290 



THE CAT. 



[CHAP, ix 



on the inner side of the antero-posterior axis of the eyeball. The 

 membranous sheath of the whole nerve becomes continuous with 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 lamina 

 cribrosa. 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 of which the sclerotic forms the larger portion) covers the 



/Scl 



r 



'Cm 



Fig. 154. DIAGRAM REPRESENTING A VERTICAL SECTION OF THE CAT'S EYE. 



Scl Sclerotic coat. 



Cn. Cornea. 



R. The attachment of the tendons of the recti 



muscles. 

 Ch. The choroid. 

 Cp. The ciliary processes. 

 Cm. The ciliary muscle. 



Jr. The iris. 



Aq. The aqueous humour. 



Cr. The crystalline lens. 



Vt. The vitreous humour. 



Rt. The retina. 



Op. The 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, with 

 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, unlike 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 MEDIAN membrane also consists of two parts : the 

 choroid and iris. The CHOROID is a membrane placed 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 ciliary processes. These project inwards towards 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 



