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The anterior or inner ends of both rods and cones are con arrangement, and the more slender form and increased tinuous with the rod and cone fibres of the external granule length, especially of their outer segments. The external layer, as already described. Each rod and cone is sub- granule layer is well marked, and the central fibres belongdivided into an outer strongly refractile and an inner feebly ing to it, instead of passing vertically forwards, incline very refractile segment. By the action of various reagents the obliquely or almost horizontally outwards to the internal outer segments both of the rods and cones exhibit a trans- granule layer, which, together with the layers anterior to it, verse striation, and ultimately break up into discs. Hensen is so thin as almost to have disappeared. In the yellow spot has described a longitudinal striation in the outer segments, surrounding the fovea the bacillary layer is also composed and Ritter has stated that both in the outer and inner of cones which are not, however, so slender or so long as segments of the rods an axial fibre exists. Max Schultze at the fovea itself. The layer of nerve cells and the inner has also seen the inner segments of both rods and cones part of the external granule layer are thicker than in the longitudinally striped on the surface. Modifications in the rest of the retina. The yellow spot is the part of the relative numbers and appearances of the rods and cones retina most sensitive to light. have been seen in the eyes of various vertebrata. In birds, At the ora serrata or anterior border of the retina the for example, the cones are much more numerous than the nervous layers, including the rods and cones, cease to rods, whilst the reverse is the case in mammals generally. exist. The radial connective tissue and internal limiting In the cartilaginous fishes the cones are entirely absent; so membrana are present; from the radial tissue a layer of also, as Schultze has shown, in the bat, hedge-hog, and cells is prolonged forward in contact with the deep surface mole ; whilst in reptiles the bacillary layer is exclusively of the ciliary processes as the pars ciliaris retinæ. composed of cones. In all the vertebrata, except the The retina is supplied with blood by the arteria centralis, mammalia, the twin or double cones described by Hannover which, traversing the axis of the optic nerve, reaches the probably exist. In the amphibia, lens-shaped bodies have retina at the optic disc. In the retina it branches dichobeen described in the inner segments of the cones. The tomously in the nerve fibre layer, avoiding however the rods and cones are the peripheral end-organs in connection yellow spot, and its terminal twigs reach the ora serrata. with the fibres of the optic nerve, and their apparent rela- The capillaries form in the more anterior layers of the tion to these fibres is as follows :The optic nerve fibres retina a distinct network, which does not enter the external are continuous with the central processes of the ganglion granule and bacillary layers, but penetrates the yellow cells of the retina, the peripheral branching processes of spot, though not the fovea centralis. The blood is conveyed which pass into the internal granulated layer, where they from the retina by the central vein which accompanies the may possibly become continuous with the central processes artery in the optic nerve, and opens either into the of the inner granular layer. The peripheral processes of ophthalmic vein or directly into the cavernous sinus. the inner granular layer enter the external granulated layer, veins and capillaries of the retina have been described by but it is difficult to say whether or not they become con His as completely invested by perivascular lymphatic tinuous with the central processes of that layer. There sheaths, whilst the arteries only possess such sheaths for a can, however, be no doubt that the peripheral processes of limited part of their course. this layer are directly continuous with the rods and cones The Optic Nerve itself passes from the orbit through the Nerve of of the bacillary layer. The entire arrangement is sometimes optic foramen into the cranial cavity, where it arises from sight. called the radial nervous fibres of the retina.
the optic commissure. This commissure is a flattened band In addition to the nervous structures just described, the formed by the junction of the two optic tracts. Each tract retina contains a delicate supporting connective tissue like winds backwards around the tuber cinereum and crus the neuroglia of the brain and spinal cord. Not only does cerebri to arise from the optic thalamus, corpora quadrigeit lie between the fibres, cells, and so-called granules in the mina, and geniculata ; and some observers also state that several nervous layers, and form in them a radial arrange- it derives fibres from the tuber cinereum and lamina cinerea. ment of supporting fibres, but it constitutes the two limitary In the commissure an interchange takes place between the membranes of the retina. The membrana limitans externa fibres of opposite nerves and tracts, so that not only does (8) is excessively thin, and appears in vertical sections an optic nerve contain fibres derived from the tract on its through the retina as a mere line between the bacillary and own side, but from the opposite tract, and it has even been external granular layers, continuous on the one hand with stated that fibres pass across the commissure from one optic the connective tissue which passes for a short distance be nerve to the other, and from one optic tract to the other. tween the rods and cones, and on the other with the connec The Aqueous Humour is a limpid watery fluid, containing Refracting tive tissue framework of the external granule layer. a little common salt in solution, which occupies the space
media, The membrana limitans interna (1) covers the anterior between the cornea and the front of the crystalline lens, surface of the retina, and lies next the vitreous body ; its In this space the iris lies, and imperfectly divides it into posterior surface blends with the radial arrangement of two chambers, an anterior and a posterior, which commuconnective tissue between the optic nerve fibres, but its nicate with each other through the pupil. The anterior anterior or hyaloid surface, as J. C. Ewart has recently chamber, of some size, is situated between the iris and shown, possesses a mosaic appearance, like that of a layer cornea ; but as the iris is in contact with the front of the of squamous endothelium.
lens, the posterior chamber is reduced to a mere chink The yellow spot exhibits some structural differences from between the circumference of the iris and that of the lens. the rest of the retina. It owes its colour to the presence The Crystalline Lens is situated behind the iris and of yellow pigment deposited in the more anterior layers of pupil, and in front of the vitreous body. It is a transthe retina. Except at its central depression, the fovea parent bi-convex lens, with its antero-posterior diameter fd centralis, it is thicker than the surrounding parts of the less than the transverse, its posterior surface more convex retina ; but it is much softer, a condition which is due to than the anterior, and with its circumference rounded. It the almost complete absence of the layer of optic nerve consists of a capsule and the body of the lens enclosed by the fibres, and a diminution in the amount of the supporting con- capsule. The lens capsule is a transparent, smooth, strucnective tissue; the membrana limitans interna is, however, tureless, and very elastic membrane, about twice as thick relatively stronger. In the fovea centralis itself the rods of on the anterior as on the posterior surface of the lens. It the bacillary layer have entirely disappeared, and are is non-vascular in the adult, though in the fætus a branch replaced by cones which are distinguished by their close of the central arterv of the retina which traverses the
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vitreous humour, ramifies in its posterior portion. A aqueous humour, lens, and vitreous body are the trans-
forwards to the proper lens fibres. The lens-body is non-vascular to the integument. By the involution and growth of the and non-nervous.
The surfaces of the lens become more skin at this spot a hollow is produced at the front of the flattened in old age, and its substance hardens and is less vesicle, which gradually deepening forms a pouch, the transparent.
secondary optic vesicle, in which the involuted part of the The Vitreous Body is much the largest of the refracting skin is lodged. From the included sub-epidermal tissue media, and occupies the largest part of the space enclosed the vitreous body is derived; from the included epidermis, by the tunics. Anteriorly it is hollowed out to receive the lens; whilst the cornea sclerotic and iris are produced the posterior convexity of the lens, but posteriorly it is by the subcutaneous connective tissue. The optic nerve convex, and the retina is moulded on it. It is as trans- and retina are formed from the primary optic vesicle and lucent as glass, jelly-like in consistency, and when punctured | its peduncle, and it is probable that the bacillary layer is a a watery fluid drains out. Its minute structure is difficult special development of its internal epithelial lining. The to ascertain, but as it, like the subcutaneous tissue of the choroid coat again is derived from the pia mater. Hence embryo, contains rounded, stellate, and fusiform cells, it is the eyeball is compounded of structures derived partly from customary to refer it to the gelatinous form of connective the integument and partly from the embryo brain. tissue; concentric lamellæ, and even a radiated arrangement ACCESSORY PARTS TO THE EYEBALL.-In relation to the of fibres, have also been described. It has been customary eyeball several accessory parts are found. also to consider it as invested by a delicate structureless The Eye-Brows are projections of the integument, from membrane, the hyaloid membrane; but this is now regarded which short, stiff hairs grow. as belonging to the retina, where it is known as the mem The Eye-Lids, or palpebræ, are two movable curtains, Eyelids. brana limitans interna. Almost opposite the ora serrata a an upper and a lower, which protect the front of the globe. membrane springs from the vitreous body, passes fcrwards Between each pair of lids is a horizontal fissure, the palpefor some distance in relation to the deep surface of the bral fissure. From the free margins of the two lids project ciliary processes, but separated from them by the pars short hairs, the eye-lashes or cilia; the upper set curve ciliaris retinæ, and then inclines inwards to become attached downwards and forwards, the lower set upwards and forto the anterior surface of the capsule of the lens close to its wards; they also protect the front of the globe. Each circumference. It is so closely connected at its origin with eye-lid consists of skin ; of the fibres of the orbicular the membrana limitans that it is difficult to recognise it as sphincter muscle; of a thin plate of fibro-cartilage, the a distinct membrane. It is named the suspensory ligament tarsal cartilage, to the inner end of which a fibrous band, of the lens, or zonule of Zinn, and contains fibres, which run the tendo palpebrarum, is attached, this tendon springing in the meridional direction. Where it leaves the vitreous from the ascending process of the superior maxilla ; and body a narrow space is enclosed between it and that body, of the conjunctiva. Between the conjunctiva and the tarsal which space surrounds the circumference of the lens, and cartilage is a layer of glands, the Meibomian glands ; each is called the canal of Petit. From the relation of the gland consists of a short duct, which expands at its sides suspensory ligament to the ciliary processes it has a plicated into small sacculi. The sacculi contain short columnar surface, and when these processes are torn away from it a cells; these secrete a sebaceous material, which escapes portion of the pigment of the processes is often left behind, through the orifice of the duct at the border of the eye-lid. so that the zonule is sometimes named the ciliary processes The Conjunctiva is a mucous membrane, which forms the Conof the vitreous body.
posterior layer of the eye-lid, and is reflected on to the junctiva The eye an The Eyeball is an optical instrument, constructed on the anterior part of the sclerotic. At the inner angle of juncoptical
plan of the camera obscura. The sclerotic forms the wall tion of the eye-lids is a soft reddish elevation of the instrument.
of the chamber. The choroid represents the black lining conjunctiva, the caruncula lachrymalis, and immediately for absorbing the surplus rays of light. The cornea, external to it is a vertical fold, the plica semilunaris, the
rudiment of the third eye-lid, or membrana nictitans, so obliquely into the upper and outer part of the sclerotic. well developed in birds. The palpebral conjunctiva has The inferior oblique arises from the lower burder of the small papillæ scattered over its surface; its opithelium is orbit, passes outwards and upwards to be inserted obliquely stratified, with scaly cells on the free surface and elongated into the sclerotic. These muscles roll the eyeball in the cells in the deepest layer. In the sub-epithelial tissue are orbit, and, without entering into a minute analysis of their small branched mucous glands, which are numerous in the actions, their office may be stated generally as follows : caruncula. Little masses of adenoid tissue (p. 849) with The internal rectus rolls it inwards, the external outwards, lymphatic vessels are also found in it, and the conjunctiva about its vertical axis; the superior rectus rolls it upwards, of the front of the eyeball is thinner than the palpebral part. the inferior downwards, about its transverse horizontal axis, It is not glandular, and its nerves terminate in end-bulbs though from the obliquity of their insertions they give it (p. 862). The palpebral conjunctiva, and in part that of at the same time a slight inward or outward movement as the eyeball, receive their blood-vessels from those of the the case may be ; the superior and inferior oblique roll the eye-lids, but the portion of the conjunctiva next the cornea globe around its antero-posterior or sagittal axis, the superior is supplied by the arteries of the sclerotic coat.
upwards and outwards, the inferior downwards and outymal The Lachrymal Apparatus is engaged in the secretion wards. atus. of the tears, and in conveying them away from the front Periosteal Muscle of the Orbit. The periosteum of the
of the globe. The lachrymal gland occupies a depression orbit contains, as H. Müller and Turner have described, a in the outer part of the roof of the orbit. It is smaller layer of non-striped muscular fibre in the part which covers than an almond, is sub-divided into lobules, and belongs to over the spheno-maxillary fissure. In man it is rudimenthe group of compound racemose glands. It consists of tary, but in the sheep, deer, elephant, &c., where the the ramifications of short ducts, which terminate in small osseous wall of the orbit is deficient, this muscle forms a sacculi. The wall of each sacculus consists of a delicate well-defined structure. It has been suggested that it acts membrana propria, and the cavity contains the polyhedral as a protractor muscle of the globe. secreting cells.
Outside the membrana propria is a capillary network derived from the lachrymal artery, but The Ear, or organ of hearing, is a complex acoustic ap- Ear. Giannuzzi and Boll have recently described a space between paratus, situated in connection with the temporal bone. this network and the membrana propria which they believe It is divided into three parts, named external, middle, and
has internal ear. described nerves as terminating in connection with the secreting cells. The excretory ducts of the gland are from the external auditory meatus. The auricle is the oblong ear. six to eight, and open on the back of the upper eye-lid. convoluted body situated at the side of the head. Its The tears are washed over the surface of the globe by the incurved outer border is named the helix. Within this lies a involuntary wink
curved ridge, the anti-helix, in front of which is a deep ing of this lid.
hollow, the concha, which leads into the external meatus. When the secre
The concha is bounded in front by a prominence, the tion is increased
tragus, and behind by a smaller prominence, the anti-tragus; in quantity, in
below the anti-tragus is the lobule, which forms the most the act of crying,
depending part of the auricle. The framework of the the tears flow over
auricle is formed of yellow elastic fibro-cartilage invested the cheek, but in
by integument, except the lobule, which consists merely of ordinary circum
a fold of integument containing fat. Attached to the carstances they are
tilage are not only the three auricular muscles referred to on conveyed away by two slender tubes, the lachrymal canals, which open by minute orifices, the puncta lachrymalia, one at the inner end of the Fig. 79.–Lachrymal canals and duct. 1, orbicular muscle; free border of each 2, lachrymal canal; 3, punctum; 4, caruncula; 5,
lachrymal sac; 6, lachrymal duct; 7, angular artery.
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
called recti, and are situated, one superior, one inferior, one
internal carotid artery; !, Eustachian tube; m, tip of petrous process; m, extersides of the sclerotic, but the superior and inferior recti nal auditory meatus; o, membrani tympani; 'p, tympanum; 1, points to mal. have oblique insertions into its upper and lower aspects.
leus; 2, to incus; 3, to stapes; 4, to cochlea; 5, 6, 7, the three semicircular
canals; 8 and 9, portio dura and portio mollis. The other two muscles are called obliqui. The superior oblique arises along with the recti, passes to the inner end page 836, but also certain smaller muscles called the proper of the upper border of the orbit, where its tendon goes muscles of the pinna. Thus the greater muscle of the helix through a pulley, and is directed back to be inserted | is placed on its anterior border ; the lesser muscle of the
helix is situated where it arises out of the concha; the don of the stapedius muscle passes ; and a foramen which
irregular chain, stretching across the cavity from the outer
to the tympanic walls. Middle ear,
The Tympanum, or Drum, or Middle Ear, is a chamber The Incus, or anvil-shaped bone, possesses a body and irregularly cuboidal in form, situated in the temporal bone two processes ; on the anterior surface of the body is a between the bottom of the meatus and the internal ear. saddle-shaped hollow in which the head of the malleus fits; The outer wall is formed of the membrani tympani, which the short process projects almost horizontally backwards, inclines obliquely downwards and inwards at the bottom and is attached by a ligament to the posterior wall of of the external meatus, at an angle of 55° to the axis of the tympanum; the long process extends at ärst downthe meatus, whilst the membranes in the two ears form wards and then inwards, to end in a rounded projection, with each other an obtuse angle of 130° to 135o. The tym- named os orbiculare, through which it articulates with the panic membrane is attached to a groove at the bottom of stapes. the meatus, and is concave on its outer, convex on its inner The Stapes, or stirrup-shaped bone, possesses a head and surface. It consists of three layers : an external tegumen-neck, a base and two crura; the head articulates with the tary, continuous with the skin of the meatus, which con os orbiculare of the incus; from the constricted neck the tains no hairs or glands; an internal mucous, continuous two crura curve inwards to the base, which is attached to with the mucous lining of the tympanum; and an inter- the fenestra ovalis. The joint between the malleus and mediate membrana propria, which consists of unyielding incus is diarthrodial and saddle-shaped, and the articular fibres arranged both radially and circularly. The radial surfaces are enclosed by a capsular ligament. The joint fibres radiate from the point of attachment of the handle between the incus and stapes is also diarthrodial, and posof the malleus. The membrana propria is usually said to sesses an investing capsular ligament. Toynbee and Rüd. be destitute both of nerves and vessels, but Kessel states inger have described the base of the stapes and the margin that nerves, blood, and lymph vessels exist in it as well as of the fenestra ovalis as each invested by hyaline cartilage. in the mucous and tegumentary layers. Immediately in Between these plates elastic fibres extend in a plexiform front of the membrana tympani is the Glaserian fissure. manner, and the intervals between them are occupied by The inner wall separates the tympanum from the labyrinth, fluid ; the joint seems, therefore, a modified amphiarthrosis. and presents the following appearances : a rounded elevation The bones are moved on each other at these joints by small or promontory caused by the first turn of the cochlea, on muscles. The tensor tympani arises from the apex of the the surface of which promontory are grooves for the petrous temporal, and the cartilage of the Eustachian tube lodgment of the tympanic plexus of nerves ; above the enters the tympanum at its anterior wall, and is inserted promontory is an oval opening closed in by a membrane, into the malleus near the root. The laxator tympani the fenestra ovalis, which corresponds with the vestibule ; muscle arises from the spine of the sphenoid, and the carbehind and below the promontory is a round opening closed tilage of the Eustachian tube enters the tympanum through in by a membrane, the fenestra rotunda, which corresponds the Glaserian fissure, and is inserted into the neck of tho with the tympanic passage in the cochlea. The floor of malleus. The stapedius arises within the pyramid, enters the tympanum is a narrow chink between the inner and the tympanum through the hole at its apex, and is inserted outer walls; and the roof is formed by the anterior surface into the neck of the stapes. The tympanum is lined by a of the petrous-temporal bone. At its anterior wall the mucous membrane continuous with that of the Eustachian tympanum opens into the Eustachian tube, a canal which tube, which invests the tympanic ossicles, ligaments, and communicates with the nasal compartment of the pharynx muscles, and is prolonged backwards so as to line the immediately behind the inferior turbinal. The wall of the mastoid air-sinuses. The epithelium covering this memtympanic end of this tube is formed of bone, that of the brane, where it lines the floor and the adjacent part of the pharyngeal end of a curved plate of hyaline cartilage, anterior, posterior, and internal walls, consists of ciliated which is connected to the bone by fibro-cartilage; its columnar cells; but the epithelium covering the roof, the pharyngeal orifice is dilated into a trumpet-shaped mouth; promontory, the membrani tympani, and the tympanic through this tube the ciliated mucous membrane of the ossicles, is tessellated. In the sub-epithelial connective nasal part of the pharynx is prolonged into the tympanum. tissue the blood and lymph vessels and nerves of the The cartilaginous wall of the tube does not completely tympanum ramify. Kessel has recently described in it surround it, but is completed by a fibrous membrane, and certain peculiar bodies, which consist of a central axial a layer of voluntary muscle, named by Rüdinger the dila- band with a series of capsules, possessing a fibrillar structator tubæ. Above the tympanic orifice of the Eustachian ture, arranged concentrically around the axis; the function tube is a fine canal, through which the tensor tympani of these bodies is not known. muscle enters the tympanum. At its posterior wall the The formation of the auricle and external meatus is well tympanum communicates with the air-sinuses in the mastoid adapted for collecting and transmitt sound-vibrations temporal; here also is found a small hollow eminence, inwards to the middle ear and labyrinth. These vibrations the pyramid, through a hole at the apex of which the ten- | strike the membrana tympani, and are propagatel by the
chain of bones across the tympanic cavity to the labyrinth. dilatations, the walls of which are directly continuous with The pressure of the vibrations on the tympanic membrane each other, though the cavities are separated by an interforces that membrane inwards, so that its inner surface mediate partition. The upper and posterior dilatation, presses on the handle of the malleus, the effect of which is to named utriculus, is larger than the lower and anterior, rotate the hammer about its axis; but by the ligamentous named sacculus. The sacculus is continuous with the attachment of the malleus to the tympanic walls and to the ductus cochlearis of the membranous cochlea, and both incus, and, as Helmholtz has shown, by the interlocking of sacculus and utriculus communicate by slender tubes with cog-like processes connected with the articular surfaces of a short diverticulum lodged in the aqueductus vestibuli, to the two bones, the range of movement is so limited that the which the name of ductus vestibuli may be given. The pressure on the malleus is transmitted through the incus membranous semicircular canals are about fd the diameter upon the stapes, which presses, therefore, on the mem
of the osseous.
Their walls are continuous with that of
sists of a delicate fibrous mem
The Vestibule lies immediately internal to the tympanum, distended with the fluid endo-
p, peripheral, and c, central canals exist. In amphibia, reptiles, and birds the cochlea ing ridge, the crista acoustica, to
inger.) is small and rudimentary in comparison with its develop- which calcareous particles, the ment in mammals. The osseous vestibule is an ovoid otoliths, which may be either amorphous or crystalline, chamber about sth inch in diameter. In its outer or are adherent. The endothelial investment of the crista tympanic wall is the fenestra ovalis ; in its inner are small is elongated into columnar cells, and intercalated between auditory foramina, which transmit from the internal them are fusiform cells, the auditory cells, each of which, as meatus the vestibular branches of the auditory nerve ; Schultze and other observers have described, possesses a behind these holes is the opening of a minute canal, the peripheral and a central process. The peripheral process aqueductus vestibuli ; its anterior wall communicates with projects beyond the plane of the free surface of the endothe scala vestibuli of the cochlea, and into its posterior thelium into the endolymph as the auditory hair, whilst wall open the five orifices of the three semicircular canals. the central process extends into the sub-endothelial tissue,
The Semicircular Canals are named superior, posterior, where the nerve plexus belonging to the terminal branches and external. The superior and posterior are sometimes of the auditory nerve ramifies, and with which it is probably called the vertical canals, and the external the horizontal continuous. These auditory cells are, therefore, the pericanal, but, as Crum Brown has shown, the superior and pheral end-organs of the vestibular branches of the auditory posterior lie in planes equally inclined to the mesial plane nerve, and their general arrangement is not unlike that of of the head, and the external is in a plane at right angles the olfactory cells of the nose. to the mesial plane. Further, the canals in the two ears The Cochlea is by far the most complex part of the have definite relations to each other; for whilst the superior labyrinth. It is about canal of each ear is nearly parallel to the posterior canal 4th inch long, and of the other, the external canals in both ears lie nearly shaped like the shell
SC in the same plane. The canals are bent, forming nearly of a common snail; şds of a circle, and would have had six openings into | its base lies near the the vestibule had not the contiguous ends of the superior internal meatus, and and posterior blended together to open by a common orifice. its apex is directed
dr. The opposite end of each of these canals and the outer end outwards.
The osseof the external canal dilate close to the vestibule to twice ous cochlea is a tube their usual diameter, and form an ampulla. The osseous wound spirally two Fig. 82.- Diagram of the membranons labyrinth.
DC, ductus cochlearis; dr, ductus reuniens; S, vestibule and semicircular canals are lined by a periosteum and a half times round
succulus; U, utriculus; dv, ductus vestibuli; invested by a tessellated endothelium, and contain a little a central pillar or
SC, semicircular canals. (After Waldeyer.) fluid, the perilymph. In this fluid the membranous laby- modiolus. Both the pillar and the tube diminish rapidly rinth is suspended.
in diameter from the base to the apex of the cochlea. The membranous vestibule is formed of two small sac-like The tube is imperfectly divided into two passages by a
FIG. 81.-cl, columnar cells
processes of auditcry cells, m, nerve fibres. (After Rüd.