FUNCTIONS OF THE CEREBELLUM. 349 



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 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 previously 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. 



456. The foregoing considerations maybe summed up in this simple state- 

 ment : that, when we voluntarily direct our eyes towards any object, the 

 actions of the several muscles concerned, are guided by the visual sensations, 

 rather than by the ordinary muscular sense, through which other voluntary 

 movements are regulated. In this manner are accomplished, not merely the 

 revolutions of the eyeballs from side to side, upwards and downwards, or in 

 any direction that is required to cause the image to fall most advantageously 

 upon the two retinae ; but also that rotation on their axes, which keeps the 

 images in the same position upon the retinae, when the head moves in a plane 

 perpendicular to their axes ; and likewise that exact convergence of the two 

 axes which shall cause them to meet in the object on which the attention is 

 fixed, and which consequently varies with its distance. Of all the movements 

 of the eyes, there is none which exhibits the necessity of the guiding visual 

 sensations so much as the revolution of both eyes inwards. Some persons 

 can effect this voluntarily to a greater extent than others ; but even then, they 

 can only accomplish it by fixing the gaze upon some object situated between 

 the eyes ; and cannot call the adductor muscles into combined action in per- 

 fect darkness, or if the lids be closed. Even those who have the least power 

 of effecting this extreme convergence, by at once directing the eyes towards 

 a very near object, can accomplish it by looking at an object placed at a mo- 

 derate distance, and gradually bringing this nearer to the nose, keeping the 

 eyes steadily fixed upon it. The unwonted character of the movement is 

 shown in this, that it can only be maintained, even for a short time, by a 

 strong effort, producing a sense of fatigue. No effort whatever can call into 

 simultaneous action the two external Recti ; and this fact is an additional 

 proof of the necessity of a guiding visual sensation ; since it is evident, that 

 no object can ever be placed in such a position, as to require this action for 

 the direction of the axes of the eyes towards it. 



6. Functions of the Cerebellum. 



457. That the Cerebellum has some special function, distinct from that of 

 the Cerebral Hemispheres, can scarcely be doubted ; since its peculiar struc- 

 ture and position, its independent connections with the Medulla Oblongata, 

 and its extremely variable size relatively to the remainder of the Encephalon, 

 point it out as an instrument adapted to some particular purpose. We shall 

 inquire briefly into the nature of the evidence respecting its function, which 

 is supplied to us by Comparative Anatomy, by Experiment, and by Patholo- 

 gical phenomena. A Cerebellum is found in all Vertebrated animals; although 

 it is in some extremely small, looking like a little prominence on the Medulla 

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