The Toxic Amblyopias, 1896. The disease is one of interstitial inflammation followed by atrophy of the axial fibres of the optic nerve, with connective tissue proliferation, which may start at different points. According to De Schweinitz there is an increase of nuclei, hypertrophy of the connective tissue and wasting of the nerve-fibres of a limited portion of the optic nerve known as the papillo-macular bundle; in fact, that there is an interstitial sclerosing inflammation comparable, according to Samelshon, to the same pathological process that alcohol produces in the liver. The papillo-macular bundle, according to Bunge consist of those fibres in the optic nerve which supply the retina, between the macula lutea and the papilla, and lie in the temporal portion of the nerve-tip in a wedge shaped segment. They occupy about one-third of the surface of the papilla with tsized retinal vessels.
Anatomy.- The optic nerve extends from its terminal expansion, the retina which receives visual impressions, to the brain centres, where perception takes place. It may be divided for its anatomical considerations into three separate regions- cranial, orbital and intra-ocular portions.
Each optic tract arises by two roots, of which the external is made up of fibres arising from the corpus geniculatum externum, from the thalamus opticus and from the anterior corpus quadrigeminum. From these ganglia radiating fibres extend to the gray matter of the occipital lobe. The inner root of the optic tract receives fibres from the corpus geniculatum internum and from both the posterior and anterior corpus quadrigeminum. Another bundle of fibres comes direct from direct from the cortex of the occipital lobe, Other fibres have been traced as coming through the crus cerebri and along the pons varolii from the posterior columns of the cord. Still other fibres of this internal root come from the corpus dentatum of the cerebellum.
The optic tract formed by the union of these two roots passes forward along the inferior surface of the thalamus opticus, crosses the crus cerebri and unites upon the olivary process of the sphenoid bone with the optic tract from the opposite side to form the optic chiasm or commissure.
In addition to the fibres of the optic tracts, the chiasm has fibres which appear to come from the corpus subthalamicum and serve to connect corresponding parts on opposite sides of the brain. They are known as the commissural fibres of Meynert and of Gudden, and have no direct visual function (Fig. 77). In the optic commissure the fibres of each optic tract undergo partial decussation, the fibres of the right optic tract supplying the right half of each retina and the left optic tract supplies the left half of each retina. The dividing line in the eye is on the vertical meridian through the macula, while at the fovea there is an intermingling of the fibres from both sides. The orbital portion of the optic Scheme of the central visual apparatus. R, retina, shaded where it is innervated by the left, clear where innervated by the right hemisphere No, nervus opticus; Ch, Chiasma; Trotractus opticus; CM, Meynert’s commissure; CG, Gudden’s commissure; L, lateral tract root; M, median tract root, Tho, thalamus opticus; Cgl, corpus geniculatum laterale; Qa, nates; Bqa, brachia anteriora; Rd, direct cortical tract root; Ss, sagittal medullary layer of occipital lobe; Co, cortex (chiefly of the cuneus); Lm, median tract (schleife).
Nerve commences where it passes through the optic foramen from its origin in the optic commissure. From the chiasm to the foramen the nerve is about 10 mm. long, and from the foramen to the eyeball it is about 28 mm. long and 4 mm. in diameter. At the optic foramen the nerve becomes invested with a sheathe from the dura meter in addition to the pial sheath in which it has been inclosed in the skull. Between the dural and pial sheaths of the optic nerve is a space which is imperfectly divided by trabeculae of connective tissue and containing lymph. This space is directly continuous with the arachnoid cavity of the brain. Another lymph space lies beneath the pial sheath but this is normally only microscopical. The arteria and vena centralis retinae pierce the nerve about 15 mm. behind the eyeball. The central artery does not supply the nerve as a whole but gives off very minute branches just behind the lamina cribrosa to supply it and the optic papilla.
The pial sheath is a fibro-vascular structure very closely adherent to the nerve and gives off connective tissue bands which from a network of trabecular tissue between the fibres of the nerve. It receives its blood supply from branches of the ophthalmic artery, and by its continuity with the pia mater, forms a communication between the intra-cranial and orbital arteries. The pial sheath terminates by becoming blended with the inner layers of the sclera. The dural sheathe forms a fibrous covering to the nerve and terminates by blending with the outer layers of the sclera.
The ocular portion of the optic nerve is that part where it penetrates the globe. On passing into the eyeball the sheaths are left behind as described above and with them the connective tissue septa separating the fibres turn aside and blend with the sclera. The nerve-fibres having lost their medullary coat are continued as naked axis cylinder, and terminate as the optic papilla.
The Lamina Cribrosa is made up of fibrous tissue interwoven with the connective tissue sheaths and septa from the optic nerve at the level of the sclerotic opening. This structure is more or less visible with the ophthalmoscope and represents the limit of an ophthlmoscopic view. The optic nerve fibres, in order to gain entrance to the globe must pierce both the sclera and choroid which they do through a circular opening. The edge of this opening may be in close contact with the nerve or a small space may be left through which the sclera may be seen. Krause estimates the number of fibres within the optic nerve as high as 400,000
The Ophthalmoscopic Appearance of the Healthy Papilla is that of a circular area, whitish in color due to the lamina cribrosa which shines through the transparent nerve-fibres, the white substance of the sheaths having terminated at this point. It generally has a pinkish tint, due to the presence of capillaries, the degree of this coloration varying in different individuals. A little to the inner side of the disc the central artery of the retina is seen to emerge, which usually divides after passing the lamina criminal cribrosa, although it may sometimes have divided before coming into view. The two chief divisions thus formed pass, one upward and the other downward, to the retina. The central vein is somewhat darker in color and larger in size than the artery and accompanies it. There are frequently small lines of pigment bordering the disc at some point.
The physiological cup is an excavation at about the centre of the disc of a varying extent but it never reaches to the edge of the disc as does the cup of glaucoma. It is usually funnel- shaped and more distinctly white in appearance than is the rest of the disc (See Chromo-Lithograph, Plate II Fig.2) This is due to an exposure of the central part of the lamina cribrosa from the divergence of the nerve-fibres as they turn to pass over into the retina.
The sclerotic Ring is a whitish ring found at the edge of the disc and is caused by the opening in the choroid being somewhat larger than that in the sclera, and thus permitting the sclera to be seen through the transparent nerve-fibres. It is generally more visible at the outer edge of the disc owing to a greater thinness of the nerve fibres at that point. The average diameter of the disc is about 1.6 mm, its apparent size varying with the refraction of the eye.
Opaque Nerve Fibres.- This is a rather frequent congenital anomaly which generally affects but one eye though it is sometimes seen in both. The condition is due to the continuance of the opaque medullary sheath of some of the fibres of the nerve for a short distance after passing through the lamina cribrosa of the sclera. Opaque fibres are most seen extending either above or below (See Chromo-Lithograph, Plate III Fig. 10) and appear as a white patch, which runs a variable distance sometimes ending abruptly but generally as a striated, fan-shaped margin. The diagnosis of the condition in not difficult and yet it is sometimes confused with atrophy of the choroid or with the white mound around the disc seen in retinitis albuminurica. In opaque nerve-fibres the white or yellowish patch is, except in extremely rare instances, continuous with and concealing the margins of the disc. The retinal vessels may be wholly concealed or will appear here and there. The striation and flame-like shape of the opaque fibres are also characteristic diagnostic points. The surrounding parts of the fundus are normal and vision is usually only affected by an enlargement of the blind spot.
Coloboma of the Sheath.- This is another very rare anomaly which depends upon an imperfect closure of the foetal fissure and is very frequently mistaken for a retraction of the choroid found in myopia. It is often accompanied by a fissure or defect in the choroid but, unlike choroidal changes it has no pigment border. There is an apparent elongation downward and backward of the nerve which has a concave look. The nerve runs into the exposed sclera or sheath. The usual location of the coloboma is at the lower part of the disc, though in three cases that came under my observation and reported in the journal of ophthalmology, Otology and Laryngology, vol. ii., p. 2, 1890 the coloboma was in each instance at the upper part of the disc.
Professor of Ophthalmology in the College of the New York Ophthalmic Hospital; Surgeon to the New York Ophthalmic Hospital. Visiting Oculist to the Laura Franklin Free Hospital for Children; Ex-President American Homoeopathic Ophthalmological, Otological and Laryngological Society. First Vice-President American Institute of Homoeopathy : President Homoeopathic Medical Society of the State of New York ; Editor Homoeopathic Eye. Ear and Throat Journal : Associate Editor. Department of Ophthalmology, North American Journal of Homoeopathy, etc.