Reference > Anatomy of the Human Body > Page 1005
Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
  The anterior chamber of the eye appears as a cleft in the mesoderm separating the lens from the overlying ectoderm. The layer of mesoderm in front of the cleft forms the substantia propria of the cornea, that behind the cleft the stroma of the iris and the pupillary membrane. The fibers of the ciliary muscle are derived from the mesoderm, but those of the Sphincter and Dilatator pupillæ are of ectodermal origin, being developed from the cells of the pupillary part of the optic cup.
  The sclera and choroid are derived from the mesoderm surrounding the optic cup.
  The eyelids are formed as small cutaneous folds (Figs. 866, 867), which about the middle of the third month come together and unite in front of the cornea. They remain united until about the end of the sixth month.
  The lacrimal sac and nasolacrimal duct result from a thickening of the ectoderm in the groove, nasoöptic furrow, between the lateral nasal and maxillary processes. This thickening forms a solid cord of cells which sinks into the mesoderm; during the third month the central cells of the cord break down, and a lumen, the nasolacrimal duct, is established. The lacrimal ducts arise as buds from the upper part of the cord of cells and secondarily establish openings (puncta lacrimalia) on the margins of the lids. The epithelium of the cornea and conjunctiva, and that which lines the ducts and alveoli of the lacrimal gland, are of ectodermal origin, as are also the eyelashes and the lining cells of the glands which open on the lid-margins.
1c. 1. The Tunics of the Eye
  From without inward the three tunics are: (1) A fibrous tunic, (Fig. 869) consisting of the sclera behind and the cornea in front; (2) a vascular pigmented tunic, comprising, from behind forward, the choroid, ciliary body, and iris; and (3) a nervous tunic, the retina.

The Fibrous Tunic (tunica fibrosa oculi).—The sclera and cornea (Fig. 869) form the fibrous tunic of the bulb of the eye; the sclera is opaque, and constitutes the posterior five-sixths of the tunic; the cornea is transparent, and forms the anterior sixth.

The Sclera.—The sclera has received its name from its extreme density and hardness; it is a firm, unyielding membrane, serving to maintain the form of the bulb. It is much thicker behind than in front; the thickness of its posterior part is 1 mm. Its external surface is of white color, and is in contact with the inner surface of the fascia of the bulb; it is quite smooth, except at the points where the Recti and Obliqui are inserted into it; its anterior part is covered by the conjunctival membrane. Its inner surface is brown in color and marked by grooves, in which the ciliary nerves and vessels are lodged; it is separated from the outer surface of the choroid by an extensive lymph space (spatium perichorioideale) which is traversed by an exceedingly fine cellular tissue, the lamina suprachorioidea. Behind it is pierced by the optic nerve, and is continuous through the fibrous sheath of this nerve with the dura mater. Where the optic nerve passes through the sclera, the latter forms a thin cribriform lamina, the lamina cribrosa scleræ; the minute orifices in this lamina serve for the transmission of the nervous filaments, and the fibrous septa dividing them from one another are continuous with the membranous processes which separate the bundles of nerve fibers. One of these openings, larger than the rest, occupies the center of the lamina; it transmits the central artery and vein of the retina. Around the entrance of the optic nerve are numerous small apertures for the transmission of the ciliary vessels and nerves, and about midway between this entrance and the sclerocorneal junction are four or five large apertures for the transmission of veins (venæ vorticosæ). In front, the sclera is directly continuous with the cornea, the line of union being termed the sclero-corneal junction. In the inner part of the sclera close to this junction is a circular canal, the sinus venosus scleræ (canal of Schlemm). In a meridional section of this


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