CRANIAL CAVITY. 



radial muscle (Dilator pupillse) fibres externally; and of cir- 

 cular (Sphincter pupillse) fibres internally, surrounding an 

 almost circular aperture (pupil) lying just to the nasal side of 

 its centre. The ciliary arteries form an anastomosing circle 

 (circulus arteriosus major) around its outer or ciliary margin; 

 and another circle (circulus arteriosus minor) around the mar- 

 gin of the pupil. [727] 



Inner Coat. The retina is a delicate membrane composed 

 of an outer pigmented layer attached to the choroid; and 

 of an inner nervous layer, the retina proper, which is in con- 

 tact with the hyaloid membrane of the vitreous body but 

 is only attached to this at the entrance of the optic nerve 

 and near the ciliary processes. It seems to end in a serrated 

 border (ora serrata) just behind the ciliary body, but the pig- 

 mented layer, thinned out, is really prolonged onto the ciliary 

 body and the iris. It presents internally, at the posterior 

 pole, a small yellowish spot (macula lutea) which is depressed 

 (fovea centralis) centrally; and also, at the entrance of the 

 optic nerve, a whitish area (optic disc) which is depressed 

 (optic cup) centrally but elevated (colliculus nervi optici) at 

 the edge. It consists of ten layers which are only separable 

 microscopically. [731] 



Refracting Media. The vitreous body is a transparent jelly- 

 like substance lying between the retina and lens; it is concave 

 anteriorly (fossa patellaris) and enclosed in a thin transparent 

 (hyaloid) membrane. The hyaloid membrane is thickened 

 (zonula of Zinn) in front of the ora serrata, presenting grooves 

 for the ciliary processes and splitting into two layers; the 

 posterior layer is the thinner and covers the fossa patellaris; 

 the anterior layer (suspensory ligament) blends with the front 

 of the capsule of the lens. A lymph space (canal of Petit) runs 

 around the lens behind the suspensory ligament. The crystal- 

 line lens is a biconvex transparent body lying between the iris 

 and vitreous body, and enclosed in a thin transparent capsule. 

 It is formed by concentric laminae and consists of a soft outer 



[57] 



