SYMMETRY AND HARMONY OF MUSCULAR MOVEMENTS. 915 



revolve downwards and inwards, by the conjoint action of the Inferior and In- 

 ternal Recti ; the other, downwards and outwards, by the conjoint action of the 

 Inferior and External Recti. In these movements, two different muscles, the 

 External and Internal Recti, are called into action on the two sides, with or, with- 

 out the superior and inferior Recti ; but they are so employed for the purpose 

 of directing the axes of the eyes towards the same point ; and although, as just 

 noticed, we can put the two Internal Recti in action together, we cannot volun- 

 tarily cause the two External Recti to contract together, it not being possible 

 that any object should be in such a position as to require this action for the direc^ 

 tion of the axes of the eyes towards it. 



918. The greater number of the foregoing movements may be performed 

 even unconsciously to ourselves, in obedience to a voluntary determination to 

 keep the direction of the eyes fixed, instead of to give motion to the eyeballs. 

 Thus, if we gaze steadily at an object in front of us, and then depress the head 

 forwards on its transverse axis, the eyeballs roll upwards upon their transverse 

 axes (1) by the action of the Superior Recti, without our being aware of it; so 

 if, while still maintaining the same fixed gaze, we raise the head into the vertical 

 position and then depress it backwards, the eyeballs are rolled downwards (2) 

 by the action of the Inferior Recti; if, under the same conditions, the head be 

 made to rotate on its vertical axis from side to side, the eyeballs will be made 

 to roll on their vertical axes in the contrary direction, by the External and 

 Internal Recti (4) of the two sides respectively ; so by causing the head to 

 move obliquely in the opposite directions, the oblique movements (5 and 6) of 

 the eyeballs are made to take place by the continued fixation of the vision upon 

 the same object. To these we have to add one more action, which cannot be 

 called forth in any other mode ; namely, that rotation of the two eyes upon 

 their antero-posterior axes which takes place, probably by the instrumentality 

 of the oblique muscles, whem we incline the head to one side or the other by 

 rotating it upon its antero-posterior axis (916). In all these movements, as 

 in the preceding, the Will directs the result; and there is no other difference 

 between them, than that which arises out of our consciousness of a change in 

 the one case, and our unconsciousness in the other. The truly Involuntary 

 movements of the eyeballs, however, are performed under very different condi- 

 tions ; there being here no purposive direction or fixation of the gaze ; and the 

 muscular contractions 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, &c. ; but many more 

 abnormal movements of the eyeballs, in which there is neither harmony nor 

 symmetry, present themselves in convulsive diseases. 



919. It is 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 

 ( 886); and that this direction is afforded by the visual sensations which 

 govern the muscular movements, the result of these merely .being determined 

 by the Will, seems sufficiently obvious from the considerations now stated. The 

 following circumstances, however, afford additional confirmation of this doctrine. 

 It is well known that, in children born blind, the movements are not harmo- 

 nious ; they are frequently very far from being so, in cases of congenital cata- 

 ract, 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 har- 

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

 therefore made upon the retinae. It is no objection to this doctrine to say, that 

 persons who have become blind may still move their eyes in an harmonious 

 manner ; since, the habit of the association of particular movements having been 



