NORMAL HISTOLOGY. 



CHAPTER XVII. 



THE EYE AND ITS APPENDAGES. . 

 THE EYEBALL. 



THE bulbus oculi consists of three coats: i, the external 

 fibrous tunic, comprising the sclerotic and the cornea, upon which 

 devolves the maintenance of the form of the organ ; 2, the middle 

 vascular tunic, made up of the choroid, the ciliary body,- and the 

 iris, to which the principal vascular supply of the eye is distributed ; 

 3, the inner nervous tunic, the retina, which receives the termi- 

 nal expansion of the optic nerve and contains the specialized neuro- 

 epithelium concerned in the perception of the visual stimulus. The 

 aqueous humor, the crystalline lens, and the vitreous body are en- 

 closed by these coats, and represent the refractive media of the eye. 



Referred to their embryonic origin, the several parts of the eye 

 may be grouped under two headings, those developed from the 

 ectoderm and those derived from the mesoderm. The mem- 

 bers of the first group may be subdivided into (a) structures derived 

 directly from the ectoderm, including the lens and its anterior epi- 

 thelium, and the epithelium of the cornea and of the adjacent scleral 

 surface, and (6) structures derived secondarily from the ectoderm 

 through the optic vesicles protruded from the involuted ectoderm of 

 the cerebral vesicles ; to this class belong the primary retinal tissues, 

 i including the pigment-layer, as well as the atrophic retinal layers 

 continued over the posterior surface of the ciliary body and the iris. 

 All other parts of the eyeball, comprising the remaining portions 

 of the sclera, the cornea, the iris, the ciliary body, the choroid, and 

 the vitreous body, as well as the connective-tissue ingrowths of the 

 retina, are developed from the mesoderm. 



THE CORNEA. 



The cornea consists of five layers : 



1. The anterior epithelium. 



2. The anterior limiting membrane. 



3. The substance proper. 



4. The posterior limiting membrane. 



5. The posterior endot helium. 



The anterior epithelium, the only part of the cornea derived 

 from the ectoderm, all others being mesodermic, is stratified squa- 



