ORGANS OF SENSE.
Organ of globe of-the eye is contained in a bony socket, formed by the bones of the cranium and of the face. It is furnished with muscles which can move it in every direction, and surround ed by a very soft and delicate kind of fat, which yields to it in all its motions. It is composed of certain membranes, called its tunics or coats, and of other parts termed humours.
Rd figure is very nearly spherical ; but the transparent portion in front is the sec tion of a smaller sphere than the globe. The optic nerve, to which the eye-ball is attached posteriorly, enters considerably on the inside of the axis of the eye.
The coats of the eye are disposed con centrically; and the exterior, which is very dense, firm, and tough, is called the stile rotica. This does not cover the whole globe, but leaves a circular opening in front, filled by the transparent cornea, which, although pellucid, is a very finn and strong membrane. Hence, the sclero tics and cornea together form a very com plete exterior case, which defends and supports the more delicate parts within. The necessity of having the front of the globe transparent, for the purpose of ad mitting the rays of light, is obvious.
Under the sclerotica a soft and vascular membrane surrounds the eye-ball, and is called the choroid coat It is connected to the sclerotica by a loose adliesio n,which can be destroyed by blowing air between the membranes ; but in front this adhesion is stronger, and forms a white circle named orhicithis or ligamentum ciliare. The of the choroid coat is a deep brown, approaching to a black, and this colour is derived from a substance called pigmen tum nigrum, which separates from the choroid by maceration, and dissolves in water so as to render it turbid.
The inner surface of the choroid coat, which is universally coloured by pigmen tum nigrum in the human subject, is sometimes called tunics ruyschiana, as Ruysch endeavoured to prove that it form ed a distinct membrane from the external part. It is this inner surface that possess es the brilliant colours observable in ani mals, whence the appellation of tapetum.
This surface lies in contact with the retina, but does not adhere to that membrane. On the front of the eye, however, and be yond the anterior margin of the retina, the choroid is closely attached by means of numerous and very delicate folds, called the ciliary processes, to the surface of the vitreous humour, round the margin of the crystalline lens.
The iris is a membrane continued trans versely across the eye-ball,behind the cor nea, and appearing as a continuation of the choroid from the orbiculus ciliaris. The round opening in the front of this membrane is called the pupil ; it allows the passage of the rays of light into the interior of the eye. This aperture varies in its dimensions according to the quantity of light to which the organ is exposed : a strong light causes the pupil to become contracted,in order to exclude a portion of the rays of light which offend the organ. The aperture is dilated in a weak light, to let in as many mays as possible. Some anatomists have thought proper to employ themselves in debating at length, whether these motions arise from a really muscular structure or no ; but we believe that they have not yet settled the point completely.
The name of iris was applied to this part, from the diversity of colours obser vable in it in different individuals ; and it is the colour of this that produces the colour of the eye, in the popular sense of the phrase. There is a remarkable cor respondence in this point between the skin and hair and the iris. A light com plexion and hair is accompanied with blue, grey, or lighter colours of the iris ; while a dark skin and black hair are at tended with the dark brown iris.
In that curious variety of the human race called the Albinos, where the skin and hair arc of a dead milk-white hue, in consequence of a total absence of the rote mucosum or colouring principle, the colouring matter of the iris and choroid is also deficient, and these parts appear red. from the numerous blood-vessels which they contain.