71 



BYE. 



i:<i K. 



:* 



put of the surface of the globe, contain* tha rat of the aqueous 

 humour, and U bounded in frout by the transparent and ahghtly- 

 prominent dim: crt in the sclerotic like a watch-glaas in iti metallic 

 rim, and known as the Cornea, from iU horny texture. Its transverse 

 chord, or the diameter of the circle of union between the cornea and 

 sclerotic i* nine-twentieths, or nearly half an inch in length. 



The globe derives its firmness to the touch from the distension of 

 the contained fluid*: it* capability to bear that distension, which 

 insures the permanence of its shape, is due to the flexible but strong 

 and inelastic outer covering or tunic, consisting, as we have said, of 

 the sclerotic and cornea. 



Sclerotic. (Fig. 5, a.)- The Sclerotic Membrane is so called from 

 it* toughneas (mrAipo't, hard, rigid). It may be considered as an 

 expanded prolongation of the sheath of the optic nerve, which it 

 resembles in iU interwoven fibrous texture. Its inner surface is 

 continuous with the lamina cribrosa already mentioned. Immediately 

 around this part it is about an eighth of an inch thick, and gradually 

 becomes thinner as it approaches the cornea, which it slightly over- 

 laps. The two structures are not separated by an abrupt line, but 

 are blended together, and adhere so closely that they cannot be torn 

 asunder without great force. The thin glistening tendons of the 

 muscles which move the globe, or rather their smooth outer laminae 

 are spread over and incorporated with the sclerotic in front, 

 approaching each other till they unite near its junction with the 

 cornea. They render it somewhat thicker in this situation than in 

 the space* between them or behind the line where they begin to be 

 inserted. This front part of the capsule of the eye is called the 

 tunica albuginea, from the whiteness characteristic of all tendinous 

 parts. When boiled the sclerotic yields gelatine. Under the micro- 

 scope it is found to consist of true connective tissue, the fibrils of 

 which can be easily demonstrated by teasing or treating transverse 

 sections with acetic acid. Numerous fine elastic elements pervade 

 the connective tissues of the sclerotic, of the same form as in tendons 

 and ligament*, that is, as a net-work. During life the elements of this 

 net-work appear to possess cavities and fluid contents, which according 

 to Virchow form a kind of nutritive system. The vessels are very few 

 in this membrane, and are derived chiefly from the ciliary arteries and 

 those of the muscles of the eyeball. Nerves have been recently 

 described in the sclerotic, but their existence, according to Kolliker, 

 is doubtful 



Fig. S 



j.i 



Section of the globe of tin. light eye through the optic nerrc. 

 a, sclerotic ; t, cornea ; c, choroiil ; d, retina ; r, Titreoiu humour ; /, ery- 

 ullinc humour or Inn ; g, aqneooa humour ; A, lri< ; i, ciliary liframent ; 

 *, ciliary proceMM ; /, on (errata of the ciliary body ; m, canal of Petit ; 

 n, foramen of .Soerarorring ; e, ihcath of the optic nerve ; f, iubtUncc of the 

 nerve,; g , arterla centralli retina). 



Conjunctiva. The albuginea is defended from contact with the air 

 by a transparent mucous membrane, continuous with that which linen 

 the interior of the eyelid*. It is called the conjunctiva reflex*, or 

 adnata, to distinguish it from the conjunctiva propria of tho 

 t is very loosely connected to the sclerotic at first to facilitate 

 the movement of the globe : n it advance* forward it becomes more 



closely attached to the albuginea ; and hence extending to the cornea 

 adheres intimately to it* margin and over it* whole surface. The 

 conjunctiva i* the moat sensitive external part of the body to all 

 painful impression*, except cold, especially where it invest* the cornea. 

 The smallest particle of foreign matter in contact with it give* intole- 

 rable pain, and make* the act of winking to clear it away imperative ; 

 and hence it* chief and most essential use a* part of the delicate 

 organ of which it may be considered as the guardian. If the nerve 

 which supplies it with sensibility be divided or injured, incidental 

 causes of irritation are suffered to produce their injurious efl'ecU 

 unheeded, and the eye toou becomes inflamed, ulcerates, and is 

 destroyed. 



Cornea. (Fig. 5, 6.) The Cornea is somewhat thicker than the 

 sclerotic except at the back of the eye, is equally tough, though 

 rather more flexible, and of a much closer and more even texture. 

 It* inner surface is concave, and nearly parallel to its outer surface ; 

 it is however rather thicker in the middle than elsewhere, and the 

 general statement that it has no share in effecting the convergence of 

 incident rays on account of the parallelism of its surfaces i* therefore 

 not quite correct It is covered externally, as we have already men- 

 tioned, by the conjunctiva, and is lined internally by a delicate elastic 

 membrane called the Membrane of Descemet or Demount. The 

 bulk of the tunic, or cornea proper, consists of several layers which 

 slide upon each other when the membrane U rubbed between the 

 finger and thumb, and are separated in the natural state by a limpid 

 fluid contained in a delicate cellular structure. This fluid give* 

 plumpness to the outer surface, which is represented by some author- 

 ities to be not exactly spherical, but of that kind which would be 

 formed by the revolution of an ellipse of small eccentricity about its 

 long axis. 



The cornea proper consists of a fibrous substance closely allied to 

 connective tissue, but which, according to Miiller, yields when boiled, 

 not gelatine, but chondriu. Its elements, pale bundles 0'002'" 0'004'" 

 in diameter, in which, at least when teased out, finer fibril* are 

 usually perceptible, sometimes more, sometimes less distinctly, are 

 united into flat bundles. These bundles, which have their flat sides 

 always parallel with the surface of the cornea, decussate in various 

 directions, and exhibit, if not complete lamellic, yet a distinctly 

 laminated structure, owing to which the cornea is very readily torn 

 and penetrated in the direction of its surfaces, and with great difficulty 

 in that of its thickness. 



The conjunctival lining of the cornea is composed principally of a 

 soft laminated epithelium 0-023'" 0-050'" thick, in which the deeper 

 layers of cells are elongated and placed vertically upon the cornea, 

 whilst the middle ones are more of a rounded form ; and as they 

 approach the surface pass into a layer O'OOS'" O'Ol"' thick, corre- 

 sponding to the horny layer of the epidermis, composed of plates 

 0-01'" 0-14'" in size, though still nucleated and soft. 



The Membrane of Descemet or Demours, as the inferior layer of 

 the cornea is called, consists of an elastic membrane rather laxly 

 attached to the cornea! tissue, and of an epithelium on its inner 

 surface. The former is as clear as glass, brilliant, quite structureless, 

 easily lacerable though tolerably firm, and so elastic that when it is 

 raised from the cornea by the scalpel and forceps, by boiling in water, 

 or by maceration in alkalies, under which treatment as under 

 reagents in general it does not lose its transparency, it always rolls 

 up strongly and towards the front. Towards the border of the 

 cornea it passes into a peculiar system of fibres, first described by 

 Bowman. This set of fibres is continued from the cornea on to the 

 iris, where they form the ligamentuni iridis pectiuaturn of Huek, the 

 pillars of the iris of Bowman. The epithelium of the membrane of 

 Demours, which in man frequently does not retain its perfect con- 

 dition, consist* of a single layer of polygonal cells, with extremely 

 fine and pale granular content* and round nuclei. This layer ceases 

 towards the border of the cornea, but isolated indications of iU 

 existence may be found along the pillars of the iris. 



Although in the embryo of man and the sheep a rich capillary 

 plexus of vessels exists in the conjunctiva, the cornea in the adult 

 human being is nearly nou- vascular. It is nevertheless not unfa - 

 ably circumstanced for nutrition. Wounds in it rapidly unite ; portions 

 of the epithelium, or even of the fibrous layer, are speedily restored 

 when removed ; and ulcers are filled up from the bottom with new 

 cornea! substance. Fatty deposits in ite tissue, particularly in its 

 cellular elements, producing a yellow zone, first accurately described 

 by Mr. Edwin Canton, and called Arcus seuilin, or Ueroutoxon Nerves, 

 have been described in the cornea by Schlemui ; they are dcrivuil 

 from the nervi ciliares, and penetrate the sclerotic at its anterior 

 border, and thence enter the fibrous layer of the cornea. They are 

 readily found at the margin of the solerotic in the form of 24 30 

 finer and thicker twigs, but scarcely exceeding 0'02'" in size. 



Chorioid or Choroid Membrane. (Fig. 6, c, fy. 6). We have now 

 to consider the internal tunics of the eye, the first of which is the 

 Choroid, or more properly Chorioid Membrane, so called from some 

 resemblance in the flocculeuce of its outer surface to the chorion, 

 or external investment of the ovum. This is a thin soft dark-bruwu 

 structure in contact with or lining nearly the whole concave surface 

 of the sclerotic. It may be said to originate around the entrance of 

 the optic uerve, which passes through it before it expands into the 



