An American text-book of diseases of the eye, ear, nose and throat / ed. by G. E. De Schweinitz and B. Alex. Randall. Illustrated with 766 engravings, 59 of them in colors.

  • De Schweinitz, G. E. (George Edmund), 1858-1938.
Date:
1899
Catalogue details

Licence: Public Domain Mark

Credit: An American text-book of diseases of the eye, ear, nose and throat / ed. by G. E. De Schweinitz and B. Alex. Randall. Illustrated with 766 engravings, 59 of them in colors. Source: Wellcome Collection.

Provider: This material has been provided by the Harvey Cushing/John Hay Whitney Medical Library at Yale University, through the Medical Heritage Library. The original may be consulted at the Harvey Cushing/John Hay Whitney Medical Library at Yale University.

    35/1336 (page 27)
    into the optic stalk from the nerve-cells located at its extremities. The great majority of fibers of the optic nerve may be regarded as the centrally directed outgrowths of the young neuroblasts situated within the developing retina: the axis-cylinder processes of these elements are guided in their journey to form central relations with the brain-centers by the supporting tissue contributed by the optic stalk. In addition, however, to the centrally growing fibers, there are others which pass in the opposite direction, and represent the peripherally directed axis-cylinder processes of the neuroblasts situated within the brain. Further complexity of structure later arises from the ingrowth of the vascular connective tissue constituting the pial sheath of the optic nerve, the extensions of which tissue form the septa subdivid- ing the nerve into the variously sized bundles which are so conspicuous in transverse sections. The posterior parts of the optic stalks become the optic tracts, while their middle portions unite to form the optic chiasm. The sheaths of the optic nerve are produced by the direct continuation of the mesodermic investment from which the cerebral dura, arachnoid, and pia are derived. Development of the Fibrous and Vascular Coats.—With the exception of the corneal epithelium, the lens, and the nervous tunic with its cerebral attachments, which are derived from the ectoderm, all parts of the eyeball are developed from the mesoderm surrounding the primary optic vesicle. Coincident!}' with the changes affecting the optic vesicle, as already noted, the surrounding mesoderm exhibits a differentiation, marked by active cell-proliferation and condensation, which results in the production of a dis- tinct envelope of actively growing embryonal connective tissue. The posterior segment of this mesodermic capsule undergoes further differentiation into an outer, relatively dense tuuic, which becomes the sclerotic, and an inner coat, which later is distinguished by a looser texture and greater vascularity. Very early in the history of the eye the lens-sac is separated from the overlying ectoderm by a thin stratum of mesodermic tissue ; later this layer becomes cleft, one part remaining as a thin mesodermic sheet over the outer surface of the young lens, the other adhering to the inner surface of the ecto- derm. The strata of mesoderm so formed constitute the pupillary membrane and the substantia propria of the cornea, the intervening cleft being the earliest indication of the future anterior chamber. The forward growth of the thin double-layered lip of the optic cup beyond the equator of the young lens and over its anterior surface is accompanied by a proliferation of the adjacent mesoderm and the extension of the primitive choroidal stratum which accompanies the retinal tissue in its growth forward. This anterior extension of the lip of the optic cup and the associated mesoderm gives rise to the rudiments of the iris and ciliary body, this expansion progressing until almost the entire anterior surface of the lens is covered : the central unoccu- pied area thus corresponds to a circular aperture within the retino-iridial sheet which remains as the pupil. In the early stages this opening is closed by the vascular pupillary membrane, a temporary structure which disappears before birth. The active growth of the thin lips of the optic cup results in still greater attenuation of the component strata of epithelial cells until these are repre- sented by the low columnar and cuboidal elements of the pars ciliaris and pars iridica retina?; the pigmentation of these epithelial cells also increases until the anterior portion of both layers is loaded with color-particles and the conspicuous pigment layer covering the posterior surface of the iris is produced. The accompanying mesodermic layer thickens and gives rise to