THE ACCESSORY ORGANS OF VISION. 



829 



Fig. 391. 



inserted into the posterior part of the external face of the sclerotic. It is 

 always more or less fasciculated, and is most frequently separated into foul- 

 portions superior, inferior, external, and internal. 



In contracting, it retracts the globe towards the back of the orbit. The 

 physiological finality of this movement will be noticed hereafter. 



2. SUPERIOR, INFERIOR, EXTERNAL, AND INTERNAL KECTI MUSCLES. These 

 four muscles are placed longitudinally on the preceding, and repeat, on a 

 large scale, the disposition of its four bundles. As their borders are in 

 contact, they constitute a fleshy sheath around it, analogous to that which it 

 forms around the optic nerve. Exactly resembling each other, these four 

 muscles compose so natural a group, that they may be described together. 

 Each is a flat band, formed of 

 parallel fibres, firmly attached 

 by its posterior extremity to 

 the back of the sheath, and to 

 the interior of the subsphe- 

 noidal canal ; anteriorly, it is 

 inserted by a thin aponeurosis 

 into the sclerotic, at the mar- 

 gin of the cornea. Isolated 

 from one another, and from 

 the retractor, by the mass of 

 fat belonging to the mem- 

 brana nictitans, these small 

 muscles are related, externally, 

 to the ocular sheath. 



There is nothing par- 

 ticular to be noted regarding 



them, their position being MUSCLES OF THE EYEBALL, VIEWED FROM ABOVE. 



1, Section of orbital process of frontal bone to which 

 the fibro-cartilaginous pulley, 4, of the superior 

 oblique muscle, 5, is attached ; 2, Zygomatic 

 process of the temporal bone ; 3, Portion of sphe- 

 noid bone into which the recti and superior 



oblique muscles are implanted ; 6, Pathetici nerve ; 

 7, Internal rectus ; 8, Superior rectus ; 9, Levator 

 palpebrse muscle; 10, External rectus; 11, Eye- 

 ball; 12, Upper eyelid; 13, Lower eyelid; 14, 

 Inner canthus of eye. 



sufficiently indicated by their 

 names. Their function is to 

 bring the pupillary opening 

 into contact with the rays 

 of light, by inclining the 

 cornea towards them, either 

 upwards, downwards, inwards, 

 or outwards; or into inter- 

 mediate positions, which 

 happens when two adjacent muscles the inferior and external rectus, for 

 instance combine their action at the same moment. 



3. GREAT OBLIQUE MUSCLE (trochlearis, or obliquus superior oculi). 

 Lying to the side of the internal and superior rectus ; and formed, like them, 

 of a fleshy band terminated by a thin aponeurosis, this muscle differs from 

 the preceding in its interrupted course. Arising from the back of the 

 orbit, and passing forward against the inner wall of that cavity, it reaches a 

 strong fibro-cartilaginous, pulley-like, process a dependency of the aponeu- 

 rosis of the orbit attached by its extremities to the frontal bone, at the 

 base of the orbital process; it passes through this loop, and then bends 

 outwards, to insinuate itself below the terminal extremity of the superior 

 rectus, and become inserted into the sclerotic, between the latter muscle 

 and the external rectus. 



This muscle pivots the eye inwards and upwards in the orbit, carrying 

 the outer aspect of the globe upwards, and its lower part outwards ; this 



