PROFESSOR ALISON ON THE NERVES OF THE EYEBALL, 77 



or disease one of these nerves should be cut across, or lose its powers, we should, 

 on the first supposition, preserve one-half of the powers, both of flexion and exten- 

 sion, which would surely be preferable to our possessing fully the power of flexion 

 without any power of extension. And thus, in the arm, where five trunks are 

 found, there would on this supposition, as to the use of a plexus, be only one-fifth 

 of the power lost, of performing any motion, by division of any one of these 

 nerves." (Obs. p. 45.) 



That this is really the effect of this arrangement in regard to the effects of 

 injury, appears to be siifficiently established by the experiments by PANIZZA on 

 frogs, in which animals the plexus supplying the inferior extremities is much less 

 intricate than in the mammalia. " If," he says, " one anterior root of the three last 

 spinal muscles be cut, the motions of the corresponding extremity are as perfect as 

 if the motiferous nervous system of the part had not been injured. Even if two 

 roots be divided, although for a moment the motions are not so energetic as at first, 

 yet they are speedily renewed, and the frog springs as if it had suffered no injury. 

 Yet by this operation, more than two-thirds of the nervous matter which presides 

 over the motion of the extremity is destroyed ; and if the third filament is divided, 

 all motion immediately ceases in the limb." " Whence, if I am not mistaken, 

 appears the use of the nervous plexuses, which, by the intermixture of the filaments 

 of different roots having a common function, establish among them, as it were, 

 such a concentrated force, that each is adequate to preserve the integrity of the 

 function, when, by means of any harm, the continuity of the other filaments js 

 interrupted." (Edin. Med. and Surg. Journal, No. 126, p. 89.) 



I am aware of experiments by CRONENBERG and by MULLER, who found that 

 by cutting one of the nerves entering the crural plexus in the frog, they could 

 paralyze or greatly enfeeble certain movements of a limb, and leave others unim- 

 paired ; and of the elaborate investigations of MULLER and others in Germany, 

 which lead to this conclusion, that every nervous fibril, whether passing through 

 a plexus or not, remains perfectly distinct from its origin to its termination. 

 Notwithstanding these observations, it is distinctly admitted by MULLER, that 

 " plexuses convey to each muscle of a limb fibres from different parts of the 

 brain and spinal cord." 



It seems to me, however, hardly possible to suppose, that this very carefully 

 adjusted piece of structure is intended merely as a guard against injury, and 

 therefore is of no use in any person or animal on whom such an injury as the 

 section of one of the nerves of an extremity has never been inflicted. But if we 

 advert to what has been said already of the evidence that any voluntary effort, 

 which excites a muscle to contraction, extends its influence over a considerable 

 portion of the cerebro-spinal axis, and at the same time to the evidence, in the 

 experiments above quoted, that every muscle supplied from a plexus, has part of 

 its motor nerves, and may be excited to contraction, from each of the nerves en- 



