Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/343

 Fart VI. A N A T O M Y. well as of .all the other bones of the human body, ex- roides, which is of a blackidi colour., more or lefs inclicept the teeth, is. covered by a particular membrane, of ned to red, and Adheres, by means of a great number of which that portion which particularly iavefts the cranium fmall veflels, to die cornea opaca, from the infertion is named pericrdniunj, and that which invefts the bones of the optic nerve, all the way to the union of the two of the face is fimply termed periojleum. cornese, where it leaves the circumference cf the globe, The pericranium is made up of two laminae clofely u- and forms a perforated feptum, by which the fmall fegnited together. The internal lamina, which has by fome ment of the globe is feparated from the greater. This been taken for a particular periofteum, covers immediate- portion goes commonly by the particular name of uvea, ly all the bohy parts of this region ; and the external la- which was formerly given to the whole fecond coat; and mina has been looked upon as a membrane diftinfl: from as it is of different colours in fevefal fubjefts, it has likethe internal, and named pericranium particularly. wife got the name of iris. The anterior portion or perforated feptum of the choroides has the name of uvea, and the hole near the cen-. Sect. -V.- Of the Eye. tre of this feptum is called/a/zV/ar. The anterior 1 amir na of the fame feptum is termed iris, and the radiated The Globe or Ball of the Eye. plicae of the pofterior lamina, procejfus ciliares. Bethe two laminae of the uvea, we find two very thin The globe of the eye is made up of feveral proper tween of fibres which appear to be flefliy, the fibres cf parts, fome of which being more or kfs folid, reprefent planes one plane being orbicular, and lying round the circum, a kind of Ihell formed by' the union of feveral membra- ference pupilla, and diofe of the other being ranous ftrata called the. coats of the globe of,the eye; and diated, ofonetheextremity of which is fixed to the orbicular the other parts being more or lefs fluid, and contained in the other to the great edge of the, uvea. particular membranous capfulse, or in the interflices be- plane, The plice or proceflus ciliarts are fmall radiated and tween the coats, are termed the humours of the globe of prominent duplicatures of the pofterior lamina of the uthe eye. Thefe capful a; are likewife termed coats. * vea, and their circumference anfwers partly to that of the The coats of the globe of the eye are of three kinds. ring of the external lamina. They are oblong Some form chiefly the fliell of the globe; fome are ad- white plates; their pofterior extremities, or thofe next ditional, being fixed only to a part of the globe; and thin choroides, being very fine and pointed; the others, fome are capfular, which contain the humours. The orthethofe. next the pupilla, broad, prominent, and ending coats which form the fbell are three in number. The external is termed tunica fclerotica or cornea; the middle in acute angles. coat is named choroides ; and the third or innermofly re- The fpace between the cornea lucida and uvea contina. The additional coats are two; one called tendi- tains the greateft part of the aqueous humour, and comnofa or albuginea, which forms the white of the eye; municates by the.pupilla with a very narrow fpace behind and the other, conjunftivafi The capfular tunics are the uvea, or between that and the cryftalline. Thefe two fpaces have been termed the two chambers of the alikewife two, the vitrea, and cryftdlina. queous humour, one anterior, the other pofterior. ' The third coat of the eye is of a very different texture The Coats of the Eye. from other two coats. It is whited foft, and The mod external, thicked, and ftrongefl coat of the tender,thatandof the in a manner medullary, or like a kind of eye is the fclerotica or cornea, and it inveds all the o- pafte fpread upon fine reticular web. It appears to bq ther parts of which the globe is compofed. It is divided thicker than - the achoroides, and reaches from the inferinto two portions, one called cornea opaca, the other tion of the optic nerve, of the ciliary cornea lucida, which is only a fmall fegrhent of a fphere, radii, being equally fixedto tothetheextremities choroides through its fituated anteriorly. whole extent. At the place which anfwers to the inferThe cornea opaca is made up of feveral drata clofely of the optic nerve, we obferve a fmall depreflion, connected together, and is.of an hard compact texture tion in which lies a fort of medullary button terminating in a refembling parchment. About the middle of its pode- point; and from this depreflion blood-veflels go out, rior convex portion, where it fudains the optic nerve, it which are ramified on all fides through the fubftancc of is in a manner perforated, and thicker than any where the retina. elfe. _ ; The cornea lucida is made tip in the like manner of feveral drata or lamina; clofely united, and appears to be The Humours of the Eye arid their Cafsula;. a continuation of tl^e opaque portion or fclerotica, though The vitreous humour is a clear and very liquid gelatinous fluid, contained in a fine tranfparsnt capfula, called of a different texture. This portion is fomething more convex than the cor- tunica vitrea, together with which it forms a mate nearly nea opaca, fo that if reprefents the fegment of a fmall of the confiftence of the white of an egg. It fills the greats eft part of the globe of the eye, that is, almoft all that, fphece added to the fegment of a greater. The cornea lucida is perforated by a great number of fpace which anfwers to the extent of the retina, except a imperceptible pores, through which a very fine fluid is con- fmall portion behind the tivea, where it forms a fpflula, in tinurdly difcharged, which foon afterwards evaporates. which the cryftalline is lodged. The fecond coat of the globe of the eye is the cho-^ The tunica vitrea^is compofed exteriorly pf twp lami4D nx Vol. I. No. 13. 3
 * 3?