THE EYE AND ITS APPENDAGES. 



173 



Fig. 150. 



The ciliary muscle is a small, gray structure, behind the ciliary 

 ligament,'^ and covering the outside of 

 the ciliary processes. Its contraction 

 presses the lens" towards the cornea.'' 



The iris^ (fig. 149,) is a thin, cir- 

 cular membrane,^ consisting of contrac- 

 tile tissue, and forming a vertical septum, 

 between the anterior and posterior cham- 

 bers of the eye. It is perforated nearly 

 in the centre by a foramen called the 

 pupil,'^ which in foetal life is closed by 

 the membrana pupillaris. Its external 

 border is attached to the ciliary ligament. 

 The anterior surface is flat, of a bluish 

 or brown colour, speckled and striated ; 



towards the pupil the colour is deeper. Its posterior surface (some- 

 times called the uvea), is covered with a thick layer of pigmentum 

 nigrum, and is in contact with the ciliary processes. It may con- 

 tain a few muscular fibres in its structure. It is supplied by the 

 ciliary arteries, branches of the ophthalmic, and by the ciliary 

 nerves, which pass from the lenticular ganglion through the choroid 

 coat to be distributed upon it. • 



The retina,^ is a soft, white membrane within the choroid coat, 

 and is considered by some to be an expansion of the optic nerve, 

 and by others as a distinct nervous structure in connexion with the 

 optic nerve. Internally it is in contact with the vitreous humour. 

 It commences at the optic nerve, and terminates anteriorly by a 

 jagged edge, called ova serrata, behind the ciliary body. It is 

 thicker behind than it is in front. When perfectly fresh it has a 

 pinkish tint, and is semi-transparent. Its inner surface is of a 

 fibrous character, consisting of radiating tubular fibres of the optic 

 nerve. Its external surface consists of a finely granulated matrix, 

 interspersed with very delicate vessels. The optic nerve,^^ enters 

 the globe of the eye on the inner side of its axis, and the point 

 where it is connected with the retina is incapable of vision. Directly 

 in the axis there exists upon the retina the yellow spot of Soemmer- 

 ing, a slight elevation with a minute perforation in its summit. The 

 central artery of the retina enters through the optic nerve, and tra- 

 versing the vitreous humour, supplies the retina and lens. 



Membrana Jacobi. — Is a thin lamina, existing between the cho- 

 roid and the retina, consisting of a number of club-shaped rods, 

 interspersed with transparent cells. 



The vitreous humour ^^ has a transparent, jelly-like appearance, 

 and is of a globular form. It consists of a viscous fluid, held in 

 cells formed from the hyaloid membrane, which surrounds it. It is 

 colourless, and its structure is best exhibited by freezing. 



15* 



