CONSENSUAL MOVEMENTS OF THE EYE. 347 



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 retinae, 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 

 Convergent 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. Although 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 re- 

 ceives the two images, frequently combining them together (as Mr. Wheat- 

 stone's ingenious experiments with the Stereoscope have most satisfactorily 

 shown, 547) to produce a picture in relief; and so long as these are con- 

 veyed to it in the accustomed manner, it reconciles them together, even if the 

 parts of the retinaa on which they are formed do not correspond; but if any 

 circumstance break this chain, and cause the images to be transmitted to the 

 sensorium through a new channel, the mind requires some little time to 

 adapt itself to this impression, as it does by habit to almost every other. 



a. That there is a greater tendency to consent between the images, when they are formed 

 upon corresponding parts of the retinae, the Author readily admits ; and he thinks that this 

 is a principle of some importance, in explaining the re-adjustment of the eyes, after the 

 operation for Strabismus. Every one who has seen much of this operation is aware, that 

 the re-adjustment of the eye is not always immediate, but that, after the muscle has been 

 freely divided, the eye often remains somewhat inverted for a few days, gradually acquir- 

 ing its straight position. The Author has known one case, in which, after such a degree of 

 temporary inversion as seemed to render the success of the operation very doubtful, ever- 

 sion actually took place for a short time to a considerable extent ; after which the axes be- 

 came parallel, and have remained so ever since. 



b. Another argument, derived from the results of this operation, in favour of the con- 

 sensual movement being chiefly dependent upon th^ place of the impressions on the retina, 

 is, that it is much more successful in those cases, in which the sight of the most displaced 

 eye is good, than in those in which, (as not unfrequently happens from long disuse) it is 

 much impaired. In cases of the latter class, the cure is seldom complete. There is another 

 curious fact, which may be adverted to in reference to this subject : Strabismus not unfre- 

 quently arises from the formation of an opaque spot on the centre of the cornea, which pre- 

 vents the formation of any images on the retina, except by the oblique rays ; and nature 

 seems to endeavour (so to speak) to repair the mischief, by causing the eye to assume the 

 portion most favourable for the reception of these. 



c. To one more point only, connected with the subject of Strabismus, would the Author 

 now allude. He is well convinced, from repeated observation, that those Surgeons are in the 

 right, who have maintained, in a recent controversy, that, in a large proportion of cases, 

 strabismus is caused by an affection of both sets of muscles or nerves, and not of one only ; 

 and that it then requires, for its perfect cure, the division of the corresponding muscle on 

 both sides. Cases will be frequently met with, in which this is evident; the two eyes 

 being employed to nearly the same extent, and the patient giving to both a slight inward 

 direction, when desired to look straight forwards. In general, however, one eye usually 

 looks straight forwards, whilst the other is greatly inverted: and the sight of the inverted 

 eye is frequently affected to a considerable degree by disuse ; so that, when the patien; 

 voluntarily rotates it into its proper axis, his vision with it is far from being distinct. Some 

 Surgeons have maintained, that the inverted eye is usually the only one in fault, and con- 

 sider that the division of the tendon of its Internal Rectus is sufficient for the cure. They 



