MUSCLES OF THE EYE. 537 



transparent substance, of the consistence of jelly, enclosed in a 

 delicate membrane, the hyaloid, from the inner surface of which 

 numerous lamella? are sent inwards, to form the compartments in 

 which the fluid portion is contained. 



The crystalline humour, or lens, is situated immediately behind 

 the pupil, in front of the vitreous humour, and is surrounded by 

 the ciliary processes, which slightly overlap its margin. It is 

 biconvex, the convexity of the posterior face being the greater, 

 and is enveloped by a transparent elastic membrane, the capsule 

 of the lens, which is connected with its surface by a layer of trans- 

 parent cells. After death, these cells absorb fluid from the eye, 

 and break down into the liquor Morgagni. The lens is con- 

 structed of concentric layers or lamina?, of which the external are 

 soft, and the internal firmer. It is supported in its place by the 

 ciliary zone. 



MUSCLES OF THE EYE. 



The intrinsic or motor muscles of the globe of the eye are 

 seven, five of which are straight muscles viz., the posterior 

 or retractor, the superior or levator, the inferior or depressor, the 

 external or abductor, and the internal or adductor ; the remain- 

 ing two are the oblique muscles, distinguished as superior, and 

 inferior.* 



The RECTUS OCULI POSTERIOR, or RETRACTOR OCULI, com- 

 pletely envelopes and forms a sheath round the extra-cranial 

 portion of the optic nerve. It originates from the edge of the 

 optic foramen, and is inserted to the posterior part of the external 

 face of the sclerotic, its action being to retract the eye within 

 the orbit. 



The remaining four RECTI muscles have their origin round 

 the margin of the optic foramen, within the orbit, and become 

 inserted by a broad expansion, which forms the tunica albuginea, 

 on the anterior portion of the sclerotic coat. These four muscles 

 form a fleshy sheath round the eyeball, similar to that which the 

 retractor forms round the optic nerve. They are all flat and 

 ovid in form, the narrowest part being at their origin. Acting 

 together, they retract the eyeball ; singly, the superior rectus 

 turns the eye upwards ; the inferior turns it downwards ; the 

 internal turns it inwards ; and the external turns it outwards. 



* We may here call attention to the very small muscle, described by 

 Strangeways as a supernumerary oblique muscle vide Appendix II. 



