82 PROFESSOR ALISON ON THE NERVES OF THE EYEBALL. 



of the motor and sensitive filaments (with the interposition of grey matter), 

 which takes place in the ganglia, instead of taking place at the extremities of the 

 nervous filaments in the muscular fibres themselves. 



It is very well worthy of notice that there is one action of the eye, in which 

 the ciliary nerves are essentially concerned, and in which there is a distinct re- 

 sulting sensation consequent on their action, and that in that action the ciliary 

 nerves and the iris may be said to act in obedience to the will : I mean that still 

 mysterious effort, whereby the eye increases its own refracting power, and so en- 

 ables the rays from an object brought gradually nearer it, to form a distinct image 

 on the retina and excite a distinct sensation in the mind ; which effort is uniformly 

 coincident with a gradual contraction of the pupil. Here an effort of volition is 

 made in the direction of the eye, and the continued gratification of the sense, re- 

 sulting from that effort, in so far as it affects the refractive power, seems to act 

 the same part there, as the gratification of the sensations in the chest, in regulat- 

 ing the contractions of the muscles of respiration. 



However, I am aware that objections may be stated to these speculations; 

 and probably it is wiser to rest at present on the general inference, deducible from 

 a comparison of the ganglionic nerves of the eye and of other parts, that when the 

 sensitive and motor filaments which connect a muscle with the spinal cord, meet 

 in a ganglion before reaching the cord, their endowments are so far modified 

 that the sensations thence resulting are rendered less precise ; that the efforts of 

 the Avill cannot be insulated on such a muscle, and, therefore, although capable 

 of being influenced by the will, it is truly involuntary. 



But it is obviously part of the design of Nature, in the construction of the gangli- 

 onic nerves, not only that they should withdraw the muscles they supply from the 

 dominion of the will, but likewise that they should facilitate and increase upon them 

 the power of what I have elsewhere called Sensorial Influence, i. e. the influence 

 attending or resulting from Sensations and Emotions of mind, which we know to 

 originate or to be excited exclusively in the larger masses of the nervous system, and 

 to act with peculiar power on muscles and other organs which have their nerves 

 through the ganglia. Here also the study of the eye gives us important information. 



The ordinary action of the iris, in obedience to the stimulus of light, is cer- 

 tainly effected by a reflex action, in which the optic nerve, the corpora quadrige- 

 mina, and the 3d nerve are concerned, and which has been fully illustrated by the 

 experiments of MAYO, FLOURENS, VALENTIN, and others. That the peculiar sen- 

 sation of light, excited by the impression on the corpora quadrigemina, not only 

 attends the action but regulates its degree, is at least highly probable ; although 

 it is right to admit, that the action occurs occasionally in cases of amaurosis, 

 where the patient expresses himself as conscious of no sensation ; and I do not 

 think that there is so good evidence of the necessary interposition of mental 

 changes in this action, performed by an involuntary muscle, as in the cases where 



