CONSENSUAL MOVEMENTS OF THE EYE. 189 



254. 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, as we have seen, involves the necessity (in a 

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

 the two eyeballs. Now it is fair to argue from the facts already stated 

 (respecting the distribution of the Third pair, and the known functions of its 

 inferior branch), that, in directing our eyes by a voluntary effort to any par- 

 ticular object, the will acts chiefly upon one eye, and that the other follows its 

 direction by an automatic movement. This automatic movement appears to 

 be governed by the relative place of the images upon 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 congenital 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 or luminous, and a more vivid impression therefore 

 made upon the retina. It is 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 movements having been once 

 acquired, the known laws of nervous action account for its continuance ; and, 

 as a matter of fact, a want of consent may be often noticed where the blind- 

 ness is total. The peculiar vacant appearance, which may be noticed in the 

 countenance of persons completely deprived of sight by amaurotic or other 

 affections which do*not alter the external aspect of the eyes, seems to result 

 from this, that their axes are parallel, as if the individual were looking into 

 distant space, instead of presenting that slight convergence, which must 

 always exist between them when the eyes are fixed upon a definite object. 

 This convergence, which is of course regulated by the Internal Recti, varies 

 in degree according to the distance of the object ; and it is astonishing how 

 minute an alteration in the axes of the eyes is perceptible to a person ob- 

 serving them. For instance, A sees the eyes of B directed towards his face, 

 but he perceives that B is not looking at him ; he knows this by a sort of 

 intuitive interpretation of the fact, that his face is not the point of convergence 

 of B's eyes. But if B, who might have been previously looking at something 

 nearer or more remote than A's face, fix his gaze upon the latter, so that the 

 degree of convergence of the axes is'altered, without the general direction of 

 the eyes being in the least affected, the change is at once perceived by the 

 person so regarded ; and the eyes of the two then meet. 



255. The foregoing considerations may be summed up in this simple state- 

 ment ; that, when the axis of one eye is voluntarily directed towards an 



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 impair- 

 ment results in part from disuse merely, seems very evident, from the great improve- 

 ment 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. 



