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from the supra-orbital and frontal arteries. The veins are arranged in two sets: (a) subconjunctival or retrotarsal, opening into the muscular tributaries of the ophthalmic veins, and (b) pretarsal, into the angular and superficial temporal veins. The lymph vessels, like the veins, form pre- and retrotarsal networks, which communicate with each other through the tarsal plates. The lymph is drained chiefly into the anterior auricular and parotid lymph glands, but partly, by vessels which accompany the anterior facial vein, into the submaxillary lymph glands. The sensory nerves of the eyelids are supplied by the trigeminal nerve-the upper lid chiefly by the supra-orbital and supra-trochlear branches of the ophthalmic; the lower, by the infraorbital branch of the maxillary. The region of the lateral commissure receives some filaments from the lacrimal nerve, that of the medial from the infra-trochlear. These sensory nerves form a marginal plexus behind the orbicularis oculi muscle. The levator palpebræ superioris muscle is supplied by the oculomotor nerve and the non-striped fibres of the eyelids by the sympathetic.
The lacrimal apparatus consists of: (1) the lacrimal gland, which secretes the tears; (2) the lacrimal ducts, by which the tears are drained from the front of the eye; and (3) the lacrimal sac and naso-lacrimal duct, which convey them into the nasal cavity.
FIG. 700.-DISSECTION TO SHOW THE LACRIMAL APPARATUS.
Glandula Lacrimalis. The lacrimal gland is a flattened, oval body situated in the superior and lateral part of the orbital cavity; it consists of two portions -superior and inferior-imperfectly separated from each other by the expansion of the tendon of the levator palpebræ superioris muscle. The glandula lacrimalis superior is firm and much larger than the inferior; it measures transversely about 20 mm., and sagittally from 12 to 14 mm. It occupies the fossa lacrimalis on the medial surface of the zygomatic process of the frontal bone, and is fixed by fibrous bands to its periosteum, while its inferior surface is in contact with the levator palpebræ superioris and rectus lateralis muscles, which intervene between it and the bulb of the eye. The glandula lacrimalis inferior consists of small, loosely aggregated lobules; it lies below and in front of the orbital portion, and projects into the posterior part of the upper eyelid, where its deep surface is in contact with the conjunctiva. The ducts draining the glandula superior are from three to five in number; they pass between the lobules of the glandula inferior, and open at the upper and lateral part of the fornix conjunctivæ superior. The ducts of the glandula inferior number from three to nine; some of them join those from the glandula superior, while others open separately at the fornix conjunctivæ superior. The lacrimal gland has a structure resembling that of the parotid, and
is supplied by the sympathetic and lacrimal nerves and by the lacrimal artery, while its veins are drained into the ophthalmic vein.
Ductus Lacrimales.—The lacrimal ducts, two in number, commence in minute orifices, termed the puncta lacrimalia, at the apices of the papillæ lacrimales (p. 821), and are directed medialwards, along the medial parts of the margins of the eyelids, above and below the lacus lacrimalis. The superior duct at first ascends for a short distance and then inclines downwards; the inferior duct descends for a short distance and then runs horizontally; at the angle where it changes its direction each duct is dilated into an ampulla. The two ducts open close together into the lateral and front part of the lacrimal sac, a little below its middle; sometimes they open separately into a pouch-like dilatation of the sac, termed the sinus of Maier. The ducts are lined with stratified epithelium placed on a tunica propria, outside which is a layer of striped muscular fibres derived from the lacrimal part of the orbicularis oculi. These muscular fibres are arranged somewhat spirally around the ducts, but at the base of each papilla lacrimalis they are circular in direction and form a species of sphincter. On contraction they serve to empty the contents of the lacrimal ducts into the lacrimal sac.
The saccus lacrimalis and ductus nasolacrimalis together form the passage by which the tears are conveyed from the lacrimal ducts to the nasal cavity.
The lacrimal sac is the upper expanded part of the passage, and measures from 12 to 15 mm. in length, about 7 mm. antero-posteriorly, and from 4 to 5 mm. transversely. It lies in the groove formed by the lacrimal bone and frontal process of the maxilla, and ends above in a rounded, blind extremity or fundus, while it narrows below into the naso-lacrimal duct. At the junction with the duct a fold of mucous membrane, named the valve of Beraud, together with a laterally directed pouch, the sinus of Arlt, are sometimes present. Near its superior extremity it is crossed, in front, by the medial palpebral ligament (O.T. tendo oculi), from the upper and lower edges of which the orbicularis oculi takes origin; the lacrimal part of the orbicularis oculi muscle is behind it.
The naso-lacrimal duct averages about 18 mm. in length, and has a diameter of from 3 to 4 mm. Rather narrower in the middle than at its extremities, it is directed downwards and slightly backwards, and opens into the inferior meatus of the nose at the junction of its anterior fourth with its posterior three-fourths, i.e. a distance of 30 to 35 mm. from the posterior boundary of the nostril. Its lower orifice is somewhat variable in form and position, and is occasionally duplicated. It is frequently guarded by a fold of mucous membrane, termed the plica lacrimalis (Hasneri). Through this orifice the mucous lining of the duct is continuous with that of the nasal cavity. The mucous membrane of the duct is thrown into inconstant folds, several of which have been described as valves. Its epithelium is columnar and in part ciliated; opening into the lower part of the duct are numerous glands, similar to those in the nasal mucous membrane.
The nerves of the lacrimal ducts and sac are derived from the infra-trochlear branch of the naso-ciliary; their arteries from the inferior palpebral and nasal. The veins of the naso-lacrimal duct are large and numerous, forming a sort of erectile tissue similar to that in the nasal cavity.
DEVELOPMENT OF THE EYE.
The retina and optic nerve are developed from a hollow outgrowth of the fore-brain, termed the optic vesicle (see pp. 54 and 33). This extends towards the side of the head, and its connexion with the brain is gradually elongated to form the optic stalk. The ectoderm overlying the optic vesicle becomes thickened, invaginated, and finally cut off as a hollow island of cells, which is named the lens vesicle. This vesicle indents the outer and lower part of the optic vesicle, converting it into a cup (optic cup), lined by two layers of cells continuous with each other at the margin of the cup. The inner of these strata, thicker than the outer, is named the retinal layer, and becomes differentiated into the nervous and supporting elements of the retina; while the outer, named the pigmentary layer, forms the stratum pigmenti. The edge of the optic cup extends in front of the equator of the lens, and bounds the future aperture of the pupil. In front of the lens, and also opposite its equator, the retinal layer is thin, and represented only by a stratum of columnar cells which becomes closely applied to the pigmentary layer, the two forming the
pars ciliaris and pars iridica retina. The indentation of the optic cup extends as a groove for some distance along the postero-inferior aspect of the optic stalk, forming what is termed the chorioidal fissure (Fig. 701). Through this fissure mesoderm passes inwards between the lens and the retina to form a part of the vitreous body, while the arteria centralis retinæ also becomes enclosed in it and so gains its future position in the centre of the optic
FIG. 701.-SECTIONS THROUGH PORTIONS OF THE HEADS OF FETAL RABBITS, to illustrate the connexion of the optic cup with the fore-brain, and the invagination of the ectoderm to form the lens.
Outer layer of optic cup
The arteria centralis is prolonged forwards from the porus opticus through the vitreous body, as a cone of branches, as far as the back of the lens. By the fifth or sixth month all these branches have disappeared except one, the arteria hyaloidea, which persists until the last month of foetal life, when it also atrophies, leaving only the canalis hyaloideus to indicate its position.
The vitreous body is developed between the optic cup and the lens, and is derived partly from ectoderm and partly from mesoderm. It consists primarily of a series of fine protoplasmic fibres which project from the cells of the retinal layer of the cup and form a delicate Optic stalk reticular tissue. At first these fibres are seen in relation to the whole of the optic cup, but later they are limited to the ciliary region, where by a process of condensation Chorioidal they appear to form the zonula ciliaris. When the
mesoderm reaches the cup through the chorioidal fissure it unites with this reticular tissue to form the vitreou body.
FIG. 702.-OPTIC CUP AND LENS VIEWED
The lens, at first in contact with the ectoderm
FROM BEHIND AND BELOW, to show from which it is derived, is soon separated from it by
formation of chorioidal fissure and enclosure of the arteria centralis retinæ (from inodel by Ziegler).
mesoderm, and then consists of a rounded vesicle with epithelial walls. The anterior wall remains as a single layer of cells-the anterior lens epithelium of the adult; the cells of the posterior wall become elongated into lens fibres, and by the forward growth of these the cavity of the vesicle is obliterated. This elongation into lens fibres is greatest at the centre of the lens, while near the equator the fibres are shorter, and here the gradual transition between the anterior epithelium and the lens fibres is seen (Fig. 696). The lens becomes enveloped in a vascular tunic, which receives its vessels from the arteria centralis retina and from the vessels of the iris. The from: part of this tunic forms the membrana pupillaris, and this, like the rest of the tunic. disappears before birth.
The hollow optic stalk becomes solid by the thickening of its walls and, acquiring nerve-fibres, is transformed into the optic nerve. These nerve-fibres are mostly centripetal, and are derived from the nerve-cells of the retina; but a few are centrifugal and have their origin in the brain. The further development of the retina resembles, in certain respects, that of the spinal medulla.
Cameron states (Journ. Anat. and Physiol., vol. xxxix.) that in the early stages of the development of the inner or retinal layer of the optic cup all the structures, described by His as being present in the spinal medulla of the human embryo, are to be found, viz., (a) spongioblasts b) germinal cells, and (c) neuroblasts.
The spongioblasts undergo ramification and form a network or myelospongium, and also give rise to the inner and outer limiting membranes; the latter is next the original cavity of the optic vesicle, and therefore corresponds to the inner limiting membrane of the spinal medulla. The spongioblasts also form the groundwork of the inner and outer molecular layers into which the processes of the neuroblasts grow and arborise.
The germinal cells are always situated beneath the external limiting membrane, and by their division give rise to the neuroblasts. The first-formed neuroblasts are larger than those of succeeding generations, and are found in the site of the future ganglionic layer. The germinal cells in the middle of the convexity of the retinal cup cease to divide at an early stage of development, and become directly transformed into the rod and cone cells from which the rods and cones develop as processes; hence these structures appear first over the middle of the convexity of the retina, and gradually extend towards the margin of the retinal cup.
The molecular layers make their appearance as plexuses of myelospongium. The internal molecular layer is first developed at the centre of the retinal cup, and gradually extends towards the cup margin, and into it the processes from the nuclei on either side grow and ramify. The rod and cone fibres, and the outer processes of the internal nuclear layer, grow into and arborise within the external molecular layer.
The condensed mesoderm surrounding the optic cup becomes the sclera and chorioid. In the portion of the mesoderm which lies in front of the lens a cleft-like fissure appears, and divides it into a thick anterior and a thin posterior layer. The former becomes the substantia propria of the cornea; the latter, the stroma of the iris and anterior part of the vascular tunic of the lens. The fissure represents the future camera oculi anterior, and its lining cells form the endothelium of this chamber.
The eyelids arise as two integumentary folds above and below the cornea, each being covered on both its surfaces by the ectoderm. By the third month the folds meet and unite with each other at their edges, the eyelids being only permanently opened shortly before birth; in many animals they are not opened until after birth. The ectoderm forms the epithelium of the conjunctiva and the epithelium corneæ. It is also invaginated at the lid margins to form the hair follicles and the lining cells of the tarsal glands and glands of Moll, and, at the fornix conjunctivæ superior, to form the lining of the alveoli and ducts of the lacrimal gland.
The naso-lacrimal duct, lacrimal sac, and ducts represent the remains of the furrow between the maxillary and lateral nasal processes (p. 49). It is at first filled with a solid rod of cells, which becomes hollowed out to form the sac and ducts.
The ear or auditory organ (Fig. 703) consists of three portions-external, middle, and internal-the last constituting its essential part, since the peripheral terminations of the acoustic nerve are distributed within it.
The external ear1 includes (a) the auricula, attached to and projecting from the side of the head; and (b) the external acoustic meatus leading inwards from the most depressed part of the auricula to the tympanic membrane.
The auricle (O.T. pinna) (Fig. 704) presents two surfaces, lateral and medial, the latter forming an angle (cephalo-auricular angle) of about 30° with the side of the head. The lateral surface is irregularly concave, but presents several wellmarked elevations and depressions. The deepest of the depressions is situated near its middle, and is named the concha auriculæ. It is divided by an almost transverse ridge, the crus helicis, into an upper, smaller, and a lower, larger portion: the former is termed the cymba concha; the latter, which leads into the meatus, the cavum concha. Anteriorly, the crus helicis is continuous with the margin of the auricula or helix, which is incurved in the greater part of its extent, and is directed at first upwards, and then backwards and downwards, to become gradually
1 Although it is usual to speak of the external, middle, and internal ear, it would be more correct to use the terms external, middle, and internal portions of the ear.
lost in the upper part of the lobule.
Near the point where the helix begins to descend a small tubercle, the tuberculum auriculæ (Darwini), is often seen.
In front of the descending part of the helix is a second elevation, the antihelix. Single below, it divides superiorly into two limbs, termed the crura antihelicis, between which is a triangular Auricula depression, the fossa triangularis. The elongated furrow between the helix and antihelix is named the scapha. The concavity of the concha is overlapped in front by a tongue-like process, the tragus, and below and behind by a triangular projection, the antitragus; the
Tympanic cavity, with chain of ossicles
FIG. 703.-DIAGRAMMATIC VIEW OF THE ORGAN OF HEARING.
notch, directed downwards and forwards between these two processes, is named the incisura intertragica. The tragus consists really of two tubercles, the upper of which constitutes the tuberculum supratragicum of His, and is separated from the helix by a groove, the sulcus auris anterior. The lobule is situated below the incisura intertragica, and is the most dependent part of the auricle.
The medial or cranial surface
also is irregular, and presents Crus antihelicis superior
Crus antihelicis inferior-
- Auricular index.
The auricula is usually smaller and more finely modelled in the female than in the male, but presents great variations in size and shape in different individuals. In the newly born child its length is about one-third of that of the adult, while it increases slightly in length and breadth in old age.
The relation of the width to the height is termed the auricular index, and is expressed as follows:
width of auricula x 100 length of auricula This index is less in white than in dark races.
The cephalo-auricular angle may be practically absent, as in those cases where the skin of the head passes directly on to the lateral surface of the auricula, or it may be increased to nearly a right angle, so that the lateral surface of the auricula looks directly forwards. The tuberculum auriculæ, the significance of which was recognised by Darwin, is a somewhat triangular prominence which projects forwards in cases where the helix is well rolled over, but backwards and upwards when the incurving of the helix has been arrested. More frequently present in men than in women, it is of developmental interest since it has been shown to be well marked at the sixth month of foetal life, the entire auricula, at this stage, resembling in appearance that of the adult macaque monkey.
FIG. 704.-VIEW OF LATERAL SURFACE OF LEFT AURICULA (half natural size).
The lobule may be small and sessile or considerably elongated; it may adhere to the skin of the cheek (i.e. webbed), or may tend to bifurcate at its lower extremity.