292 



THE CAT. 



[CHAP. ix. 



Ch.s 



opening contracts more and more till it is reduced to an extremely 

 narrow vertical chink. This is probably due (as has been sug- 

 gested to me by my friend Mr. Henry Power) to the greater con- 

 traction of the superior and inferior radiating fibres than of those 



which radiate outwards and inwards. A 

 muscle called the ciliary muscle consists 

 of a ring of radiating organic fibres which 

 take origin in front from the inner surface 

 of the sclerotic, close to the cornea, and 

 pass outwards and backwards to the cho- 

 roid membrane, opposite to the ciliary pro- 

 cesses. By their contraction these fibres 

 pull forward the choroid, and so render the 

 ciliary ligament less tense. The muscle 

 thus tends to draw the choroid and retina 

 together like a bag round the vitreous 

 body, pressing the lens forwards, which 

 is supposed to be thus indirectly made 

 less flat. There are a few more inwardly 

 situated fibres which run circularly (nearly 

 at right angles to the rest), and form 

 part of the ciliary ligament as before men- 

 tioned. 



Inmost of all the coats of the eyeball 

 lies a soft delicate membrane of nervous 

 matter with connective tissue the RETINA. 

 It extends forwards almost to the ciliary 

 processes, and ends anteriorly in a finely 

 toothed margin called the ora serrata, and 

 thence to the tips of those processes the 

 retina is continued on by a transparent 

 layer, the pars ciliaris retinae, consisting 

 of nucleated cells which are not nervous, 

 but part of the membrana Umitans interna, 

 Slightly on the inner side of the middle of 

 the retina, at the back of the eye, is an 

 oval prominent spot, the poms opticus, 

 where the optic nerve enters, and whence 

 the vessels of the retina radiate. This 

 is a blind spot. 



The retina is an extremely complex membrane, consisting of about 

 seven layers of different forms of nervous tissue, supported and con- 

 nected by a most delicate framework of connective tissue. 



The outermost layer, that which comes next to the pigment layer 

 just described, is what has been called the membrane of Jacobson. 

 It consists of a multitude of minute and delicate nervous rods, 

 placed with their ends directed outwards and inwards. Amongst the 

 rods are scattered at ^regular intervals a less number of nervous cones, 

 placed with their apices outwards. 



Fig. 135. DIAGRAM REPRESENT- 

 ING A SECTION OF THE RE- 

 TINA. 



c/i. s. Outer, or clioroidal surface. 

 in. s. Inner surface, or that next 



the vitreous humour. 

 a. Layer of nerve-fibres at inner 



surface of retina. 

 6. Ganglionic layer of large nerve- 



c. Inner molecular layer. 



d. Inner nuclear layer. 



c. Outer molecular (inter-nuclear) 



layer. 



/. Outer nuclear layer. 

 g. Layer of rods and cones. 

 h. Layer of pigment cells next 



the choroid. 

 M I. Membrana limitans. 



