836 OF THE STRUCTURE AND FUNCTIONS OF MUSCULAR TISSUE. 



evidently symmetrical, but might seem at first sight not to be harmonious, 

 because the eyes do not move together towards one side or the other; it is, 

 however, really harmonious, since it directs their axes towards the same 

 point. 1 Now it is to be observed, with regard to these movements, that we 

 can never effect them in antagonism with each other, or with those of other 

 muscles. We cannot, for example, raise one eye and depress the other; nor 

 can we raise or depress one eye, when we adduct or abduct the other. The 

 explanation of this will be found in the fact, that we can never, by so doing, 

 direct the eyes to the same point. The harmonious but unsyra metrical move- 

 ments, forming the second class, are those in which the Internal and Exter- 

 nal Recti of the two sides are made to act together, either alone or in con- 

 junction with the Superior and Inferior Recti. They are as follows: 4. One 

 eye is made to revolve directly imvards by the action of its Internal Rectus, 

 whilst the other is turned outwards by the action of its External Rectus. 5. 

 One eye is made to revolve upwards and inwards, by the conjoint action of 

 the Superior and Internal Recti ; the other upwards and outwards, by the con- 

 joint action of the Superior and External Recti. 6. One eye is made to re- 

 volve downwards and imvards, by the conjoint action of the Inferior and 

 Internal 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 without 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 voluntarily 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 direction of the axes of the eyes towards it. 



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

 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, whilst 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 

 directions by the External and Internal Recti (4) of the two sides respect- 

 ively ; so, by causing the head to move obliquely in the opposite directions, 

 the reverse 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 mus- 

 cles, when we incline the head to one side or the other by rotating it upon 

 its antero-posterior axis. In all these movements, as in the preceding, the 



1 Some persons can effect this voluntarily to a greater extent than others ; but <'ven 

 then, they can only accomplish it by fixing the gaze upon some object situated be- 

 tween tin- eyes ; and cannot call the adductor muscles into combined action in perfect 

 darknr-s, 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 "far object, can 

 !icconi|)li>h it by looking at an object placed at a moderate distance, and gradually 

 bringing this nearer to the nose, keeping the eyes steadily fixed upon it. The un- 

 wonted 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. 



