MOVEMENTS OF THE EYEBALL. 837 



Will directs the result ; and there is no other difference between theru than 

 that which arises out of our consciousness of a change iu the one rase, and 

 our unconsciousness in the other. It may here he remarked that Professor 

 Helmholtz 1 has recently adopted the same view as that just expressed in 

 regard to the nature of the volitional direction and the influence of the guiding 

 sensations; in corroboration of which he further mentions the important fact 

 ascertained by Bonders and himself, that by the use of a prism before one eye, 

 both eyes may be made to move outwards, or one up and the other down, still 

 under the same fundamental law. The truly involuntary movements of the 

 eyeballs, however, are performed under very different conditions ; there being 

 here no purposive direction or fixation of the gaze; and the muscular contrac- 

 tions not being determined by visual sensations, but being called forth by nerve 

 force excited in some remote part. Of this we have an example in the normal 

 revolution of both eyes upwards and inwards, which takes place in the acts of 

 coughing, sneezing, winking, etc.; but far more remarkable illustrations are 

 presented in those abnormal movements of the eyeballs, occurring in Convul- 

 sive diseases, in which there is neither harmony nor symmetry. 



686. It has been stated to be a condition of single and distinct vision, that 

 the usual axes of the eyes should be directed towards the object, in order 

 that its picture should be thrown upon the parts of the two retinae which are 

 accustomed to act together ( 621); but as this cannot take place without the 

 guidance of visual sensations, the movements of the eyeballs are wanting in 

 harmony whenever the visual power has been deficient from birth. This is 

 most remarkably the case, where the deficiency has been so complete that 

 not even light can be distinguished; but the movements are frequently very 

 far from being harmonious, iu cases of congenital cataract, where a consid- 

 erable 'amount of light is evidently admitted, but where no distinct image 

 can be formed; and in such cases, the movements are most harmonious where 

 the object is bright or luminous, and more vivid impressions are therefore 

 made upon the retinse. It is no objection to this doctrine to say, that persons 

 who have become blind may still move their eyes in a harmonious manner; 

 since the habit of the association of particular movements 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 laryugeal 

 movements of the deaf and dumb ( 537) ; 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 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 iu the 

 axes of the eyes becomes perceptible to a person observing them. For in- 

 stance, 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 the CQnverg- 

 ence 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 ; 



1 In his Croonian Lecture, Proceedings of the Koyal Society, April 14th, 1864, 

 p. 193. 



