890 ANATOMY [ORGANS or SENSE vitreous humour, ramifies in its posterior portion. A single layer of polygonal cells lies between the body of the lens and the anterior portion of the capsule. The lens body is softer at its periphery than in its centre. It is built up of concentric layers, and on both the anterior and posterior surfaces lines are to be seen radiating from the central pole of each surface towards the circumference of the body. The radiated pattern varies in different animals. In the human foetus there are usually three lines, but in the adult they are more numerous. The lines on one surface do not lie immediately opposite those on the other, but are inter mediate. By the action of strong spirit and other reagents the body of the lens can be split up from the periphery towards the centre in the direction of these lines, so that they mark the edges of apposition of its concentric laminae. Each lamina consists of numerous hexagonal fibres about yg^th inch wide, which extend from one surface to the other over the circumference of the lens, so that a fibre which begins at the polar end of a radius on the one surface terminates at the circumferential end of a radius on the opposite. The edges of the fibres are sinuous in man, but denticulated in many animals, especially fishes, so that the fibres, not only in the same, but in superimposed layers, are closely interlocked. The lens fibres are nucleated, a structural fact which gives a clue to their true nature, and they are now regarded as peculiarly modified elongated cells. Babuchin states that he can trace the transition from the cells of the layer between the lens-body and capsule to the proper lens fibres. The lens-body is non-vascular and non-nervous. The surfaces of the lens become more flattened in old age, and its substance hardens and is less transparent. The Vitreous Body is much the largest of the refracting media, and occupies the largest part of the space enclosed by the tunics. Anteriorly it is hollowed out to receive the posterior convexity of the lens, but posteriorly it is convex, and the retina is moulded on it. It is as trans lucent as glass, jelly-like in consistency, and when punctured a watery fluid drains out. Its minute structure is difficult to ascertain, but as it, like the subcutaneous tissue of the embryo, contains rounded, stellate, and fusiform cells, it is customary to refer it to the gelatinous form of connective tissue; concentric lamellae, and even a radiated arrangement of fibres, have also been described. It has been customary also to consider it as invested by a delicate structureless membrane, the hyaloid membrane ; but this is now regarded as belonging to the retina, where it is known as the mem- brana limitans interna. Almost opposite the ora serrata a membrane springs from the vitreous body, passes forwards for some distance in relation to the deep surface of the ciliary processes, but separated from them by the pars ciliaris retinae, and then inclines inwards to become attached to the anterior surface of the capsule of the lens close to its circumference. It is so closely connected at its origin with the membrana limitans that it is difficult to recognise it as a distinct membrane. It is named the suspensory ligament of the lens, or zonule of Zinn, and contains fibres, which run in the meridional direction. Where it leaves the vitreous body a narrow space is enclosed between it and that body, which space surrounds the circumference of the lens, and is called the canal of Petit. From the relation of the suspensory ligament to the ciliary processes it has a plicated surface, and when these processes are torn" away from it a portion of the pigment of the processes is often left behind, so that the zonule is sometimes named the ciliary processes of the vitreous body. Tie eye an The Eyeball is an optical instrument, constructed on the plan of the camera obscura. The sclerotic forms the wall of the chamber. The choroid represents the black lining for absorbing the surplus rays of Light. The cornea, i
aqueous humour, lens, and vitreous body are the trans lucent media which, like the glass lens of the camera obscura, bring the rays of light to a focus. The retina is the sensitive plate on which the optical picture is thrown. In considering the relation of the retina to the visual rays, it must be kept in mind that the place of entrance of the optic nerve is insensible to light, and that the most sensitive part of the retina is the yellow spot, with its fovea centralis, where the optic nerve fibres are absent, but where the bacillary layer reaches its maximum size. It is clear, therefore, that the rods and cones of this layer, and not the optic nerve fibres, are the structures in the retina which are stimulated by the light ; and it is probable, as was suggested many years ago by Goodsir, that these rods and cones are impressed by the light, not as it enters the eye directly, but as it is reflected backwards from the choroid along their axes. The iris is the diaphragm which, by opening or closing the pupil, admits or cuts off the rays of light. The ciliary muscle represents the adjusting screw of the camera ; through its attachment to the ciliary processes and their relation to the suspensory ligament of the lens, it is able to act upon the lens and modify the curvature of its anterior surface ; for when the eye is to be accommodated to the vision of near objects the anterior surface of the lens becomes more convex than when distant objects are being examined. It has already been stated on p. 864 that the retina is formed in the j )T ^ mar y optic vesicle, which grows forwards ment to the integument. By the involution and growth of the skin at this spot a hollow is produced at the front of the vesicle, which gradually deepening forms a pouch, the secondary optic vesicle, in which the involuted part of the skin is lodged. From the included sub-epidermal tissue the vitreous body is derived ; from the included epidermis, the lens ; whilst the cornea sclerotic and iris are produced by the subcutaneous connective tissue. The optic nerve and retina are formed from the primary optic vesicle and its peduncle, and it is probable that the bacillary layer is a special development of its internal epithelial lining. The choroid coat again is derived from the pia mater. Hence the eyeball is compounded of structures derived partly from the integument and partly from the embryo brain. ACCESSORY PARTS TO THE EYEBALL. In relation to the eyeball several accessory parts are found. The Eye-Brows are projections of the integument, from which short, stiff hairs grow. The Eye-Lids, or palpebrae, are two movable curtains, Eyelie an upper and a lower, which protect the front of the globe. Between each pair of lids is a horizontal fissure, the palpe- bral fissure. From the free margins of the two lids project short hairs, the eye-lashes or cilia; the upper set curve downwards and forwards, the lower set upwards and for wards ; they also protect the front of the globe. Each eye-lid consists of skin ; of the fibres of the orbicular sphincter muscle ; of a thin plate of fibro-cartilage, the tarsal cartilage, to the inner end of which a fibrous band, the tendo palpebrarum, is attached, this tendon springing from the ascending process of the superior maxilla ; and of the conjunctiva. Between the conjunctiva and the tarsal cartilage is a layer of glands, the Meilomian glands ; each gland consists of a short duct, which expands at its sides into small sacculi. The sacculi contain short columnar cells ; these secrete a sebaceous material, which escapes through the orifice of the duct at the border of the eye-lid. The Conjunctiva is a mucous membrane, which forms the Con- posterior layer of the eye-lid, and is reflected on to the juncti anterior part of the sclerotic. At the inner angle of junc tion of the eye-lids is a soft reddish elevation of the conjunctiva, the camncula lachrymalis, and immediately
external to it is a vertical fold, the plica semilunaris, the
