ORGANS OF SENSE, 171 
5. Tue 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 distange 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 pigment named wvea, 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, concen- 
tric with the pupil. When enlargement is necessary, the sphincter or circu- 
lar fibres relax, and the radiated acting from the circumference to the centre, 
approximate 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 fetus is closed by 
the membrana 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 lamine, extending from the surface of the 
iris. It is most distinct about the fifth month, and becomes absorbed 
towards the period of birth. 
6. Tue 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 Semmering or the punctum aureum. 
The retina consists of three lamine, each differing in texture and function. 
The exterior is the tunica serosa or Jacob’s 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. Tue Aqueous Humor is a perfectly colorless liquid, occupying the 
interval between the cornea and the lens, and inclosed by a fine secreting 
877 
