CONSENSUAL MOVEMENTS OF THE EYE. 187 



other downwards and outwards, the Inferior Rectus Is probably operating, 

 along with the Internal Rectus, on one side, whilst the Superior Oblique is the 

 chief cause of the latter movement. All these movements may be voluntarily 

 performed by Man ; but it is not so clear that the muscles by which they are 

 effected, are equally influenced by volition in each case ; and there are some 

 curious diversities in our power of operating on different muscles, which throw 

 some light on the matter. Of those which are entirely subjected to the will, 

 we can only put that pair in action together which will operate without de- 

 stroying the symmetrical position of the two eyes, namely, the Superior Recti. 

 We cannot voluntarily abduct both eyes, nor roll them downwards and out- 

 wards by the conjoint action of the two External Recti or Superior Obliques. 

 Nor, again, can we bring any of these voluntary muscles the Superior 

 Obliques and Superior Rectus, for example to act against each other in the 

 two eyes, so as to destroy their symmetry. Thus, as remarked by Valentin, 

 in almost every movement, in which the harmony of the two eyes is preserved, 

 whilst the symmetry is destroyed (as in those of the 1st, 5th, and Oth of the 

 foregoing classes), one or more muscles of voluntary motion are acting on one 

 eye, and one, or more of the automatic grouj5 are chiefly concerned in produc- 

 ing the rotation of the faker. This idea is an extremely ingenious one, and 

 will be found to be supported by other facts. 



252. But there are two kinds of movement of the Eyeballs which are not 

 at all voluntary. In the first of these, both eyeballs are rotated upwards and 

 imuards, by the action of both Inferior Obliques. In the other, both eyeballs 

 are directed inwards, by the action of both Internal Recti. Now in both these 

 cases, the harmony of the movements is destroyed : but it is by two similar 

 muscles, both acting automatically, and subjected, therefore, to the same stimu- 

 lus. In the first of these cases, the stimulus may originate in some part very 

 distant from the eye itself, and may be of a purely reflex kind ; as when the 

 eye is rotated under the lid, in the acts of .sneezing, coughing, winking, &c. 

 The latter we shall find to be another result of the same cause as that which 

 secures the usual harmonic movements of the eyeball ( 255). 



253. It may be stated as a physiological fact, that single vision with two 

 eyes is dependent upon the formation of the image upon parts of the two 

 retinae which are accustomed thus to act with each other. In many physio- 

 logical works, it is asserted, that single vision is the result of the impressions 

 being made on corresponding parts of the two retina,- that is to say, on parts 

 equally distant from the axis, on one side or the other: but this seems to be 

 disproved by the fact, that patients who have been long affected with Con- 

 vergent Strabismus, and who see equally well with both eyes (as many do), 

 are not troubled with double vision. On the other hand, when a person whose 

 eyes look straight before him, is the subject of a disorder which renders their 

 motions in any degree irregular, he is at once affected with double vision ; and 

 the same has been noticed to be a common immediate result of the successful 

 operation for the cure of strabismus, where vision is good in both eyes. Al- 

 though the images were previously formed on parts of the retinae which were 

 very far from corresponding with each other, yet no sooner is the position of 

 the eyes rectified (so that the relation between the situation of the images is 

 the same as it would have been in a sound eye"), than the patient sees double. 

 Now in these cases the difficulty very speedily diminishes, and the patient 

 soon learns to see single. It can scarcely be imagined, then, that to any other 

 cause than habit, is to be attributed the long-discussed phenomenon of single 

 vision with two eyes. The mind receives the two images, frequently com- 

 bining them together (as Mr. Wheatstone's ingenious experiments with the 

 Stereoscope have most satisfactorily shown, 339) to produce a picture in 

 relief; and so long as these are conveyed to it in the accustomed manner, it / 



