78 Orbit 



THE 'MUSCLES OF THE ORBIT 



The levator palpebras superioris, the four straight muscles, and the 

 superior oblique one arise around the optic foramen ; the straight 

 muscles pass forwards, closely applied round the optic nerve, to 

 their insertion into the sclerotic about a quarter of an inch behind the 

 cornea. The third nerve supplies the superior, inferior, and internal 

 recti, the external one being supplied by the sixth. Acting together, 

 the straight muscles retract the eye-ball, whilst the oblique muscles 

 draw it forwards. 



The external rectus of one eye acts in concert with the internal 

 rectus of the other. 



If the long axis of the orbit were in the straight line with that of 

 the eye-ball, the superior and inferior recti would simply turn the 

 cornea upwards or downwards ; but, because these muscles pass for- 

 wards obliquely to their insertion, that is, in the axis of the orbit, they 

 turn the cornea slightly inwards as well. The inward inclination 

 caused by the superior rectus is counteracted by the action of the in- 

 ferior oblique, and that of the inferior rectus is checked by the superior 

 oblique. 



The superior oblique passes forwards to the upper and inner 

 part of the orbit, where its slender tendon runs through a fibre-carti- 

 laginous ring which is lubricated by a delicate synovial membrane. 

 The tendon then passes downwards, outwards, and backwards, be- 

 tween the superior rectus and the sclerotic, to be inserted between the 

 superior and external recti, in the hemisphere of the globe behind the 

 transverse equator. The supply is from the fourth nerve, which has 

 been called * patheticus ' on the supposition that it supplied the muscle 

 which gave a ' pathetic ' turn to the eye-ball. The pulley can be 

 obscurely felt by thrusting the finger into the inner and upper part of 

 the orbit. 



Chronic serous effusion into the synovial membrane of the pulley 

 causes a prominent cyst ; it is often seen in those beyond middle 

 life. 



The inferior oblique arises from the superior maxilla just external 

 to the nasal duct, and, passing obliquely upwards, between the in- 

 ferior rectus and the floor of the orbit, and then between the sclerotic 

 and the outer rectus, is inserted just below the superior oblique, behind 

 the equator of the globe. It is supplied by the third nerve. 



The action of the oblique muscles. If an imaginary pin be 

 driven vertically through the centre of the globe, movement becomes 

 possible only in the horizontal plane, and the superior oblique, coming 

 from the inner side, to be inserted on the outer side of the globe, 

 behind the transverse axis, or equator, on contracting must draw the 

 posterior hemisphere inwards and so turn the cornea outwards. 



