ORGANS OF SENSE. 171 



5. The Iris is the circular, vertical partition dividing the intervening 

 space which contains the aqueous humor into two chambers, an anterior and 

 a posterior, these communicating by the central circular aperture in the iris 

 named the pupil. This opening is circular in the main, and a little nearer 

 to the nasal than to the temporal side of the iris ; its size varies with the 

 distance of the object in view, the sensibility of the retina, and the intensity 

 of the light. There is often a correspondence in color between the iris and 

 the hair. The posterior surface is coated with a thick lamina of dark pig- 

 ment named uvea, from a fancied resemblance to a ripe purple grape. The 

 iris is highly organized, and receives its blood posteriorly from the long 

 ciliary arteries, and anteriorly from the ophthalmic. The nerves of the iris 

 are derived from the ophthalmic ganglion. The structure of the iris is 

 pretty generally considered to be muscular, the exceedingly minute fibres 

 being arranged in two series, one radiated, the other circular, concentric with 

 the pupil. When enlargement is necessary, the sphincter or circular fibres 

 relax, and the radiated acting from the circumference to the centre approxi- 

 mate the inner margin of the iris to the outer, the reverse taking place when 

 the aperture or pupil is to be diminished. 



During the greater part of uterine life the pupil of the foetus is closed by 

 the memhrana pupillaris, a delicate membrane filling up the aperture of the 

 pupil, and completely separating the anterior from the posterior chamber. 

 It actually consists of two thin lamina?, extending from the surface of the 

 iris. It is most distinct about the fifth month, and becomes absorbed towards 

 the period of birth. 



6. The Retina is the third or innermost, or nervous tunic of the eye, and 

 is in immediate contact with the vitreous humor which distends the posterior 

 part of the globe. During life it is transparent, but soon becomes clouded. 

 It is the expansion of the optic nerve into a membrane forming the portion 

 of a sphere, and extending between the choroid and the vitreous humor to 

 within about one eighth of an inch of the ciliary ligament. The optic nerve 

 pierces the posterior part of the globe about a line and a half internal to 

 the centre. About a line and a half to the inside of the entrance of the 

 optic nerve, and in the part of the retina which would be pierced by the 

 imaginary antero-posterior axis of the eye, there is a small circular or oval 

 spot with a dark centre and a yellowish border. This has been called the 

 foramen of Soeinmei^ng or the 'punctum aureum. 



The retina consists of three laminae, each difiering in texture and function. 

 The exterior is the tunica serosa or JacoVs membrane ; it supports the nervous 

 tissue and separates it from the pigmentary surface of the choroid. The 

 internal is a vascular layer, composed of the capillary ramifications of the 

 centralis artery and vein. The middle layer is the expanded medullary or 

 fibrous substance of the optic nerve. 



The cavity of the eye is occupied by refracting media, the humors differ- 

 ing from each other; these are the aqueous, the crystalline, and the vitreous, 

 and are placed in this order between the cornea and the optic nerve. 



7. The Aqueous Humor is a perfectly colorless liquid, occupying the 

 interval between the cornea and the lens, and inclosed by a fine secreting 



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