650 



DISSECTION OF THE ORBIT. 



insertion. 



Between 

 heads of 

 outer rectus. 



Use of all ; 



inner and 

 outer, 



upper and 

 lower, 



and two 

 adjacent. 



Common 

 tendinous 

 origin of 

 the recti. 



Dissect 

 inferior 

 oblique. 



Lower 

 oblique 

 muscle : 



origin ; 

 course ; 



insertion ; 

 relations. 



Action of 

 oblique 

 muscles : 

 alone, 



between the two heads. All the muscles are directed forwards, the 

 lower ones also obliquely outwards, and have a tendinous insertion 

 into the ball of the eye about a quarter of an inch from the cornea, 

 and in front of the greatest transverse diameter of the ball. 



Between the heads of origin of the external rectus, the different 

 nerves before mentioned are transmitted into the orbit, viz., the 

 third, the nasal branch of the fifth, and the sixth, together with the 

 ophthalmic veins. 



Action. The four recti muscles are attached to the eyeball at 

 opposite sides in front of the greatest transverse diameter and are 

 able to turn the pupil in opposite directions. 



The inner and the outer muscles move the ball horizontally 

 around a vertical axis, the former directing the pupil towards the 

 nose and the latter towards the temple. 



The upper and lower recti elevate and depress respectively the 

 fore part of the ball around a transverse axis ; but as the muscles 

 are directed obliquely outwards, the upper muscle turns the pupil 

 upwards and inwards, and the lower muscle turns it downwards and 

 inwards. 



By the simultaneous action of two adjacent recti, the ball will 

 be moved to a point intermediate to that to which it would be 

 directed by either muscle singly. 



Dissection. By opening the optic foramen, the attachment of 

 the recti muscles will be more fully laid bare, and they will be seen 

 to arise from a tendinous ring which passes above, outside and inside 

 the optic foramen, and bridges across the sphenoidal fissure from 

 below the inner and outer sides of the foramen, the two fibrous 

 bands meeting below at a small spicule of bone on the upper margin 

 of the great wing of the sphenoid. To dissect out the inferior 

 oblique muscle, let the eyeball be replaced in its natural position ; 

 then by separating from the facial aspect the lower eyelid from the 

 margin of the orbit, and removing some fat, the muscle will appear 

 beneath the eyeball arching from the inner to the outer side : if the 

 external tarsal ligament be divided, it may be followed upwards to 

 its insertion into the ball. 



The INFERIOR OBLIQUE MUSCLE (fig. 230, H) is placed near the 

 anterior margin of the orbit, and differs from the other muscles in 

 being directed across, instead of parallel to the axis of the orbit. 

 It arises from the superior maxillary bone immediately outside the 

 opening of the nasal duct. From this spot the muscle passes out- 

 wards between the inferior rectus and the bone and then between the 

 eyeball and the external rectus, to be inserted into the sclerotic coat 

 between the outer and upper recti. 



The borders of the muscle look forwards and backwards, and the 

 posterior receives its branch of the third nerve. The insertion of 

 the tendon is near that of the superior oblique muscle, but rather 

 closer to the optic nerve. 



Action of the oblique muscles. The superior oblique acting alone 

 would draw the back of the eyeball upwards and inwards, and 



