87 EYE. 



The structural peculiarities of the eye, as well as the presence of 

 that organ, may be Inferred with more certainty from the circum- 

 stances of an tini"!* 1 than from the place it occupies in any zoological 

 scale : in fact, no part has a closer relation to the habits and mode of 

 existence. The eye may be simple or compound, single or multiplied, 

 fixed or moveable : it may be encased in a hard transparent shell ; or 

 lie deeply imbedded within the protection of a bony socket; or 

 project from the surface of the head at the extremity of a sensitive 

 and retractile horn : it may be adapted for near or distant, oblique or 

 direct, vision ; for seeing in a strong or a weak light, in a dense or in a 

 rare medium ; or it may be formed so as to accommodate itself to 

 each of these conditions in it* turn ; and these peculiarities will all 

 be found upon examination to be in strict accordance with the 

 exigencies of the animal. Mere difference in bodily size, and the pro- 

 portionate reduction or increase in the bulk of the eye, is sufficient to 

 constitute a reason for a difference in its structure, and may suggest 

 an explanation when such discrepancies are observed to exist in 

 animals otherwise alike. 



Yet with all the varieties in configuration to which we have 

 alluded, it is rather in form than in substance that the eyes of animals 

 differ from each other. The organ has always a common purpose, and 

 is essentially the same in all cases : that is, we find an assemblage of 

 the same fundamental part*, generally arranged in the same order, 

 even when our powers of observation are assisted by the microscope, 

 and until all traces of organisation are lost in extreme tenuity of 

 texture and the transparency which result* from it And although 

 there are refinements m the structure of the organ of which we do 

 not know the purpose, and certain delicate adjustment* in the 

 exercise of the faculty of which we do not know the instruments, yet 

 upon the whole we can deduce the principles upon which the eye is 

 constructed, and assign the uses of its several parts with great 

 certainty from our knowledge of optical and physiological laws. 



The human eye is the most complete type of this organ, and the 

 structure and functions of the human eye will be here more parti- 

 cularly described, with occasional remarks upon the structure of the 

 eyes of the lower animals. 



The object, or what may be called the general problem, of the 

 human eye, is to combine distinctness and extent of vision with the 

 security and maintenance of the organ, and the utmost convenience in 

 using it The parts associated for these purposes are the orbits, or 

 sockets, of the eye ; the optic nerve ; the eyeball, or globe, with its 

 content*, and the external muscles which move and suspend it ; the 

 eyelids ; the lachrymal apparatus ; the nerves and vessels which supply 

 these parts, and the mass of fatty and cellular substance which 

 isolates and supports them. 



Orbits. The eyes with their appendages are lodged in two symme- 

 trical roomy cavities in the skull, completed in front by the eyelids, 

 but elsewhere entirely circumscribed by bone, the office of which it 

 need hardly be said is to protect them from injury, and from any 

 pressure that might embarrass the perfect freedom and precision of 

 their movements. These cavities are called the Orbits, Orbital Fossic, 

 or Sockets of the eye. Seven bones of the cranium or face, which we 

 need not enumerate, enter into the composition of each. They are 

 separated from each other in their whole depth, which is about two 

 inches, by the posterior chambers of the nose. They are conical in 

 shape, or, more strictly speaking, pyramidal, and obscurely quad- 

 rangular. The apex is directed backwards ; the base, about nn inch 

 and a quarter in width, is directed forwards, with a considerable 

 inclination outwards or towards the temple. The margin is less 

 prominent at the outer side than elsewhere, so that when viewed 

 laterally it present* a wide semicircular notch, with the concavity 

 forwards. One object of the divergence of the orbits, and of this 

 retreating curvature of the outer margin, is obviously to increase the 

 extent of vision. If the point of the finger be held before the eye, 

 and carried gradually back towards the ear, it will be observed that 

 in consequence of this arrangement it can be seen long after it has got 

 behind a vertical plane touching the front of both eyes, which taken 

 together are thus enabled to sweep over an angle of about 320, or 

 SO on each side behind the tangent plane. Above and below, the 

 edge U undercut as well as prominent, and the socket is therefore a 

 little wider within than at the margin itself, so that it slightly over 

 bang! the eyeball at these points. The inner or nasal sides pass 

 directly backwards and are parallel to each other, and the roof is 

 horizontal ; consequently the conical form of the cavity arises from 

 the inclination of the outer side and floor. In the angle between 

 these sides, and in that between the first and the roof, there are two 

 long irregular slit*. The former opens into the deep hollow between 

 the temple and the back of the upper jaw ; it U called the foramen 

 lacerum inferim, or spheno-maxillary fissure, and give* passage to a 

 branch from the fifth pair of nerves, which, piercing the bone, passes 

 beneath the floor of the orbit, and emerges upon the cheek through a 

 hole just beneath the lower edge of the orbit, about a third part from 

 the inner angle of the eye. The other slit, which is called the sphcn- 

 oidal fissure, or foramen lacerum superius, opens into the cavity of 

 the head, and transmit* another branch of the fifth pair, which passing 

 within the orbit along the rpof comes out through an opposite notch 

 in it* upper margin, and is distributed upon the forehead and upper 

 lid. Theee branches of the fifth pair, called the supra and infra- 



,.., 



orbitary nerves, are the most frequent seats of that excruciating 

 affliction the tic-douloureux. Through the aphenoidal fissure are 

 likewise transmitted the ophthalmic veins, and all the other nerve* 

 except the optic destined to the eye and its appendages. A third 

 opening, which is circular, called the foramen opticum, of the size of 

 a large quill, and leading also from the cavity of the skull, give* 

 passage through the sphenoid bone to the ophthalmic artery and the 

 optic nerve. It is directed obliquely outwards and forwards, and is 

 situated at the apex or back part of the orbit, in the angle between 

 the nasal side and the roof. In the same angle, close to the margin, 

 that is, just within the corner of the eye near the nose, there is a deep 

 groove leading into the lachrymal canal, to which we shall have 

 occasion to recur hereafter. 



Optic Nerves. The Optic Nerves, arising at the back part of the 

 brain, with which they have extensive and important couni , 

 not only whore they seem to originate in tin- corpora quuilrii: 

 but throughout the whole of the first part of their course within the 

 cranium, pass horizontally forward above the floor of that cavity, 

 converging towards each other till they meet, when they become 

 closely united. It is probable that they not merely meet but cross 

 each other, the greater part, if not thu whole, of the nerve from the 

 right side of the brain goiug to the left eye, and vice verso. It has 

 been ingeniously supposed by Dr. Wollaston ('Phil. Trans.,' 1824), in 

 order to account for some singular phenomena of disordered and 

 healthy vision, that this decussation or crossing takes place only with 

 respect to those parts of each nerve which lie towards tho other ; so 

 that each supplies the outer half of one eye and tho inner half of the 

 other. This he conceives would explain, among other things, the 

 correspondence between the homologous points of the two eyes, 

 which may be defined as those points which see the same object at 

 the same time. It is scarcely possible to verify such speculations by 

 dissection, from the softness and apparent homogeneity of the parts. 

 In fish the optic nerves cross each other entirely without touching ; 

 and in man, when the sight of one eye has been lost, the nerve beyond 

 the point of union within the cranium has been observed to be wasted 

 or diseased on the side opposite to that of the affected eye. [Hit A i N ; 

 NKHVK.] Beyond the point of junction the nerves again diverge from 

 each other, and passing into the optic foramen become invested in a 

 tough, flexible, and fibrous sheath, which is a tubular production of 

 the strong membrane called the dura mater which lines the cavity of 

 the skull. The outer part of this sheath is reflected off as the nerve 

 enters the orbit, and expanding, adheres to the bony surface of that 

 cavity throughout, becoming its periosteum. The nerves, continuing 

 to diverge, reach the eyeball after a somewhat tortuous course of an 

 inch in length. The curvature and laxity of the optic nerve give 

 facility to the movements of the globe, and preserve the delicate 

 structures within it as well as the nerve itself from the injurious 

 effects of tension. Its length is such as to allow the eyeball to 

 project slightly beyond the edge of the socket in front, and to afford 

 space behind for the action of the muscles which move it, and a 

 suitable distance between their points of attachment. Including the 

 thickness of the sheath, it is about one-sixth of an inch in diameter. 

 It does not consist like other nerves of a bundle of distinct fibn 

 of a medullary pulp inclosed in minute transparent tubes. The 

 sheath is pierced half an inch from the globe. by a vessel called '!:. 

 arteria centralU retina;, which accompanied by several small 

 reaches the axis of the nerve, and passes with it into the into* i 

 the eye. The nerve does not enter the back of the globe exactly in 

 the axis of vision, but about the fifth port of an inch from it, in a 

 horizontal line, on the inner or nasal side, and subtending an angle of 

 about 23 at the centre of the eye. At this point the dimensions of 

 the sheath are suddenly contracted, and it terminates in a thin cul- 

 de-sac pierced with minute holes or pores, hence called tho lamina 

 cribroaa (sieve-like plate). Through these pores the pulp of thu IXTVC-, 

 divested of its tubular involucra, passes into the interior of the globe 

 in divided portions ; but immediately re-uniting expands at the back 

 of the eye into a delicate cup-shaped membrane, with the concavity 

 directed forwards. This expansion of the optic nerve is called the 

 Retina ; it is the most important part of the eye, having a peculiar and 

 exclusive sensibility to the impressions of light, of which immediate 

 notice is conveyed from it along the collected nerve to the ln-.un. 

 All other parts of tho mechanism of vision are subordinate to this ; 

 and their whole office, independently of the conservation of the organ 

 as a part of a living body, is to regulate the quantity of light admitted 

 into the eye, and to distribute it in such a way upon the surface of 

 the retina that the impression, which if immediately received would 

 be confused and general, may bo an exact counterpart of the vj.-ilil.- 

 surface of the object 



Mechanism of Distinct Vision. The most elementary fact that wo 

 know respecting light is, that it proceeds in straight lines or rays from 

 every point of a luminous or illuminated body. A sensitive surface 

 or retina presented nakedly to such a body would therefore intercept 

 innumerable cones or pencils of light, each diverging from a did. -,. nt 

 point of the object But each point of tho retina must also bo con- 

 sidered in that case as the apex of a cone of rays converging upon it 

 from every part of the object ; and it is manifest that tho various 

 impressions thus received upon the same point at the game CM,- 

 would be undistinguuhable from each other. All therefore that we 



