78 PROFESSOR ALISON ON THE NERVES OF THE EYEBALL. 



tering that plexus, we can hardly miss the conclusion, that this contrivance not 

 merely provides against injury, but multiplies the power which acts on each of these 

 muscles, and enables the mind to vary the degree of energy which it can expend 

 on each, in a degree much greater than in any case where it can act on a muscle 

 only from a single point of the spinal cord. 



Then, if we remember farther, that by means of the plexus, each sensitive 

 nerve which supplies any muscle of the extremities, consists of fibrils coming 

 from different points of the cord, we can easily perceive that, by this arrangement, 

 the sensations resulting from each portion of the muscle may be more distinct, and 

 more easily discriminated from each other, than those which are excited by ner- 

 vous fibrils bound in the same sheath throughout their course, and originating 

 beside each other in the cord. 



Thus the effect and use of a plexus will be, to make the muscular sensations 

 more precise and distinct, and to make the power which the will can exert over 

 the muscles greater, and capable of greater increase at pleasure, than where such 

 arrangement does not exist ; and therefore, to increase the force and precision 

 with which the efforts of volition may be directed and insulated on the muscles 

 which are thus supplied with nerves. And I think that any one who attends to 

 the subject may observe that he is actually conscious of these differences, when 

 he compares the effects of his voluntary exertions in his extremities with the 

 motions of his head and trunk. 



I think, therefore, that Sir CHARLES BELL was right in asserting that the 

 plexus enables the acts of the will to form combinations of muscular motions for 

 definite ends, in greater variety and with greater precision than they otherwise 

 could : but I apprehend the reason to be, not that each combination is eifected by 

 an impulse emanating from a single point, nor that the different combinations are 

 formed in the plexus, but that the plexus, rendering the muscular sensations more 

 distinct, and the acts of the will more energetic, enables the mind to act on all 

 the muscles thus supplied with more power and precision, and to recollect and 

 resume the action at any subsequent time with more certainty and uniformity, 

 and thus facilitates combinations. 



III. Let us next attend to the information given by the study of the nerves 

 of the eye, as to the influence and use of the Ganglia of the Sympathetic nerve, of 

 which it is generally admitted that the ciliary ganglion, furnishing the ciliary 

 nerves, and through which the iris is moved, is a specimen and representative. 



On this subject there has been much discussion at different times, which may 

 be set aside as irrelevant or hypothetical, because proceeding on the supposition, 

 that part of the office of the sympathetic, as of other nerves, is to give the vital 

 power or energy to the muscles it supplies. It has always seemed to me ex- 

 tremely improbable, that any one of the solid textures of the living body should 



