174 



EYE. 



adopted from not making allowance for the 

 projection of the cornea, which is a portion of 

 a smaller sphere than the globe itself, and con- 

 sequently projects beyond its circumference. 

 From the flaccid state of the eye even shortly 

 after death, it must be very difficult to measure 

 it accurately. The question is, however, for- 

 tunately of little practical importance. The 

 eyeball of the male is generally a little larger 

 than that of the female; and if a close inquiry 

 be made into the matter, much difference in this 

 respect might probably be detected in different 

 individuals. I have seen the eyeball in an 

 adult of full size not larger than that of a child 

 of five years old ; and there is much apparent 

 difference in consequence of the difference in 

 the depth of the orbit, and in the gape of the 

 eyelids. Although the human eyeball is nearly 

 a perfect sphere, that precise form is obviously 

 not an essential requisite in the construction of 

 a perfect organ of vision. In all the vertebral 

 animals the bottom of the eye, where the retina 

 is expanded, is probably a portion of a correct 

 ^sphere, but in many the anterior part is com- 

 pressed, or in other words the sphere is trun- 

 cated, to adapt it to the form and dimensions of 

 the head, or to bring the cornea and lens nearer 

 4o the retina. In the mysticete whale the axis 

 as to the diameter as 20 to 29 ; in the swan as 

 7 to 10 ; in the turtle as about 8 to 10 ; and in 

 the cod as 14 to 17. This deviation from the 

 >spherical form demands a corresponding provi- 

 sion in the construction of the sclerotic, to be 

 noticed when describing that membrane. For 

 ;a fuller account of the comparative proportional 

 measurements of the eye, the student is referred 

 to the works of Cuvier and D. W. Sommer- 

 ring, as quoted at the end of this article ; the 

 limits of which do not admit of a greater detail 

 of facts derived from comparative anatomy 

 than the illustration of the description of the 

 human organ absolutely demands. 



Having attempted to give a general notion 

 of the mechanism of the eye in the preceding 

 paragraphs, it remains to consider each com- 

 ponent part separately, and to determine its 

 organization, properties, and application, as 

 well as the changes to which it is liable from 

 age, disease, or other circumstances. 



Of the sclerotic membrane. This, as has 

 been stated, constitutes, with the transpa- 

 rent cornea, the external case upon which 

 the integrity of the more delicate inter- 

 nal parts of the organ depends, otherwise in- 

 capable of preserving their precise relations to 

 each other : without such support the compo- 

 nent structures must fall to pieces, or be crushed 

 by external pressure. The name is derived 

 from the Greek <rxfa)qou, and it has also been 

 called cornea and cornea opaca in contradistinc- 

 tion to the true or transparent cornea, a structure 

 to which it bears no resemblance whatsoever ; 

 it is the same animal material which exists in 

 all parts of the body where strength with flexi- 

 bility is required, the material which in modern 

 times has been denominated fibrous mem- 

 brane. When carefully freed from all ex- 

 traneous matter by clipping with a pair of 

 scissors under water, it presents the brilliant 

 silvery-white appearance so characteristic of 



the fibrous membranes. The white streaks 

 which give the fibrous appearance appear ar- 

 ranged concentrically as the lines on imper- 

 fectly polished metallic surfaces. It is inelastic 

 as other fibrous membranes, and so strong that 

 it does not tear or yield unless exposed to the 

 greatest violence. Although penetrated by the 

 vessels going into and returning from the in- 

 ternal parts of -the eye, it does not appear to 

 have much more red blood circulating through 

 its texture than other tendinous expansions 

 distinguished for their whiteness. The vas- 

 cularity of the anterior part, however, where it 

 is exposed in the living body, constituting the 

 tunica albuginea, or white of the eye, is 

 different from that of. the rest of the mem- 

 brane. The four straight muscles are pene- 

 trated by small branches of the ophthalmic 

 artery, the delicate ramifications of which con- 

 verge to the circumference of the cornea, for 

 the nutrition of which membrane they appear 

 to be destined. In the natural state they can 

 scarcely be detected, but when enlarged by in- 

 flammation, present a remarkable appearance, 

 considered by practical writers one of the most 

 characteristic symptoms of inflammation of the 

 eyeball, or, as it is called, iritis. They then 

 appear as numerous distinct vessels, and as they 

 approach the margin of the cornea, become so 

 minute and subdivided, that they can no longer 

 be distinguished as separate vessels, but merely 

 present a uniform red tint, described as a pink 

 zone. The colour of this inflammatory vascula- 

 rity is also characteristic. Whether from the 

 vessels being more arterial than venous, or 

 from their distribution in so white a structure, 

 they present a brilliant pink appearance very 

 different from the deep red of conjunctival in- 

 flammation, which often enables the practi- 

 tioner to pronounce an opinion as to the nature 

 of the disease before he makes a close examin- 

 ation. 



The inner surface of the sclerotic where it 

 is in contact with the choroid, does not present 

 the same brilliant silver-white appearance that 

 it does externally, being stained with the black 

 colouring matter ; it is also obscured by a thin 

 layer of cellular membrane, by means of which 

 it is united to the external surface of the cho- 

 roid.* This layer of cellular membrane was 

 described by Le Cat, and more particularly by 

 Zinn, as a distinct membrane, and considered 

 to be a continuation of the pia mater ; it is, how- 

 ever, obviously nothing more than the connect- 

 ing material applied here as in other parts of 

 the body where union is requisite. 



The thickness of the sclerotic is greater in 

 the bottom of the eye than at its anterior part, 

 where it is so thin that it allows the black colour 

 of the choroid to appear through it, giving to this 

 part of the eye a blue tint, particularly remark- 

 able in young persons of delicate frame. The at- 

 tachments of the four straight muscles, how- 

 ever, appear to increase the thickness in this 



* [Arnold and others describe and figure a serous 

 membrane in this situation ( Spinnwebenhaut , arach- 

 noidea oculi ). See the figure of a vertical section 

 of the eye in Arnold liber das Auge, tab. iii. fig. 2, 

 and couied into Mr. Mackenzie's work on the 

 Eye. ED.] 



