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condition of the optic disc, and of the yellow spot region, the rest of the fundus must be examined in all its parts. By directing the patient to look strongly upwards, inwards, etc., even the extreme periphery

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FIG. 32.-Head of the optic nerve (figure and description from Fuchs). A. Ophthalmoscopic view.-Somewhat to the inner side of the centre of the papilla the central artery rises from below, and to the temporal side of it rises the central vein. To the temporal side of the latter lies the small physiological excavation with the grey stippling of the lamina cribrosa. The papilla is encircled by the light scleral ring (between c and d), and the dark choroidal ring at d.

B. Longitudinal section through the head of the optic nerve. - Magnified 14 × 1. The trunk of the nerve up to the lamina cribrosa has a dark colour, because it consists of medullated nerve fibres, n, which have been stained black by Weigert's method. The clear interspaces, se

separating them, correspond to the septa composed of connective tissue. The nerve trunk is enveloped by the sheath of pia mater, p, the arachnoid sheath, ar, and the sheath of dura mater, du. There is a free interspace remaining between the sheaths, consisting of the subdural space, sd, and the subarachnoid space, sa. Both spaces have a blind ending in the sclera at e. The sheath of dura mater passes into the external layers, sa, of the sclera; the sheath of pia mater into the internal layers, s, which latter extend, as the lamina cribrosa, transversely across the course of the optic nerve. The nerve is represented in front of the lamina as of light colour, because here it consists of nonmedullated, and hence transparent, nerve fibres. The optic nerve spreads out upon the retina, r, in such a way that in its centre there is produced a funnel-shaped depression, the vascular funnel, ¿, on whose inner wall the central artery, a, and the central vein, v, ascend. The choroid, ch, shows a transverse section of its numerous blood vessels, and toward the retina a dark line, the pigment epithelium; next the margin of the foramen for the optic nerve, and, corresponding to the situation of the choroidal ring, the choroid is more darkly pigmented. Ci is a posterior, short, ciliary artery, which reaches the choroid through the sclera. Between the edge of the choroid, d, and the margin of the head of the optic nerve, c, there a narrow interspace in which the sclera lies exposed, and which corresponds to the scleral ring visible by the ophthalmoscope.

may be inspected. It is well, here, as elsewhere, to be methodical. Commencing above, direct the gaze of the patient" up," then "up and in,” “in,” "down and in," "down," and so through the entire circle. In this way only can every part of the fundus be with certainty brought into view.

Note generally any alterations in the choroid or retina, such as hæmorrhages, exudations, deposits of pigment, or absorption of the same. A broad distinguishing feature between affections of these two coats is, that retinal disturbances tend to interfere with the vessels in their course, whilst the deeper choroidal affections allow the retinal vessels to pass over them unaffected.

The illustration (Fig. 32) shows the anatomy of

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the optic nerve, exhibited at the optic disc or nerve head, as observed with the ophthalmoscope. The arrangement of the retinal vessels on the disc

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varies a great deal. Usually the artery bifurcates into a superior and inferior division (Fig. 33), which again divides into a temporal and a nasal branch. The veins run more or less parallel to the arteries, and are also

crossed by the latter. Pulsation may be sometimes noticed in the veins, or slight pressure on the globe will induce it. Pulsation in the arteries is always

pathological.

The colour of the disc is not uniform, the inner half being darker tinted than the outer.

The centre where the vessels issue is white and somewhat depressed. The depression is occasioned

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FIG. 34. The three kinds of excavation of the optic nerve.

Schematic.-FUCHS.

A. Physiological excavation.-Funnel-shaped, partial, with normal lamina
cribrosa.

B. Atrophic excavation.-Bowl-shaped, total, with normal lamina cribrosa.
C. Glaucomatous excavation.-Ampulliform, total, with the lamina cribrosa
bulged out posteriorly.

by the separation of the optic nerve fibres as they enter the eye (Fig. 32); the light colour is due to the white fibres of the lamina cribrosa visible at the bottom of the depression. Not infrequently this depression (physiological cup or excavation) is much exaggerated, involving the outer half of the papilla, and, it may be, extending to its external border. A physiological excavation or cup never involves the whole of the optic disc, and this distinguishes it from the excava

tion of glaucoma, which affects the entire disc, and from the cup of optic atrophy, in which the excavation is shallow, and the whole papilla white.

Differences in level in the fundus can be recognised by both methods of employing the ophthalmoscope. With the indirect method a difference of level will give rise to parallax. It will be seen in certain cases that, when the objective lens is moved to and fro, different parts of the image move at unequal rates. The vessels, for example, at the edge of a glaucomatous cup, being on a nearer level, will move in front of, and more quickly than those at the bottom of the excavation. In optic neuritis again, the papilla projects forwards, and the summit and base present the same unequal parallax.

For the accurate estimation of differences in level, the direct method should be used, and the measurement effected by placing lenses behind the sight hole of the mirror. Each dioptre of refraction will indicate a difference of 0.3 mm. of level.

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Routine examination with the ophthalmoscope.-First employ the mirror only-stand at about 16 or 18 in. from the patient, and illuminate the eye. In this way the degree of transparency of the refractive media is ascertained, and opacities in the cornea, or lens, or "floaters in the vitreous are revealed. If, also, there be marked ametropia, the retinal blood vessels will be visible, and the nature of the abnormal refraction may be diagnosed by observing whether they go "with," or "against," the observer as he moves his head from side to side.

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