914 OF MUSCULAR MOVEMENTS. 



the object under survey upon the same part of the retina, when the head is 

 inclined sideways, or bowed towards one shoulder and then towards the other. 

 It appears, from the experiments of Dr. Gr. Johnson, that the action of the 

 Oblique muscles is exactly adapted to produce such a rotation ; the Inferior 

 oblique, in its contraction, causing the eyeball to move upon its antero-poste- 

 rior axis, in such a manner that a piece of paper, placed at the outer margin of 

 the cornea, passes downwards and then inwards towards the nose ; and the 

 Superior oblique eifecting precisely the reverse action, the paper at the outer 

 margin of the cornea passing first upwards and then inwards. There was not 

 the slightest appearance, in these experiments, of elevation, depression, abduc- 

 tion, or adduction of the cornea, as a result of the action of the Oblique muscles ; 

 all these movements being attributable to the Recti alone. 1 



917. On studying the Voluntary movements of the Eyeballs, we are led to 

 perceive that they are not so much symmetrical as harmonious; that is to say, 

 the corresponding muscles on the two sides are rarely in action at once j whilst 

 such a harmony or consent exists between the actions of the muscles of the two 

 orbits, that they work to one common purpose, namely, the direction of both 

 eyes towards the required object. They may be arranged under two groups : 

 the first comprising those which are alike harmonious and symmetrical ; the 

 second including those which are harmonious but not symmetrical. To the 

 first group belong the following : 1. Both eyeballs are elevated by the contrac- 

 tion of the two Superior Recti. 2. Both eyeballs are depressed by the conjoint 

 action of the Inferior Recti muscles. 3. Both are drawn directly inwards or 

 inwards and downwards, as when we look at an object placed on or near the 

 nose ; this movement is effected by the action of the Internal Recti of the two 

 sides, with or without the Inferior Recti. It is 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 their 

 axes are directed towards the same point. 3 Now it is to be observed, with re- 

 gard 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 

 unsymmetrical movements, forming the second class, are those in which the 

 Internal and External Recti of the two sides are made to act together, either 

 alone, or in conjunction with the Superior and Inferior Recti. They are as 

 follows : 4. One eye is made to revolve directly inwards, 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 conjoint action of the Superior and External Recti. 6. One eye is made to 



1 The Author has been informed by his friend Mr. Bowman that he has met with two 

 cases of double vision, in which the defect was not experienced when the head was held 

 erect or turned upon its vertical axis, but only when it was inclined to the one shoulder or 

 the other. Such a peculiarity is readily explained on the above hypothesis, by the suppo- 

 sition that one or both of the oblique muscles of one eye was paralyzed, so that the normal 

 rotation was not performed on that side. 



2 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 perfect 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 moderate 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. 



