ORGANS OF SPECIAL SENSE. 349 



The arteries are from the long and anterior ciliary and from 

 the ciliary processes (from the short ciliary). 



The veins join those of the ciliary processes and anterior 

 ciliary veins. 



The nerves are the long ciliary from the nasal branch of the 

 ophthalmic (first division of fifth), and ciliary branches from 

 the lenticular or ophthalmic ganglion. 



The circular fibres are supplied by the fibres from the third 

 or motor oculi, the radiating fibres are supplied by the sympa- 

 thetic. 



The membrana pupillaris is a delicate, vascular membrane, 

 closing the pupil in the foetus, continuous at its margin with the 

 iris. It usually disappears about the eighth month, but occa- 

 sionally persists. 



The ciliary muscle is a grayish, circular band of unstriated 

 fibres, attached to the fore part of the choroid. It consists of 

 two sets of fibres the external radiating fibres, dilator pupillce 

 arising from the junction of the sclerotica and cornea, and is 

 inserted into the choroid opposite the ciliary processes; the in- 

 ternal circular fibres, sphincter pupillce_, pursue a circular course 

 around the insertion of the iris. 



It is the muscle of accommodation, its contraction drawing 

 on the ciliary processes, relaxing the circular fibres described as 

 the ligament of the lens, and allowing the anterior surface of the 

 lens to become more convex by its inherent elasticity. 



THE RETINA is a delicate, white, nervous membrane, the ex- 

 pansion of the optic nerve. It lines the eyeball, being in con- 

 tact externally with the. choroid and internally with the vitreous 

 body. It terminates in front near the ciliary ligament in an 

 irregular margin, the ora serrata, but is prolonged forward as the 

 pars ciliaris to the iris. Its internal posterior surface presents at 

 a point corresponding to the axis of the eyeball a small, round, 

 elevated spot of yellowish color, the macula lutca, or yellow spot 

 "of Sommering, in the centre of which is the fovea centralis, a 

 central depression, the region of most acute vision, and about one- 

 tenth of an inch to its nasal side, at a point corresponding to the 

 axis of the orbit, the entrance of the optic nerve, an oval, bluish- 

 white depression, with distinct, often pigmented margins, a re- 

 gion destitute of vision and called the optic disk. Its centre 

 presents the arteria centralis retinae, giving branches to the nasal 

 upper and lower side of the macular region. 



The structure of the retina is exceedingly complex, con- 

 sisting microscopically of ten distinct layers from within out- 

 ward, as follows: 



