916 OP MUSCULAR MOVEMENTS. 



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 deaf and dumb ; and, as a matter of fact, a want of consent 

 may often be observed where the blindness is total. The peculiar vacant 

 appearance, which may be noticed in the countenances of persons, completely 

 deprived of sight by amaurotic or other affections, which do not alter the" ex- 

 ternal 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 per- 

 ceptible to a person observing 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 pre- 

 viously looking at something nearer or more remote than A's face, fix his gaze 

 upon the latter, so that the degree of the 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. 



920. The physiological principles which have now been stated have an im- 

 portant application in the treatment of Strabismus by operation ; a practice 

 whose frequent want of success is due in great part to the injudicious selection 

 of cases, and to the wrong measures pursued. The degree in which habit 

 accustoms parts of the retinae that did not originally correspond, to work 

 together harmoniously, is remarkably shown 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 frequently noticed as an 

 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 be 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. That there is a greater tendency to 

 consent between the images, however, when they are formed upon the parts of 

 the two retinae which normally correspond, may be freely admitted ; and this 

 seems to be a principle of some importance in determining the readjustment of 

 the eyes, after the operation for Strabismus. This readjustment is not always 

 immediate j for 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 inver- 

 sion 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 favor of the consensual movement 

 being chiefly regulated by the correspondence in the seats of the impressions on 

 the two retinae, 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 unfre- 

 quently 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 



