The anterior or inner ends of both rods and cones are continuous with the rod and cone fibres of the external granule layer, as already described. Each rod and cone is subdivided into an outer strongly refractile and an inner feebly refractile segment. By the action of various reagents the outer segments both of the rods and cones exhibit a transverse striation, and ultimately break up into discs. Hensen has described a longitudinal striation in the outer segments, and Ritter has stated that both in the outer and inner segments of the rods an axial fibre exists. Max Schultze has also seen the inner segments of both rods and cones longitudinally striped on the surface. Modifications in the relative numbers and appearances of the rods and cones have been seen in the eyes of various vertebrata. In birds, for example, the cones are much more numerous than the rods, whilst the reverse is the case in mammals generally. In the cartilaginous fishes the cones are entirely absent; so also, as Schultze has shown, in the bat, hedge-hog, and mole; whilst in reptiles the bacillary layer is exclusively composed of cones. In all the vertebrata, except the mammalia, the twin or double cones described by Hannover probably exist. In the amphibia, lens-shaped bodies have been described in the inner segments of the cones. The rods and cones are the peripheral end-organs in connection with the fibres of the optic nerve, and their apparent relation to these fibres is as follows:-The optic nerve fibres are continuous with the central processes of the ganglion cells of the retina, the peripheral branching processes of which pass into the internal granulated layer, where they may possibly become continuous with the central processes of the inner granular layer. The peripheral processes of the inner granular layer enter the external granulated layer, but it is difficult to say whether or not they become continuous with the central processes of that layer. There can, however, be no doubt that the peripheral processes of this layer are directly continuous with the rods and cones of the bacillary layer. The entire arrangement is sometimes called the radial nervous fibres of the retina.

In addition to the nervous structures just described, the retina contains a delicate supporting connective tissue like the neuroglia of the brain and spinal cord. Not only does it lie between the fibres, cells, and so-called granules in the several nervous layers, and form in them a radial arrangement of supporting fibres, but it constitutes the two limitary membranes of the retina. The membrana limitans externa (8) is excessively thin, and appears in vertical sections through the retina as a mere line between the bacillary and external granular layers, continuous on the one hand with the connective tissue which passes for a short distance between the rods and cones, and on the other with the connective tissue framework of the external granule layer.

The membrana limitans interna (1) covers the anterior surface of the retina, and lies next the vitreous body; its posterior surface blends with the radial arrangement of connective tissue between the optic nerve fibres, but its anterior or hyaloid surface, as J. C. Ewart has recently shown, possesses a mosaic appearance, like that of a layer of squamous endothelium.

The yellow spot exhibits some structural differences from the rest of the retina. It owes its colour to the presence of yellow pigment deposited in the more anterior layers of the retina. Except at its central depression, the fovea centralis, it is thicker than the surrounding parts of the retina; but it is much softer, a condition which is due to the almost complete absence of the layer of optic nerve fibres, and a diminution in the amount of the supporting connective tissue; the membrana limitans interna is, however, relatively stronger. In the fovea centralis itself the rods of the bacillary layer have entirely disappeared, and are replaced by cones which are distinguished by their close

arrangement, and the more slender form and increased length, especially of their outer segments. The external granule layer is well marked, and the central fibres belonging to it, instead of passing vertically forwards, incline very obliquely or almost horizontally outwards to the internal granule layer, which, together with the layers anterior to it, is so thin as almost to have disappeared. In the yellow spot surrounding the fovea the bacillary layer is also composed of cones which are not, however, so slender or so long as at the fovea itself. The layer of nerve cells and the inner part of the external granule layer are thicker than in the rest of the retina. The yellow spot is the part of the retina most sensitive to light.

At the ora serrata or anterior border of the retina the nervous layers, including the rods and cones, cease to exist. The radial connective tissue and internal limiting membrana are present; from the radial tissue a layer of cells is prolonged forward in contact with the deep surface of the ciliary processes as the pars ciliaris retinæ. The retina is supplied with blood by the arteria centralis, which, traversing the axis of the optic nerve, reaches the retina at the optic disc. In the retina it branches dichotomously in the nerve fibre layer, avoiding however the yellow spot, and its terminal twigs reach the ora serrata. The capillaries form in the more anterior layers of the retina a distinct network, which does not enter the external granule and bacillary layers, but penetrates the yellow spot, though not the fovea centralis. The blood is conveyed from the retina by the central vein which accompanies the artery in the optic nerve, and opens either into the ophthalmic vein or directly into the cavernous sinus. The veins and capillaries of the retina have been described by His as completely invested by perivascular lymphatic sheaths, whilst the arteries only possess such sheaths for a limited part of their course.

The Optic Nerve itself passes from the orbit through the Nerve of optic foramen into the cranial cavity, where it arises from sight. the optic commissure. This commissure is a flattened band formed by the junction of the two optic tracts. Each tract winds backwards around the tuber cinereum and crus cerebri to arise from the optic thalamus, corpora quadrigemina, and geniculata; and some observers also state that it derives fibres from the tuber cinereum and lamina cinerea. In the commissure an interchange takes place between the fibres of opposite nerves and tracts, so that not only does an optic nerve contain fibres derived from the tract on its own side, but from the opposite tract, and it has even been stated that fibres pass across the commissure from one optic nerve to the other, and from one optic tract to the other. The Aqueous Humour is a limpid watery fluid, containing Refracting a little common salt in solution, which occupies the media. between the cornea and the front of the crystalline lens. In this space the iris lies, and imperfectly divides it into two chambers, an anterior and a posterior, which communicate with each other through the pupil. The anterior chamber, of some size, is situated between the iris and cornea; but as the iris is in contact with the front of the lens, the posterior chamber is reduced to a mere chink between the circumference of the iris and that of the lens.

space

The Crystalline Lens is situated behind the iris and pupil, and in front of the vitreous body. It is a transparent bi-convex lens, with its antero-posterior diameter d less than the transverse, its posterior surface more convex than the anterior, and with its circumference rounded. It consists of a capsule and the body of the lens enclosed by the capsule. The lens capsule is a transparent, smooth, structureless, and very elastic membrane, about twice as thick on the anterior as on the posterior surface of the lens. It is non-vascular in the adult, though in the foetus a branch of the central artery of the retina which traverses the

I. — 112

vitreous humour, ramifies in its posterior portion. A single layer of polygonal cells lies between the body of the lens and the anterior portion of the capsule. The lens body is softer at its periphery than in its centre. It is built up of concentric layers, and on both the anterior and posterior surfaces lines are to be seen radiating from the central pole of each surface towards the circumference of the body. The radiated pattern varies in different animals. In the human fœtus there are usually three lines, but in the adult they are more numerous. The lines on one surface do not lie immediately opposite those on the other, but are intermediate. By the action of strong spirit and other reagents the body of the lens can be split up from the periphery towards the centre in the direction of these lines, so that they mark the edges of apposition of its concentric laminæ. Each lamina consists of numerous hexagonal fibres about 8th inch wide, which extend from one surface to the other over the circumference of the lens, so that a fibre which begins at the polar end of a radius on the one surface terminates at the circumferential end of a radius on the opposite. The edges of the fibres are sinuous in man, but denticulated in many animals, especially fishes, so that the fibres, not only in the same, but in superimposed layers, are closely interlocked. The lens fibres are nucleated, a structural fact which gives a clue to their true nature, and they are now regarded as peculiarly modified elongated cells. Babuchin states that he can trace the transition from

the cells of the layer between the lens-body and capsule to the proper lens fibres. The lens-body is non-vascular and non-nervous. The surfaces of the lens become more flattened in old age, and its substance hardens and is less transparent.

The Vitreous Body is much the largest of the refracting media, and occupies the largest part of the space enclosed by the tunics. Anteriorly it is hollowed out to receive the posterior convexity of the lens, but posteriorly it is convex, and the retina is moulded on it. It is as translucent as glass, jelly-like in consistency, and when punctured a watery fluid drains out. Its minute structure is difficult to ascertain, but as it, like the subcutaneous tissue of the embryo, contains rounded, stellate, and fusiform cells, it is customary to refer it to the gelatinous form of connective tissue; concentric lamellæ, and even a radiated arrangement of fibres, have also been described. It has been customary also to consider it as invested by a delicate structureless membrane, the hyaloid membrane; but this is now regarded as belonging to the retina, where it is known as the membrana limitans interna. Almost opposite the ora serrata a membrane springs from the vitreous body, passes forwards for some distance in relation to the deep surface of the ciliary processes, but separated from them by the pars ciliaris retina, and then inclines inwards to become attached to the anterior surface of the capsule of the lens close to its circumference. It is so closely connected at its origin with the membrana limitans that it is difficult to recognise it as a distinct membrane. It is named the suspensory ligament of the lens, or zonule of Zinn, and contains fibres, which run in the meridional direction. Where it leaves the vitreous body a narrow space is enclosed between it and that body, which space surrounds the circumference of the lens, and is called the canal of Petit. From the relation of the suspensory ligament to the ciliary processes it has a plicated surface, and when these processes are torn away from it a portion of the pigment of the processes is often left behind, so that the zonule is sometimes named the ciliary processes of the vitreous body.

The Eyeball is an optical instrument, constructed on the plan of the camera obscura. The sclerotic forms the wall of the chamber. The choroid represents the black lining for absorbing the surplus rays of light. The cornea,

aqueous humour, lens, and vitreous body are the translucent media which, like the glass lens of the camera obscura, bring the rays of light to a focus. The retina is the sensitive plate on which the optical picture is thrown. In considering the relation of the retina to the visual rays, it must be kept in mind that the place of entrance of the optic nerve is insensible to light, and that the most sensitive part of the retina is the yellow spot, with its fovea centralis, where the optic nerve fibres are absent, but where the bacillary layer reaches its maximum size. It is clear, therefore, that the rods and cones of this layer, and not the optic nerve fibres, are the structures in the retina which are stimulated by the light; and it is probable, as was suggested many years ago by Goodsir, that these rods and cones are impressed by the light, not as it enters the eye directly, but as it is reflected backwards from the choroid along their axes. The iris is the diaphragm which, by opening or closing the pupil, admits or cuts off the rays of light. The ciliary muscle represents the adjusting screw of the camera; through its attachment to the ciliary processes and their relation to the suspensory ligament of the lens, it is able to act upon the lens and modify the curvature of its anterior surface; for when the eye is to be accommodated to the vision of near objects the anterior surface of the lens becomes more convex than when distant objects are being examined.

eye,

It has already been stated on p. 864 that the retina is Developformed in the primary optic vesicle, which grows forwards ment of to the integument. By the involution and growth of the skin at this spot a hollow is produced at the front of the vesicle, which gradually deepening forms a pouch, the secondary optic vesicle, in which the involuted part of the skin is lodged. From the included sub-epidermal tissue the vitreous body is derived; from the included epidermis, the lens; whilst the cornea sclerotic and iris are produced by the subcutaneous connective tissue. The optic nerve and retina are formed from the primary optic vesicle and its peduncle, and it is probable that the bacillary layer is a special development of its internal epithelial lining. The choroid coat again is derived from the pia mater. the eyeball is compounded of structures derived partly from the integument and partly from the embryo brain.

Hence

ACCESSORY PARTS TO THE EYEBALL.-In relation to the eyeball several accessory parts are found.

The Eye-Brows are projections of the integument, from which short, stiff hairs grow.

The Eye-Lids, or palpebræ, are two movable curtains, Eyelids. an upper and a lower, which protect the front of the globe. Between each pair of lids is a horizontal fissure, the palpebral fissure. From the free margins of the two lids project short hairs, the eye-lashes or cilia; the upper set curve downwards and forwards, the lower set upwards and forwards; they also protect the front of the globe. Each eye-lid consists of skin; of the fibres of the orbicular sphincter muscle; of a thin plate of fibro-cartilage, the tarsal cartilage, to the inner end of which a fibrous band, the tendo palpebrarum, is attached, this tendon springing from the ascending process of the superior maxilla; and of the conjunctiva. Between the conjunctiva and the tarsal cartilage is a layer of glands, the Meibomian glands; each gland consists of a short duct, which expands at its sides into small sacculi. The sacculi contain short columnar cells; these secrete a sebaceous material, which escapes through the orifice of the duct at the border of the eye-lid.

The Conjunctiva is a mucous membrane, which forms the Conposterior layer of the eye-lid, and is reflected on to the junctiva. anterior part of the sclerotic. At the inner angle of junction of the eye-lids is a soft reddish elevation of the conjunctiva, the caruncula lachrymalis, and immediately external to it is a vertical fold, the plica semilunaris, the

rudiment of the third eye-lid, or membrana nictitans, so well developed in birds. The palpebral conjunctiva has small papillæ scattered over its surface; its epithelium is stratified, with scaly cells on the free surface and elongated cells in the deepest layer. In the sub-epithelial tissue are small branched mucous glands, which are numerous in the caruncula. Little masses of adenoid tissue (p. 849) with lymphatic vessels are also found in it, and the conjunctiva of the front of the eyeball is thinner than the palpebral part. It is not glandular, and its nerves terminate in end-bulbs (p. 862). The palpebral conjunctiva, and in part that of the eyeball, receive their blood-vessels from those of the eye-lids, but the portion of the conjunctiva next the cornea is supplied by the arteries of the sclerotic coat.

achrymal The Lachrymal Apparatus is engaged in the secretion paratus. of the tears, and in conveying them away from the front of the globe. The lachrymal gland occupies a depression in the outer part of the roof of the orbit. It is smaller than an almond, is sub-divided into lobules, and belongs to the group of compound racemose glands. It consists of the ramifications of short ducts, which terminate in small sacculi. The wall of each sacculus consists of a delicate membrana propria, and the cavity contains the polyhedral secreting cells. Outside the membrana propria is a capillary network derived from the lachrymal artery, but Giannuzzi and Boll have recently described a space between this network and the membrana propria which they believe to be continuous with the lymphatic system. Pflüger has described nerves as terminating in connection with the secreting cells. The excretory ducts of the gland are from six to eight, and open on the back of the upper eye-lid. The tears are washed over the surface of the globe by the involuntary winking of this lid. When the secretion is increased in quantity, in the act of crying, the tears flow over the cheek, but in ordinary circumstances they are conveyed away by two slender tubes, the lachrymal canals, which open by minute orifices, the puncta lachrymalia, one at the

Muscles of yeball.

inner end of the FIG. 79.-Lachrymal canals and duct. 1, orbicular muscle; free border of each 2, lachrymal canal; 8, punctum; 4, caruncula; 5, lachrymal sac; 6, lachrymal duct; 7, angular artery. eye-lid. These

tubes open at their opposite ends into a small reservoir, the lachrymal sac, situated in a hollow in the lachrymal bone. From this sac a duct, the nasal or lachrymal duct, proceeds which opens into the inferior meatus of the nose, and here the tears mingle with the mucous secretion of that cavity.

Muscles of the Eyeball.-The sclerotic coat of the eyeball has six muscles inserted into it. Four of the muscles are called recti, and are situated, one superior, one inferior, one external to, another internal to, the globe. They all arise from the rim of bone which bounds the optic foramen; the external and internal muscles are inserted vertically into the sides of the sclerotic, but the superior and inferior recti have oblique insertions into its upper and lower aspects. The other two muscles are called obliqui. The superior oblique arises along with the recti, passes to the inner end of the upper border of the orbit, where its tendon goes through a pulley, and is directed back to be inserted

obliquely into the upper and outer part of the sclerotic. The inferior oblique arises from the lower border of the orbit, passes outwards and upwards to be inserted obliquely into the sclerotic. These muscles roll the eyeball in the orbit, and, without entering into a minute analysis of their actions, their office may be stated generally as follows:The internal rectus rolls it inwards, the external outwards, about its vertical axis; the superior rectus rolls it upwards, the inferior downwards, about its transverse horizontal axis, though from the obliquity of their insertions they give it at the same time a slight inward or outward movement as the case may be; the superior and inferior oblique roll the globe around its antero-posterior or sagittal axis, the superior upwards and outwards, the inferior downwards and outwards.

Periosteal Muscle of the Orbit.-The periosteum of the orbit contains, as H. Müller and Turner have described, a layer of non-striped muscular fibre in the part which covers over the spheno-maxillary fissure. In man it is rudimentary, but in the sheep, deer, elephant, &c., where the osseous wall of the orbit is deficient, this muscle forms a well-defined structure. It has been suggested that it acts as a protractor muscle of the globe.

The EAR, or organ of hearing, is a complex acoustic ap- Ear. paratus, situated in connection with the temporal bone. It is divided into three parts, named external, middle, and internal ear.

The External Ear consists of the pinna or auricle and External the external auditory meatus. The auricle is the oblong ear. convoluted body situated at the side of the head. Its incurved outer border is named the helix. Within this lies a curved ridge, the anti-helix, in front of which is a deep hollow, the concha, which leads into the external meatus. The concha is bounded in front by a prominence, the tragus, and behind by a smaller prominence, the anti-tragus; below the anti-tragus is the lobule, which forms the most depending part of the auricle. The framework of the auricle is formed of yellow elastic fibro-cartilage invested by integument, except the lobule, which consists merely of a fold of integument containing fat. Attached to the cartilage are not only the three auricular muscles referred to on

k

e

FIG. 80.-The ear as seen in section. a, helix; b, anti-tragus; c, anti-helix; & concha; e, lobule; f, mastoid process; g, portio dura; h, styloid process; k internal carotid artery; 1, Eustachian tube; m, tip of petrous process; n, external auditory meatus; o, membrani tympani; p, tympanum; 1, points to malleus; 2, to incus; 3, to stapes; 4, to cochlea; 5, 6, 7, the three semicircular canals; 8 and 9, portio dura and portio mollis.

page 836, but also certain smaller muscles called the proper muscles of the pinna. Thus the greater muscle of the helix is placed on its anterior border; the lesser muscle of the

The Internal Ear, named the Labyrinth, from its complex construction, is the part of the auditory apparatus in which the nerve of hearing is distributed, and where the peripheral end-organs are situated. It is imbedded in the petrous bone, and is divided into three parts, viz., vestibule, semicircular canals, and cochlea, each of which consists of an osseous and a membranous portion (Pl. XIX. figs. 8, 9, 10). The Vestibule lies immediately internal to the tympanum, between it and the bottom of the internal auditory meatus; behind it are the semicircular canals, and in front is situated the cochlea. It is the part of the labyrinth which first appears in animals, and is therefore the most constant part of the organ. In the myxinoid fishes a single semicircular canal is superadded to the vestibule, in the lamprey two canals, but in other fishes and in the higher vertebrates three canals exist. In amphibia, reptiles, and birds the cochlea is small and rudimentary in comparison with its development in mammals. The osseous vestibule is an ovoid chamber about th inch in diameter. In its outer or tympanic wall is the fenestra ovalis; in its inner are small auditory foramina, which transmit from the internal meatus the vestibular branches of the auditory nerve; behind these holes is the opening of a minute canal, the aqueductus vestibuli; its anterior wall communicates with the scala vestibuli of the cochlea, and into its posterior wall open the five orifices of the three semicircular canals. The Semicircular Canals are named superior, posterior, and external. The superior and posterior are sometimes called the vertical canals, and the external the horizontal canal, but, as Crum Brown has shown, the superior and posterior lie in planes equally inclined to the mesial plane of the head, and the external is in a plane at right angles to the mesial plane. Further, the canals in the two ears have definite relations to each other; for whilst the superior canal of each ear is nearly parallel to the posterior canal of the other, the external canals in both ears lie nearly in the same plane. The canals are bent, forming nearly ds of a circle, and would have had six openings into the vestibule had not the contiguous ends of the superior and posterior blended together to open by a common orifice. The opposite end of each of these canals and the outer end of the external canal dilate close to the vestibule to twice their usual diameter, and form an ampulla. The osseous vestibule and semicircular canals are lined by a periosteum invested by a tessellated endothelium, and contain a little fluid, the perilymph. In this fluid the membranous labyrinth is suspended.

The membranous vestibule is formed of two small sac-like

con

οι

each other, though the cavities are separated by an intermediate partition. The upper and posterior dilatation, named utriculus, is larger than the lower and anterior, named sacculus. The sacculus is continuous with the ductus cochlearis of the membranous cochlea, and both sacculus and utriculus communicate by slender tubes with a short diverticulum lodged in the aqueductus vestibuli, to which the name of ductus vestibuli may be given. The membranous semicircular canals are about d the diameter of the osseous. Their walls are continuous with that of the utriculus, and they open by five orifices into it. Each has an ampulla within the ampulla of the osseous canal. Both the sacculus and utriculus are in places attached to the periosteal linings of the osseous vestibule, and delicate ligamentous bands connect the membranous semicircular canals to the periosteal lining of the tubes in which they are tained. The wall of the membranous vestibule and canals consists of a delicate fibrous membrane lined by a tessellated endothelium. The inner part of this membrane has a vitreous or hyaline lustre, and gives origin in the canals to short papilla which project into the lumen. The membranous vestibule and canals are distended with the fluid endolymph. The sacculus, utriculus, and ampullæ are specially modified in connection with the peripheral termination of the vestibular branches of the auditory nerve. The membranous wall forms in each of these dilatations a projecting ridge, the crista acoustica, to which calcareous particles, the otoliths, which may be either amorphous or crystalline, are adherent. The endothelial investment of the crista is elongated into columnar cells, and intercalated between them are fusiform cells, the auditory cells, each of which, as Schultze and other observers have described, possesses a peripheral and a central process. The peripheral process projects beyond the plane of the free surface of the endothelium into the endolymph as the auditory hair, whilst the central process extends into the sub-endothelial tissue, where the nerve plexus belonging to the terminal branches of the auditory nerve ramifies, and with which it is probably continuous. These auditory cells are, therefore, the peripheral end-organs of the vestibular branches of the auditory nerve, and their general arrangement is not unlike that of the olfactory cells of the nose.

DC

n

FIG. 81. cl, columnar cells covering the crista acoustica; P, peripheral, and c, central processes of auditory cells, n, nerve fibres. (After Rüdinger.)

The Cochlea is by far the most complex part of the labyrinth. It is about 1th inch long, and shaped like the shell of a common snail; its base lies near the internal meatus, and its apex is directed outwards. The osse

dr

dv

ous cochlea is a tube wound spirally two FIG. 82.-Diagram of the membranous labyrinth. DC, ductus cochlearis; dr, ductus reuniens; S, and a half times round succulus; U, utriculus; dv, ductus vestibuli;

a central pillar or SC, semicircular canals. (After Waldeyer.) modiolus. Both the pillar and the tube diminish rapidly in diameter from the base to the apex of the cochlea. The tube is imperfectly divided into two passages by a