ORGANS OF THE SENSES. THE EYE.

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IV. ORGANS OF THE SENSES.

In this place it is intended to describe the organs of sight, hearing and smelling, which, considered with reference to their anatomy and development, are regarded as the higher organs of special sense. The description

of the organ of touch is given along with the skin in the histological part of the work, and that of the organ of taste along with the descriptive anatomy of the digestive system.

THE EYE.

The organ of vision, strictly speaking, consists only of the ball or globe of the eye, a spheroidal structure enclosed by strong membranous coverings, receiving the optic nerve posteriorly, and containing the sensitive terminations of that nerve, together with a series of transparent media, which constitute an optical instrument of variable focus, through which the rays of light are transmitted to the sensitive part, and so brought into focus as to form upon it a distinct inverted image of the objects from which they proceed. But there are likewise various structures external to the eyeball which contribute to the production of perfect vision, such as the straight and oblique muscles by which the eyeball is moved in different directions, and the various supporting and protective structures known as appendages of the eye (tutamina oculi), including the eyebrows, eyelids, and conjunctiva, and the lachrymal apparatus.

APPENDAGES OF THE EYE.

THE EYELIDS AND CONJUNCTIVA.

The eyelids (palpebræ) are moveable folds of integument, strengthened toward their margins by a thin lamina of cartilage. The mucous membrane, which lines their inner surface, and which is reflected thence in the form of a pellucid covering on the surface of the eyeball, is named membrana conjunctiva.

The upper lid is larger and more moveable than the lower the transparent part of the globe is covered by it when the eye is closed; and the eye is opened chiefly by the elevation of this lid by a muscle (levator palpebræ) devoted exclusively to this purpose. The eyelids are joined at the outer and inner angles (canthi) of the eye. The interval between the angles, fissura palpebrarum, varies in length in different persons, and, according to its extent, the size of the globe being nearly the same, gives the appearance of a larger or a smaller eye. The greater part of the edge of each eyelid is flattened, but towards the inner angle it is rounded off for a short space, at the same time that it somewhat changes its direction; and, where the two differently formed parts join, there exists on each lid a slight conical elevation—papilla lachrymalis, the apex of which is pierced by the aperture or punctum of the corresponding lachrymal canalicule.

In the greater part of their extent the lids are applied to the surface of the eyeball; but at the inner canthus, opposite the puncta lachrymalia, there intervenes a vertical fold of conjunctiva, the plica semilunaris, resting on the eyeball; while, occupying the recess of the angle internal to the border of this fold, is a spongy-looking reddish elevation, formed by a group

of glandular follicles, and named the caruncula lachrymalis. The plica semilunaris is the rudiment of the third eyelid (membrana nictitans) found in some animals.

Structure of the lids.-The skin covering the eyelids is thin and delicate; and at the line of the eyelashes, altered in its character, it joins the conjunctival mucous membrane which lines the inner surface of the lids. Beneath the skin, and between it and the conjunctiva, the following structures are successively met with, viz. :-The fibres of the orbicularis muscle; loose connective tissue; the tarsal cartilages, together with a thin fibrous membrane, the palpebral ligament, which attaches them to the margin of the orbit; and, finally, the Meibomian glands. In the upper eyelid there is, in addition, the insertion of the levator palpebræ superioris, in the form of a fibrous expansion fixed to the anterior surface of the tarsal cartilage.

Fig. 456.-VERTICAL SECTION OF THE LEFT ORBIT AND ITS CONTENTS.

The section has been carried through the middle of the optic foramen and optic nerve obliquely as far as the back of the eyeball, and thence forward through the eyeball, eyelids, &c., in an antero-posterior direction. a, the frontal bone; b, the superior maxillary bone; c, the eyebrow with the orbicularis palpebrarum, integument, &c., divided; d, the upper, and d', the lower eyelid, partially open, showing the section of the tarsal cartilages and other component parts, the eyelashes, &c. ; e, e, the reflection of the conjunctiva from the upper and lower eyelids to the surface of the eyeball; f, the levator palpebræ superioris muscle; g, the upper, g', the lower rectus muscle; h, the inferior oblique muscle divided; 1, 1, the optic nerve divided in its sheath; 2, the cornea; 2', the sclerotic; 3, the aqueous chamber; 4, the crystalline lens; 5, the centre of the vitreous humour.

The fibres of the orbicularis muscle are closely adherent to the skin by fine connective tissue, entirely devoid of fat. A marginal fasciculus of its fibres has been found within the line of the eyelashes, separated by the bulbs of the lashes from the other fibres, and constituting the ciliary muscle of Riolan. The fibres of the orbicular muscle, while adherent to the skin, glide loosely on the tarsal cartilages.

The tarsal cartilages (tarsi) are two thin elongated plates of cartilage of the yellow kind placed one in each lid, and serving to give shape and firmness to those parts. The upper cartilage, the larger, is half oval in form,

THE EYELIDS AND CONJUNCTIVA.

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being broader near the centre and narrowing towards the angles of the lids. The lower is thinner, much narrower, and more nearly of a uniform breadth throughout. The free or ciliary edge of the cartilages, which is straight, is thicker than any other part. At the inner canthus the cartilages are fixed by the fibrous slips of the tendo palpebrarum (p. 172); and at the outer angle they are attached to the malar bone by a fibrous band belonging to the palpebral ligament, and named the external tarsal ligament.

The palpebral ligament is a fibrous membrane placed beneath the orbicularis muscle, attached peripherally to the margin of the orbit, and internally to the tarsal cartilages near the inner free edge. The membrane is thickest at the outer part of the orbit.

Meibomian glands.-On the ocular surface of each lid are seen from twenty to thirty parallel vertical lines of yellow granules, lying immediately under the conjunctival mucous membrane. They are compound sebaceous follicles,

Fig. 457.-MEIBOMIAN GLANDS,
LACHRYMAL GLAND, &C., AS SEEN
FROM THE DEEP SURFACE OF THE
EYELIDS OF THE LEFT SIDE.

a, palpebral conjunctiva; 1, lachrymal gland; 2, openings of seven or eight glandular ducts; 3, upper and lower puncta lachrymalia; 6, 6, shut ends of the upper and lower Meibomian glands, of which the openings are indicated along the margins of the eyelids.

embedded in grooves at the back of the tarsal cartilages; and they open on the free margin of the lids by minute orifices, generally as many in number as the lines of follicles

themselves. These glands consist of nearly straight excretory tubes, each of which is closed at the end, and has numerous small cæcal appendages projecting from its sides. The tubes are lined by mucous membrane, on the surface of which is a layer of scaly or pavement epithelium cells.

According to Heinrich Müller there is likewise a layer of unstriped muscular fibre contained in each eyelid; that of the upper lid arising from the under surface of the levator palpebræ, that of the lower lid arising from the neighbourhood of the inferior oblique muscle, and each being inserted near the margin of the tarsal cartilage. It may also be mentioned in this place that the same writer describes a layer of unstriped muscle crossing the spheno-maxillary fissure, corresponding to a more largely developed layer found in the extensive aponeurotic part of the orbital wall of various mammalia. This set of fibres has been more particularly described by Turner (H. Müller, in Zeitschr. f. Wiss. Zool. 1858, p. 541; W. Turner, in Nat. Hist. Rev. 1862, p. 106).

The eyelashes (cilia) are strong short curved hairs, arranged in two or more rows along the margin of the lids, at the line of union between the skin and the conjunctival mucous membrane. The lashes of the upper lid, more numerous and longer than the lower, have the convexity of their curve directed downwards and forwards; whilst those of the lower lid are arched in the opposite direction. Near the inner canthus these hairs are weaker and more scattered.

Structure of the conjunctiva.—The conjunctiva consists of the palpebral part, along with which may be grouped the plica semilunaris and caruncula lachrymalis, and of the ocular part or conjunctiva bulbi, in which may be distinguished the sclerotic and corneal portions: each of these several parts presents peculiar and distinctive characters. The epithelium is stratified and thick; the cells of the superficial strata scaly, delicate, and each with a distinct nucleus.

The palpebral portion of the conjunctiva is opaque and red, is thicker and more vascular than any other part of the membrane, and presents numerous fine papillæ freely supplied with nerves. At the margins of the lids the palpebral conjunctiva enters the ducts of the Meibomian glands; through the puncta lachrymalia it passes into the canaliculi, and is continuous with the lining membrane of the lachrymal sac; and it is prolonged into the orifices of the ducts of the lachrymal gland.

The sclerotic portion of the conjunctiva, changing its character at the line of reflection from the eyelids, becomes thinner, and loses its papillary structure it is loosely connected to the eyeball by submucous tissue. It is also transparent and nearly colourless, but a few scattered branches of bloodvessels are generally visible on it in the healthy condition, and under the influence of inflammatory congestion a copious network of vessels very irregularly disposed comes into view. This network is derived from the palpebral and lachrymal arteries. It may be easily made to glide loosely on the surface of the eyeball by pressing the eyelid against it. But another set of vessels likewise exists on the surface of the sclerotic, and may be brought into view by congestion. The position of this set is entirely sub-conjunctival, adherent to the sclerotic coat; they are less tortuous than the conjunc tival set, and are derived from the muscular and anterior ciliary branches of the ophthalmic artery: they remain immoveable on pressure of the eyelid. They dip into the sclerotic near the cornea, and appear to unite with a more deeply connected minute network disposed in closely set straight lines, radiating from the margin of the cornea, and the gorged condition of which is well known to ophthalmic surgeons as characteristic of sclerotitis.

The corneal conjunctiva consists almost entirely of epithelium, any underlying membrane being extremely thin, transparent, and adherent to the anterior elastic layer of the cornea, in connection with which it will be again referred to. Vessels lie between it and the cornea, and form a circle of anastomotic capillary loops around the circumference. This plexus of vessels extends farther inwards in the foetus.

A well developed network of lymphatics exists throughout the sclerotic and palpebral portions of the conjunctiva; but at the margin of the cornea a sudden diminution takes place in the size of the meshes and diameter of the vessels. Of the network referred to, only a narrow circle th of an inch in diameter exists on the corneal conjunctiva, and this circle has a well defined inner margin within which no lymphatics exist (Teichmann).

The nerves in the membrane, as far as the cornea, seem to have the same arrangement as in the skin in general.

In the submucous tissue of the eyelids there are small follicular glands spread over the whole surface of the conjunctiva palpebrarum, and in the vicinity of the reflection of the conjunctiva upon the eyeball a set of larger more complex glands of a racemose structure, somewhat similar to that of the lachrymal gland (Sappey, C. and W. Krause).

Closed follicles have also been observed in the conjunctiva by Bruch, and, after him, by other observers.

LACHRYMAL GLAND AND CANALS.

THE LACHRYMAL APPARATUS.

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The parts which constitute the lachrymal apparatus are the following, viz:-The gland by which the tears are secreted, situated at the upper and outer side of the orbit, together with its excretory ducts; the two canals into which the fluid is received near the inner angle; and the sac with the nasal duct continued from it, through which the tears pass into the inferior meatus of the nose.

The lachrymal gland, an oblong flattened body, about the size of a small almond, is placed in the upper and outer part of the orbit, a little behind the anterior margin. The upper surface of the gland, convex, is lodged in a slight depression in the orbital plate of the frontal bone, to the periosteum of which it adheres by fibrous bands; the lower surface is adapted to the convexity of the eyeball, and is in contact with the upper and the outer recti muscles. The fore part of the gland, separated from the rest by a slight depression, and sometimes described as a second lobe, or as a distinct gland, is closely adherent to the back of the upper eyelid, and is covered on the ocular surface only by a reflection of the conjunctiva. The glandular ducts, usually from six to eight in number, are very small, and emerge from the thinner portion of the gland. After running obliquely under the mucous membrane, and separating at the same time from each other, they open in a row by separate orifices, the greater number in the fold above the outer canthus, and two of them (Hyrtl) in the fold below.

at these apertures commence two small canals, canaliculi, which convey the tears from the eye to the lachrymal sac. The upper canal is rather the smaller and longer of the two: it first ascends from the punctum; then makes a sudden bend, and is directed in wards and downwards to join the lachrymal sac. The lower canal descends from the corresponding punctum; and soon changing its direction like the upper one, takes a nearly horizontal course inwards. Both canals are dilated where they are bent. In some cases they unite near the end to form a short common trunk; more commonly they open separately, but close together, into the sac.

The lachrymal sac and nasal duct constitute together the passage by which the tears are conveyed from the lachrymal canals to the cavity of the nose.