



THE TUNICS OF THE EYE 1005 



ie 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 pupillse 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, nasooptic 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 naso- 

 lacrimal 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. 



The Tunics of the Eye (Fig. 869). 



From without inward the three tunics are: (1) A fibrous tunic, 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 hard- 

 ness; 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 con junctival mem- 

 brane. 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 sclerse; 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 pro- 

 cesses 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 (venae vorticosae) . 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 sclerae (canal of Schlemm). In a meridional section of this 



