348 FUNCTIONS OF THE NF.RVOUS SYSTEM. 



would even divide its other tendons, if the parallelism be not restored, rather than touch 

 the other eye. The Author is himself satisfied, however, that the restriction of the abnor- 

 mal 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 fre- 

 quently be observed in them very irregular movements; the axes of the two being some- 

 times extremely convergent, find then very divergent. This irregularity is rarely or never 

 see^i 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. 



d. 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 eye 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 looking 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 in 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. 

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

 can only be checked by division of the tendon of the other eye, A; after which the cure is 

 generally complete and permanent. That it has not been so, in many of the cases on 

 which operations have been performed, the Author attributes, without the slightest doubt in 

 his own mind, to the neglect of the second operation. As just now stated, the sight of the 

 most inverted eye is frequently very imperfect ; indeed it is sometimes impaired to such an 

 extent, that the patients speak of it as entirely useless. That this impairment results' in part 

 from disuse merely, seems very evident, from the great improvement which often succeeds 

 the rectification of the axes. The Author cannot help thinking it probable, however, that 

 the same cause which produced the distortion of the eye may, in some instances at least, 

 have affected the Optic nerve, as well as the Motor nerves of the orbit; and this idea is 

 borne out by the fact of the restoration of sight, in certain cases of Amaurosis, by division of 

 one or more tendons, where no Strabismus previously existed. (See Adams on Muscular 

 Amaurosis.) It is interesting to remark that, in these cases, Strabismus was usually the 

 first effect of the operation ; but that the eye generally recovered its ordinary position within 

 a short time, especially when the sight was improving. 



455. If this be admitted, we gain an important step in the explanation of 

 the Consensual movements of the Eye. The object to be attained is evidently 

 this, that the usual axes of the eye should always be directed towards the 

 object to be viewed; and this, asfve have seen, involves the necessity (in a 

 great majority of cases), of unsymmetrical movements being performed by 

 the two eyeballs. The combination of these movements is involuntary or 

 automatic ; and appears to be regulated by the sensations received through 

 the retinae. It is well known that, in children born blind, the movements are 

 not consensual ; they are frequently very far from being so, in cases of con- 

 genital cataract, where a considerable amount of light is evidently admitted, 

 but where no distinct image can be formed ; and in such cases, the movements 

 are most consensual where the object is bright and luminous, and a more vivid 

 impression therefore made upon the retina. It ie no objection to this theory 

 to say, that persons who have become blind may still move their eyes in a 

 consensual manner ; since, the habit of the association of particular move- 

 ments having been once acquired, the guidance of the muscles may be effected 

 by sensations derived from themselves, in the manner in which it takes place 

 in the laryngeal movements of the deal' and dumb ; and, as a matter of fact, 

 a want of consent may be often noticed where the blindness is total. The 

 peculiar vacant appearance, which may be noticed in the countenances of per- 

 sons completely deprived of sight by amaurotic or other affections, which do 



