EYE. 



175 



situation ; but that there is no general thick- 

 ening in this part from this cause is proved 

 by the thinness of the membrane in the inter- 

 vals between and beneath these tendons. The 

 consequence of this greater thinness of the 

 membrane anteriorly is, that when the eyeball 

 is ruptured by a blow, the laceration takes 

 place at a short distance from the cornea. In 

 animals in whom the eyeball deviates much 

 from a true sphere, as in the horse, ox, sheep, 

 and above all, in the whale, the sclerotic is 

 much thicker posteriorly than anteriorly, being 

 in the latter animal from three quarters to an 

 inch in thickness, while it is not more than a 

 line at its junction with the cornea. The rea- 

 son for the existence of this provision is, that 

 the form of the perfect sphere is preserved by 

 the uniform resistance of the contents, but when 

 these contents are spherical in one part, and 

 flattened in another, the external case must pos- 

 sess strength sufficient to preserve this irregu- 

 larity of form. It is remarkable that this 

 strength is conferred in the class mammalia 

 by giving to the sclerotic increase of thickness, 

 the fibrous structure remaining nearly the same 

 in its nature, while in birds, reptiles, and fishes, 

 the requisite strength is derived from the pre- 

 sence of a cartilaginous cup or portion of sphere, 

 disposed within a very thin fibrous sclerotic. 

 This cartilaginous sclerotic, as it is often- 

 called in the books, exists, as far as 1 have been 

 able to ascertain, in these three classes, and is in 

 some individuals very remarkable. In birds it 

 is thin and flexible, giving a degree of elasticity, 

 which distinguishes the eyeball in this class. 

 In fishes, as has been observed by Cuvier and 

 others, the cartilage is always present, and is 

 particularly thick in the sturgeon ; it is even 

 osseous in some, as the sea-bream, from the 

 eye of which animal I have often obtained it 

 in the form of a hard crust by putrefactive 

 maceration. Among the reptiles the turtle 

 presents a good example of this structure. 

 Where the deviation from the spherical form is 

 very great, as in birds, additional provision is 

 made to sustain the form of the organ. This 

 consists of a series of small osseous plates ar- 

 ranged in a circle round the margin of the cor- 

 nea, lapping over each other at the edges, and 

 intimately connected with the fibrous and car- 

 tilaginous layers of the sclerotic. A similar 

 provision exists in the turtle, and also in the 

 chameleon, and mny other lizards, but not 

 perhaps so neatly and perfectly arranged as in 

 birds. It is found in the great fossil reptiles 

 Ichthyosaurus and Plesiosaurus. 



The sclerotic, like other fibrous membranes, 

 being inelastic and unyielding, does not be- 

 come stretched when fluids accumulate in the 

 eyeball in consequence of inflammation, or in 

 other words, the eyeball does not become en- 

 larged from effusion of serum or secretion of 

 purulent matter into its chambers. To this 

 probably may be attributed the intolerable 

 torture and sense of tension experienced when 

 the eyeball suppurates, as well as the severe 

 pain extending to the temple in some forms of 

 inflammation. The pain in such cases must 

 not, however, be wholly attributed to this dis- 

 tension of an unyielding membrane. The 



fibrous membranes in general, when affected by 

 rheumatic or arthritic inflammation, become 

 acutely sensible, and the cause of much suffer- 

 ing ; and the sclerotic, when similarly affected, 

 acquires the same description of painful sen- 

 sibility, apparently independent of distension 

 from effusion. In certain forms of inflam- 

 mation and other morbid changes of the 

 eyeball, the sclerotic appears to yield to 

 distension, as in scrofulous inflammation and 

 hydrophthalmia ; but this is not a mechanical 

 stretching, but an alteration in structure at- 

 tended with a thinning of the membrane, and 

 consequent alteration in the shape of the globe. 

 It appears that the cornea and sclerotic are 

 peculiarly, if not in many instances almost ex- 

 clusively, the seat of the disease in chronic 

 scrofulous inflammation of the eyeball. This 

 inference may, I think, be justly drawn from 

 the fact, that in such cases the sclerotic becomes 

 so much thinned that the dark choroid projects, 

 in the form of a tumour, and the eye loses its 

 spherical form ; yet the pupil remains regular,, 

 the lens transparent, and the retina sensible to 

 light. When the cornea is destroyed by slough 

 or ulceration in severe ophthalmia, allowing 

 the lens and more or less of the vitreous hu- 

 mour to escape, the sclerotic does not accom- 

 modate itself to the diminished contents by a 

 uniform contraction, but merely falls in ; and 

 when the eye has been completely emptied, it 

 is found many years after the injury folded up 

 into a small irregular mass in the bottom of the 

 orbit. When the organization of the eye is 

 completely destroyed by idiopathic, rheumatic, 

 or syphilitic inflammation, the sclerotic becomes 

 flaccid, and the whole eyeball soft, allowing 

 the contraction of the four straight muscles to 

 produce corresponding depressions, and thus 

 convert the sphere into a form somewhat cu- 

 bical. 



Of the cornea. This is the transparent body 

 which fills the circular aperture in the anterior 

 part of the spherical sclerotic ; it is called cornea 

 from its supposed resemblance to transparent 

 horn, and cornea transparens in contradistinction 

 to the sclerotic, which, as has been stated, is 

 called cornea opaca. It is generally described 

 as a transparent structure, serving to the eye the 

 same purpose as the crystal to the watch ; but 

 this is not a correct comparison : the crystal 

 merely transmits the light without changing 

 the direction of the rays ; the cornea, whether 

 it be considered in itself a lens, or as the sphe- 

 rical surface of the aqueous humour, refracts 

 the rays and causes them to converge to a 

 focus. Haller, although he does not directly 

 say that it is a lens, yet states that if held over 

 a book it magnifies the letters, which of course 

 results from its lenticular form; and Cuvier 

 and Biot distinctly call it a meniscus. On the 

 other hand, the Sommerrings, both father and 

 son, describe it as a mere segment of a sphere, 

 the curve of the convexity corresponding to 

 that of the concavity, as in the watch crystal. 

 I consider it to be a lens and, a meniscus. If 

 it be removed from the eye a short time after 

 death with a portion of the sclerotic, and dipped 

 in water to smooth its surfaces, it magnifies ob. 

 jects when held between them and the eye, a s 





