Human Eye Anatomy

Human Eye AnatomyThe essential receptive part of the organ of vision is a delicate membrane composed of several layers of nerve cells, and known as the retina. Accurate images of external objects are thrown on this membrane by means of a series of dioptric mechanisms. The whole apparatus is incorporated in a globular structure, the eyeball.

The human eyeball is made up of segments of two spheres, one in front of the other. The posterior sphere, which is the larger, forms about five-sixths of the eyeball; it contains a transparent jelly-like substance, the vitreous humour. The anterior sphere constitutes about one-sixth of the eyeball; it projects forwards, and within it is a watery fluid termed the aqueous humour. Between the aqueous humour and the vitreous humour is a transparent biconvex body the lens (Fig. 80).

The wall of the eyeball consists of three coats or tunics named from without inwards:

(1) the fibrous tunic, composed of the sclera and cornea;
(2) the vascular tunic, consisting of the choroid, ciliarv body, and iris; and
(3) the nervous tunic, the retina.

(1) The sclera forms the posterior five-sixths of the outer tunic; it is tough in texture, opaque, and white in colour. The cornea is the projecting anterior part of the outer tunic; it is continuous with the sclera, the line of junction being practically circular. Unlike the sclera, however, it is transparent, and forms one of the refracting media of the eye.

(2) The posterior part of the vascular tunic is loosely adherent to the sclera and is known as the choroid. It consists largely of an interlacement of small bloodvessels, and contains also cells charged with a brownish-black pigment.

The anterior part of the vascular tunic hangs free behind the cornea, as the iris, a circular curtain or diaphragm; a little to the nasal side of its centre is the rounded aperture of the pupil.

The iris gives the characteristic colour to the eye and varies, therefore, in different individuals; the colour is due to cells containing a brownish-black pigment, and the amount and distribution of this determines the tint. In the substance of the iris, muscular fibres are arranged in two groups, circular and radiating. The aperture of the pupil diminishes in size when the circular fibres contract, while contraction of the radiating fibres dilates it. Between the choroid and iris is an intermediate portion called the ciliary body; it is thrown into a series of folds - the ciliary processes - which are closely connecied to a ligament (suspensory ligament) round the lens. Within the ciliary processes are numerous muscular fibres; when these, contract they pull the ciliary body forwards and relax the suspensory ligament of the lens.

(3) The retina, the innermost tunic of the eyeball, is closely applied to the choroid, and extends forwards on to the back of the ciliary processes and iris. The nerve cells, which are its chief characteristic, are found only in the posterior portion extending forwards almost to the ciliary processes. The margin between the nervous and non-nervous portions is jagged in outline and is termed the ora serrata. In the centre of the posterior segment of the retina is a small oval area with a yellowish tinge - the yellow spot (macula lutea); within this area is a central depression, the fovea centralis.

About 3 mm. to the nasal side from the front and 1 mm. below the level of the yellow spot is a slight elevation which marks the area of entrance of the optic nerve (Fig. 81).

The structure of the retina is very complicated, but for practical purposes it may be said to consist of three layers of bipolar neurons, interlacing with supporting structures and cells of association (Fig. 82). The terminal neurons, applied to the choroid, are specially modified to receive the light waves, and exhibit two forms, viz. rods and cones.

The rods are cylindrical thickenings of the dendrites, and project backwards at right angles from an outer limiting membrane; the cell bodies are situated a little distance in front of this membrane. The cones are somewhat flask-shaped thickenings of the dendrites, with their bases resting on the outer limiting membrane; the cell body is in contact with the base of the cone.

Between the straturn of rods and cones and the choroid is a layer of pigmented cells, processes from which extend between and envelop the rods and cones.

The axons of the rod bipolars and cone bipolars pass into the substance of the retina and meet and arborise, with the dendrites of the second layer of neurons. The axons of these in turn run towards the internal surface of the retina to arborise with a layer of large ganglionic cells.