluntarily and forcibly 

 rolled inwards by the action of the third nerve. And MULLER relates experiments 

 in his own person, distinctly shewing that this effect takes place even on the pu- 

 pil of the right eye, in consequence of forcible voluntary exertions made through 

 the third nerve of the left eye, and when the right eyeball is not moved. 



I think it impossible to doubt that MULLER is so far right in ascribing these 

 phenomena to what he calls " the conducting power of the cerebral substance at 

 the origin of the nervous fibres, whereby those which are contiguous to each other 

 are liable to be affected simultaneously, and the influence of the will (or of any 

 mental act) is with difficulty confined or insulated on individual fibres," or some- 

 thing is required to insulate it ; and that these observations put us in possession 

 of an important fact regarding the influence, either of volition or of sensation, or 

 of the changes in the nervous matter attending these mental acts, in exciting mus- 

 cular action, viz. that this influence naturally extends to some distance in the larger 

 masses of the nervous matter, and requires the action of some additional cause, 

 to insulate it on individual muscles, or portions of muscles. And in so far as the 

 motor influence dependent on sensation is concerned, this is strictly in accordance 

 with what is observed as to the imitation of that influence, in experiments on 

 the reflex function in paralyzed or decapitated animals. 



I think MULLER is also certainly right in supposing that the tendency to con- 

 sentient movement in the similar or corresponding portions of any pair of nerves, 

 is the reason why the third nerve is not employed to give the movement outward 

 to the eyeball ; two other nerves (the fourth and sixth) being employed to give 

 this movement, because it is a movement which must always be consentient with 

 that excited in a dissimilar part, and therefore through dissimilar nerves, on the 

 other side of the body. And although this tendency to consentient motion is 

 much less seen in the nerves of the same pair going to the extremities, yet MUL- 

 LER justly observes, that the extreme difficulty always felt in rotating one arm in 

 one direction, and the other in the opposite at the same moment, must be ascribed 

 to the violation implied in that effort, of this tendency to consentient action in the 

 corresponding portions of the same pairs of nerves. 



But I think it also certain, particularly from what we see in the eye, that 

 this observation goes but very little way in explaining the general phenomenon 

 of Consentience. The tendency to consentient action in the nerves of the same 

 pair in any part of the extremities, is so slight as to shew, that the conducting 



