76 PROFESSOR ALISON ON THE NERVES OF THE EYEBALL 



which the muscles enter ; while the filaments of nerves which go to the skin re- 

 gularly diverge to their destination. From the fin of a fish to the arm of a man, 

 the plexus increases in complexity in proportion to the variety or extent of mo- 

 tions to be pea-formed by the extremity. By the interchange of filaments, the 

 combination among the muscles is formed ; not only are the classes of extensors 

 and flexors constituted in the plexus, but all the varieties of combinations are there 

 formed, and the curious relations established which exist between opposing muscles, 

 or rather between the contractions of one class and the relaxation of another." In 

 short, it appears to be his idea, that a plexus is necessary to enable a single effort 

 of the mind to throw into action a combination of muscular contractions, and a 

 succession of efforts to excite such a succession of these combinations as exists 

 in every complex movement. 



But the case of the muscles of the eyeball seems quite sufficient to set aside 

 this opinion. None of these nerves on the opposite sides of the body are con- 

 nected by plexuses, yet no nerves can combine their actions more perfectly or 

 more surely. There is no more perfect consentience in the living body than that 

 between the 6th nerve of the right eye, and the inner portion of the 3d of the left, 

 and both are often exerted in varied combinations with many other nerves and 

 muscles ; but no nerves in the body can have less connection, so far as anatomy 

 informs us, either at their origin or in their course. 



In fact, when we reflect on what passes within us when we throw into ac- 

 tion any two muscles at the same moment, we shall see that when such a vo- 

 luntary effort is made, it is just as easy for us to excite simultaneously the most 

 widely distant or the most closely contiguous muscles ; and again, when we at- 

 tend to the necessary selection of so many different and distant muscles, in any of 

 the requisite combinations which are apparently under the influence of Sensation, 

 as in coughing, sneezing, vomiting, &c. we shall perceive that, in the entire state 

 of our faculties, any intense sensation may be said to have at its command all 

 the muscles of the body ; and although, as I have stated, I believe all mental acts 

 to be guided by sensations in the selections which they make, yet I think it quite 

 plain that neither proximity of origin, nor connection in their course, can be as- 

 signed as the cause of any of these selections. 



I believe that Dr MONRO made a nearer approach to the true statement of 

 the use of a plexus, and put it in a simpler view, when he said, that " the chief 

 intention of Nature in this very solicitous intermixture of the nervous fibrils, is to 

 lessen the danger by which accidents or diseases affecting the trunks of the nerves 

 would, without these contrivances, have been attended. Thus let us suppose, that 

 two nerves are sufficient to supply the flexors and extensors of the forearm, it is 

 evidently better for us that the one-half of each nerve should go to the flexors, and 

 the other half of each to the extensors, than the whole of the first nerve should have 

 gone to the flexors, and the whole of the second to the extensors. For if by accident 



