THE VISUAL APPARATUS. 815 



THE VISUAL APPARATUS. 



PRELIMINARY ANATOMICAL AND HISTOLOGICAL OBSERVA- 

 TIONS. THE INTRAOCULAR PRESSURE. 



The following anatomical and histological sketch can refer only to the physio- 

 logically important points; it presupposes, naturally, a knowledge of the anatomical 

 structure of the eye. 



I. External or fibrous tunic of the bulb, consisting of the cornea and the solera. 



The cornea is, for the sake of simplicity, assumed to have a uniformly 

 spherical curvature, although in reality it deviates from this form. It represents 

 rather the vertical segment of a somewhat oblate ellipsoid, which must be conceived 

 as produced by the rotation of an ellipse about its long axis ; numerous deviations 

 from such a regular figure occur, however. It is approximately of the same 

 thickness throughout; except that in the newborn the central portion is some- 

 what thicker, while in adults it is rather thinner. The cornea is composed of 

 the following layers: (i) The anterior, stratified, nucleated epithelium, 0.03 mm. 

 thick (Fig. 266, a), consists of numerous layers of cells, all of which are connected 

 by delicate processes of protoplasm. The deepest cells are rather cone-shaped, 

 are arranged vertically side by side and are known as supporting cells. The 



Corneal corpuscles in the 

 lymph-spaces in man. 



Lymph-spaces 

 communicating with "\^^ \U'^ *> ^ L^-f*f ^40 ^9 ^\ Lymph-spaces 



one another. ^^-j"t^ '- - . ; ~I^' for the corneal 



corpuscles. 



FIG. 265. 



roundish cells of the middle layers are more arched, and possess tooth-like processes, 

 which fit into corresponding depressions in the adjoining cells. The deeper cells 

 contain a diplosoma. The superficial cells are flat, entirely smooth, and hard 

 squamous epithelium, containing keratin. The conjunctiva contains scattered 

 goblet-cells, 'which produce mucus. (2) The epithelial layer rests on the anterior 

 surface of the rather uneven anterior elastic membrane, or Bowman's membrane, 

 a structureless, hyaloid membrane (6) o.oi mm. thick, under the action of re- 

 agents having a fibrillar appearance, and posteriorly passing gradually in- 

 to: (3) The true corneal tissue, which consists of doubly refracting fibers, 

 constituted of exceedingly delicate connective-tissue fibrils. These fibers are 

 woven into about 60 mat-like lamellae (/), which overlie one another and are 

 cemented together in layers. Near the anterior elastic membrane these bundles 

 bend forward as supporting fibers. Some fibers pass through this entire layer as 

 "perforating" fibers. In the interstices of the mesh work there is a system of 

 intercommunicating passages, which possess a sort of parietal layer. These 

 anastomosing canals are lymphatic in nature and communicate with the lymph- 

 vessels of the conjunctiva. The fixed corneal corpuscles (Fig. 266, c) lie in these 

 spaces; they are provided with anastomosing processes, and possess the character 

 of protoplasmic cells. Kiihne saw these cells retract upon stimulation of the 

 corneal nerves; the anatomical connection of the nerves with the cells has also 

 been shown. According to v. Recklinghausen wandering cells may also penetrate 



