188 FUNCTIONS OF THE NERVOUS SYSTEM. 



reconciles them together, even if the parts of the retinae on which they are 

 formed do not correspond ; but if any circumstance break this chain, and 

 cause the imag'es 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.* 



* 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 

 acquiring 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, eversion actually took place for a short time to a considerable extent; after 

 which the axes became parallel, and have remained so ever since. 



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

 sensual movement being chiefly dependent upon the 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 unfrequently arises from the formation of an opaque spot on the centre 

 of the cornea, which prevents 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 position most favourable for the reception of these. 



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 patient 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 consider that the division of the tendon of its Internal Rectus 

 is sufficient for the cure. They would even divide its other tendons, if the parallelism 

 be not restored rather than touch the other eye. The Author is himself satisfied, how- 

 ever, that the restriction of the abnormal state to a single eye, is the exception, and not 

 the rule, in all but very slight cases of strabismus; and to this opinion he is led both by 

 the consideration of the mode in which strabismus first takes place, and by the results 

 of the operations which have come under his notice. If the eyes of an infant affected 

 with cerebral disease be watched, there will frequently be observed in them very irregu- 

 lar movements; the axes of the two being sometimes extremely convergent, and then 

 very divergent. This irregularity is rarely or never seen to be confined to one eye. 

 Now, in a large proportion of cases of Strabismus, the malady is a consequence of some 

 cerebral affection during infancy or childhood, which we can scarcely suppose to have 

 affected one eye only. Again, in other instances we find the Strabismus to have resulted 

 from the constant direction of the eyes to very near objects, as in short-sighted persons; 

 and here, too, the cause manifestly affects both. Now it is easy to understand, why one 

 eye of the patient should appear to be in its natural position, whilst the other is greatly 

 inverted. The cause of strabismus usually affects the two eyes somewhat unequally, 

 so that one is much more inverted than the other. We will call the least inverted eve 

 A, and the other B. In the ordinary acts of vision, the patient will make most use "of 

 the least inverted eye, A, because he can most readily look straight forwards or outwards 

 with it; but to bring it into the axis, or to rotate it outwards, necessitates a still more 

 decided inversion of B. This remains the position of things, the patient usually look- 

 ing straight forwards with A, which is the eye constantly employed for the purposes of 

 vision, and frequently almost burying under the inner canthus the other eye, B, the 

 vision ip which is of very little use to him. When, therefore, the tendon of the internal 

 rectus of B is divided, the relative position of the two is not entirely rectified. Some- 

 times it appears to be so for a time; but the strabismus then begins to return, and it can 



