654 



PHYSIOLOGY 



W. sup. 



sup* 



in a tendinous expansion, which is inserted under the external rectus 

 muscle into the posterior and outer part of the eyeball, somewhat 

 behind the line of attachment of the superior oblique muscle. 



In discussing the actions of these muscles we may assume the 

 eyes to be in what is known as their primary position, i.e. with the 

 visual axes parallel and directed to a point on the horizon. It is 

 evident that if the muscles rotate the eyeball about an axis in a 

 plane at right angles to the visual axes, the pupils will move inwards, 

 outwards, or in any direction, but will not rotate. On the other hand, 

 if the axis of rotation of a movement produced by any muscle lies in a 



plane which is not vertical to the 

 visual axes, then the movement of the 

 pupil will be associated with rotation 

 of the eyeball. The first of these con- 

 ditions is practically fulfilled by the 

 external and internal rectus muscles, 

 as is shown in the diagram (Fig. 297). 

 These muscles by their contraction will 

 move the pupil directly outwards or 

 directly inwards. The axis of rotation 

 in the diagram would run vertically 

 through the eyeball. The other four 

 muscles produce movements with axes 

 of rotation which are not at right 

 angles to the visual axis. The rectus 

 superior, for instance, produces rotation 

 round the axis joining r.sup. and 

 r.inf., and will therefore produce move- 

 ment of the eyeball upwards and 



inwards with a certain amount of rotation of the eyeball on its 

 antero-posterior diameter. The rectus inferior in the same way 

 moves the eyeball downwards and inwards with rotation. The obliquus 

 superior moves the eyeball downwards and outwards, and the obliquus 

 inferior upwards and outwards, in both cases with some rotation on 

 its antero-posterior axis (Fig. 298). The only movements of the eyeball 

 therefore which can be carried out by the action of one muscle, or by 

 the reciprocal action of a pair of muscles, are the movements outwards 

 and inwards, which are involved in the common actions of convergence 

 of the visual axes and conjugate deviation of both eyeballs. Move- 

 ments upwards or downwards will require the co-operation of at least 

 two muscles. In order to direct the gaze upwards a contraction of the 

 superior rectus which moves the eyeball upwards and inwards must 

 be associated with a contraction of the inferior oblique muscle 

 which rotates the eyeball upwards and outwards. In the same 



r.ext. r.sup. r.zsre 



FIG. 297. Diagram to show points 

 of attachment and lines of action 

 of extrinsic ocular muscles. 



