Ophthalmology

Vitreous Chamber and Retina.

Behind the lens is the vitreous chamber, which is occupied by a perfectly transparent, colourless substance, much like white of egg. Clothing the back of the eye, and extending forward some distance in front of its equator, is the retina. It rests upon a highly vascular membrane which is responsible for the nutrition of the greater part of it, namely the choroid ; in man, however, the retina has its own blood vessels clearly visible with the ophthalmoscope. The retina may be likened to the sensitive plate of the camera for upon it images are formed which initiate impulses which, when conveyed to the cortex of the brain, give rise to the sensations of sight. Its sensi tive layer is placed posteriorly, and is composed of delicate structures known as the rods and cones.

At the central spot or yellow spot of the retina, the point of distinct vision, cones alone are present. The rods are believed to be concerned with lights of lower intensities, and in accordance with this they alone are present in night-flying birds (see COLOUR VISION ).

Optic Nerves and the Cerebral Cortex.

The nerve fibres from the retina converge upon the optic disk, and leave the eye as the optic nerve, which traverses the orbit to enter the skull. Inside the skull the two optic nerves meet and each is divided into two parts, one part continuing to the mid-brain on the same side, the other part crossing over to the opposite side. This crossing forms the chiasma ; in many animals, the birds for instance, the whole of each optic nerve crosses over in this way. Beyond the chiasma the nerve fibres are again collected into a compact bundle known as the optic tract, which terminates in the mid-brain. From here nervous impulses are relayed in two chief directions; some connect up with the nerves which control the movements of the eyes and others, forming the so-called optic radiations, make a long sweep backwards to reach the cerebral cortex, where, as already stated, the impulses are transformed to sensations of sight.

The exact area of the cerebral cortex, in which these fibres end, is known with great accuracy and forms the visual cortex. It is placed at the extreme hind end of the brain, and the adjoining mesial surface of each hemisphere, in the region of the calcarine fissure (see BRAIN). Should a minute portion of this cortex be cut out as the result of injury or blocking of its blood supply, the precise area of the defect which will be found in the vision can be stated with certainty. This accurate localisation was much advanced as a result of observations made during the War. General Diseases and the Eye.—Amongst the general dis eases of the body in which important manifestations occur in con nection with the eye a few may be mentioned.

In brain tumours or abscesses, swelling of the optic disk is sel dom absent, and forms perhaps the most important sign in the diagnosis of the condition. In addition the ocular nerves may have their functions interrupted, so that the movements of the eyes are interfered with, they no longer move in unison and double vision occurs. If nothing can be done for the brain tumour the optic nerve atrophies and blindness ensues. In advanced renal disease changes in the retina develop and convey a particularly grave prognosis, for patients seldom live so long as two years after their discovery. In diabetes a similar change may also arise, which may greatly spoil the sight, and although of less serious import than in renal cases, it must be considered a bad omen. Venereal disease frequently attacks the eye, whether in the early or late stages, and is perhaps the most prolific source of blindness. Tuberculous disease occasionally occurs. In diseases of the blood in general, eye signs are very common, and there are many other diseases where the diagnosis may be greatly assisted by the dis covery of changes in some part of the ocular apparatus. The in troduction of the "slit lamp"' has made many investigations possible which previously were quite outside our scope.

(R. F.

M.)