EYE MOVEMENTS 497 



internal rectus ; thus co-ordinating the adjustments required for near vision, 

 namely convergence, accommodation for near objects, and reduced pupil 

 diameter. (3) Between the superior recti muscles of the two eyes ; thus 

 causing symmetrical upward deviation. (4) Between the inferior recti of 

 the two eyes for similar reasons. (5) Between the superior oblique of one 

 eye and the inferior oblique of the other ; thus permitting conjugate rotation 

 of the eyes. (6) Between the superior rectus and the inferior oblique of 

 the same eye ; thus permitting the deviation caused by the one to be 

 corrected by the other. (7) Between the inferior rectus and the superior 

 oblique of the same eye for a similar reason. (8) Between the nucleus of 

 the superior rectus and that of the levator palpebrse of the same eye. This 

 association permits simultaneous raising of the eyelid with the upward 

 deviation of the eyes, thus preventing any restriction of vision. 



Besides these connections between the muscles producing like or associ- 

 ated action there are others equally important between the brain and these 

 centres, namely those which connect antagonistic muscles. Sherrington 

 showed that, as the muscle on one side of a limb contracts, its antagonist 

 at the same time relaxes, so as to allow the movement to take place smoothly 

 and witho'ut waste of energy. This is called ' reciprocal innervation.' The 

 eye muscles show the phenomenon very well. If the right frontal cortex 

 be stimulated, the eyes perform co-ordinate deviation to the left. If now all 

 the muscles of the right eye are divided except the external rectus, it is found 

 that this eye still moves in co-ordination as far as the middle line, through 

 the relaxation of the external rectus muscle. 



The orbicularis palpebrarum is also supplied by the 3rd nerve, for in 

 lesions of its nucleus paralysis of this muscle is found. The fibres innervating 

 it probably travel all the way with the 7th nerve. 



CAUSES AND DIAGNOSIS OF STRABISMUS 



Squint or strabismus may be caused by a number of conditions : (1) by congenital 

 abnormality ; (2) by interference with the proper rotation of the eyeball ; (3) by injury to 

 one of the external eye muscles ; (4) by injury to or stimulation of one of the nerves sup- 

 plying these muscles ; (5) by the presence of certain errors of refraction. With regard to 

 nerve injury the following description may be given. Injury to the third nerve causes (a) 

 drooping of the upper lid owing to paralysis of the levator palpebrae; (6) external 

 strabismus from paralysis of the upper, inner and lower recti and the unopposed action 

 of the external rectus ; (c) rotation of the eye about its visual axis from paralysis of 

 the superior oblique and therefore unopposed action of the inferior ; (d) dilatation of 

 the pupil from paralysis of its sphincter and the unopposed action of the dilator fibres 

 which are innervated by the sympathetic ; (e) loss of the power of accommodation 

 from paralysis of the ciliary muscle ; (/) exophthalmos or protrusion of the eye, caused 

 by the paralysis of so many of its muscles and the unopposed action of the smooth 

 muscle fibres in Tenon's capsule. Owing to the fact that for a considerable portion 

 of its course the 3rd nerve lies beside the 4th, 5th and 6th nerves, there is usually 

 also some associated symptoms of paralysis in the structures which these nerves supply. 

 Injury to the 4th nerve causes paralysis of the superior oblique, which shows itself 

 by defective movements in a downward and outward direction. Injury to the 6th 

 norvo causes internal strabismus owing to paralysis of the external rectus. This fre- 

 quently occurs when tumours, haomorrhage or injuries involve the base of the brain. 



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