290 



THE CAT. 



[chap. IX 



on the inner side of tlie antcro-posterior axis of tho_ eyeball.^ The 

 membranous sheath of the whole nerve becomes continuous -^ith the 

 sclerotic, as do also the investments of the different bundles of nerve 

 fibres of which the whole optic nerve is composed. On this account 

 the part of the sclerotic where the fibres enter is called the himina 

 cribrom. There are a few blood-vessels in the sclerotic, especially- 

 near the margin of the cornea. 



The CORNEA (or the transparent and anterior part of the fibrous 

 coat cf Avliich the sclerotic forms the larger portion) covers the 



Fi''. lil.— DiAGRAJI REPUESENTIXG A VeKTICAL SiXTlOK OF THE C'AT's EvE. 



,SV;?. S( ]er(itii; cu;it. 



Cii. Cornea. 



II. Tli« iitl<i('limt'7it iif Uiu tciiiloiis of the recti 



iiuisclcs. 

 (7(. Till; <-lioi-oi(]. 

 ('l>. Th<; ciliaiy jiroccsses. 

 Cm. The ciluuy luu.sele. 



/)•. The iris. 



,■1'/. The aiiueous huiiumr. 



Cr. Tlie ciystalliiie lens. 



Vt. The vitreous humour. 



Rt. The retina. 



Op. Tlic optic nerve. 



anterior part of the eyeball, and has its surface rather more curved 

 than is that of the sclerotic. It is also composed of fibres which are 

 softer and much more indistinct than those of the sclerotic, Avith 

 which they are nevertheless continuous, some fibres being opaque at 

 one part of their course and transparent at the other part. Between 

 the layers of fibres there are fusiform nucleated cells. The cornea 

 yields chondrin on boiling, imlike the sclerotic, which yields gelatine. 

 Each surface of the cornea is invested by a most delicate, structure- 

 less transparent membrane or elastic lamina. 



The SECOND or isiedian membrane also consists of two parts : the 

 choroid and ifis. The ciiouou) is a membrane ])laced within the 

 sclerotic, covering the sides and back of the wall of the eyeball, 

 except where the optic nerve pierces it. It extends forwards nearly 

 to the margin of the cornea, where it ends in a number of irregular 

 folds, called ri/iar// pj-occaaca. These project inwards toAvards the 

 centre of the eyeball. 



The choroid is tough externally where it is connected with the 

 sclerotic by loose connective tissue ; internally, it is smooth and dark 

 coloured, being lined, by a layer of dark pigment cells everywhere 



