A text-book of veterinary anatomy

THE SENSE ORGANS AND COMMON INTEGUMENT

The organs of the senses (Organa sensuum) receive external stimuli and conduct impulses to the brain which result in sensations of sight, hearing, taste, smell, and touch. They consist essentially of specially differentiated cells, the neuro-epithelium, and a conduction path which is simple in the more generalized sense organs, elaborate in those which are highly specialized—the eye and the ear.

THE SENSE ORGANS AND SKIN OF THE HORSE

The Eye

The eye or organ of vision (Organon visus) in the broader sense of the term comprises the eyeball or globe of the eye, the optic nerve, and certain accessory organs associated therewith. The accessory organs (Organa oculi accessoria) are the orbital fasciæ and muscles, the eyelids and conjunctiva, and the lacrimal apparatus. These structures will be considered in the order in which they may be most conveniently examined, taking the horse as a type. The bony walls of the orbit have been described in connection with the skull; the periorbita, a fibrous membrane which incloses the eyeball together with its muscles, vessels, and nerves, may be appropriately included in the account of the fasciæ.

The eyelids, upper and lower (Palpebra superior et inferior), are movable folds of integument situated in front of the eyeball. When closed, they cover the entrance to the orbit and the anterior surface of the eyeball. The upper lid is much more extensive and more movable than the lower one, and its free edge is more concave. The interval between the lids is termed the palpebral fissure (Rima palpebrarum). When the eye is closed, it is an oblique slit about two inches (ca. 5 cm.) in length; when open, it is biconvex in outline. The ends of the fissure are the angles or canthi, and are distinguished as internal or nasal, and external or temporal (Angulus oculi medialis, lateralis). The external angle is rounded when the eye is open, but the internal angle is narrowed to form a ᑐ-shaped bay or recess, termed the lacrimal lake (Lacus lacrimalis). In this there is a rounded, pigmented prominence known as the lacrimal caruncle (Caruncula lacrimalis); it is about the size of a small pea and is covered with modified skin, connected with that of the inner commissure, from which project a number of hairs provided with sebaceous glands. The lids unite on either side and form the commissures, internal and external (Commissura palpebrarum medialis, lateralis). The anterior surface of the lids (Facies anterior palpebrarum) is convex and presents a variable number of long tactile hairs in addition to the ordinary but very short hairs. A considerable number of tactile hairs are scattered over the lower part of the lower lid, but on the upper lid they are very scanty. The infrapalpebral depression (Sulcus infrapalpebralis) indicates somewhat indistinctly the limit of the lower lid. The upper lid is marked by two furrows when raised. The posterior surface (Facies posterior palpebrarum) is adapted to the free surface of the eyeball and is covered by the palpebral conjunctiva. The free border of the lid is smooth and usually black. It has a well defined posterior margin (Limbus palpebralis posterior), along which the ducts of the tarsal glands open. The anterior margin (Limbus palpebralis anterior) bears stiff hairs termed the cilia or eyelashes. On the upper lid the cilia are long and numerous except at its inner third, where they are very small or absent. On the lower lid the cilia are often scarcely distinguishable from the ordinary hairs; in other cases they may be clearly seen except near the external canthus, and are much finer and shorter than those of the upper lid. The edge of each lid is pierced near the internal angle by a minute, slit-like opening, the punctum lacrimalis, which is the entrance to the lacrimal duct or canaliculus.

Structure of the Eyelids.—The skin of the eyelids is thin and freely movable, except near the free edge, where it is more firmly attached. The underlying subcutaneous tissue is destitute of fat. The muscular layer consists chiefly of the elliptical bundles of the orbicularis oculi, with which are associated fibers of the corrugator supercilii in the upper lid and the malaris in the lower lid. At the inner side there is a fibrous band, the palpebral ligament, which is attached to the lacrimal tubercle and furnishes origin to some fibers of the orbicularis. At the inner commissure a bundle detached from the orbicularis passes inward behind the lacrimal sac, and is known as the pars lacrimalis or Horner’s muscle. At the outer side an indistinct palpebral raphé occurs where fibers of the orbicularis decussate. The fibrous layer is thicker and denser along the free edge of the lid, forming here the tarsus. The tarsus furnishes insertion to a layer of unstriped muscle known as the tarsal muscle (of Müller). The tarsal glands (Glandulæ tarsales) are partly embedded in the deep face of the tarsus, and are visible when the lid is everted if the conjunctiva is not too strongly pigmented. They are arranged in a linear series, close together, and with their long axes perpendicular to the free edge of each lid. In the upper lid they number forty-five to fifty; in the lower, thirty to thirty-five. Each consists of a tubular duct beset with numerous alveoli, in which a fatty substance, the palpebral sebum, is secreted. The palpebral conjunctiva lines the posterior surface of the eyelids.

The conjunctiva is the mucous membrane which lines the lids as palpebral conjunctiva (C. palpebrarum) and is reflected upon the anterior part of the eyeball as bulbar conjunctiva (C. bulbi); the line of reflection is termed the fornix conjunctivæ. The palpebral part is closely adherent to the tarsus, but is loosely attached further back. It is papillated and is covered with stratified cylindrical epithelium in which many goblet cells are present. In the fornix and its vicinity there are tubular glands (of Henle). Near the inner angle there are numerous lymph nodules. The conjunctiva of the outer part of the upper lid is pierced near the fornix by the orifices of the excretory ducts of the lacrimal gland. The bulbar conjunctiva is loosely attached to the anterior part of the sclera and is pigmented in the vicinity of the corneo-scleral junction. On the cornea it is represented by a stratified epithelium.

When the lids are in apposition, the conjunctiva (including the epithelium of the cornea) forms a complete conjunctival sac, as the capillary space between the lids and the eyeball is termed.

The third eyelid (Palpebra tertia) is situated at the inner angle of the eye (Figs. 551, 556). It consists of a semilunar fold of the conjunctiva, known as the membrana nictitans, which covers and partly incloses a curved plate of hyaline cartilage. Its marginal part is thin and usually more or less pigmented. The cartilage has an elongated quadrilateral outline. The part of it which lies in the membrana is wide and thin. The deep part is narrower and thicker and is embedded in fat at the inner side of the eyeball. Numerous minute lymph nodules occur in the membrana nictitans, and the deep part of the cartilage is surrounded by a gland which resembles the lacrimal gland in structure (Glandula superficialis palpebæ tertiæ).

Ordinarily the third eyelid extends very little over the inner end of the cornea, but when the eyeball is strongly retracted (as in tetanus) the membrana is protruded over it so as to measure about an inch (ca. 2 to 3 cm.) in its middle. This effect results from the pressure of the eyeball and its muscles on the fat which surrounds the deep part of the cartilage.

Vessels and Nerves.—The arteries which chiefly supply the eyelids and conjunctiva are branches of the ophthalmic and facial arteries, and the blood is drained away by corresponding veins. The sensory nerves are branches of the ophthalmic and maxillary divisions of the trigeminus. The motor nerves to the orbicularis oculi, corrugator supercilii, and malaris come from the facial nerve, the levator palpebræ superioris is innervated by the oculomotor, and the unstriped muscle of the lids by the sympathetic.

THE LACRIMAL APPARATUS

The lacrimal apparatus (Apparatus lacrimalis) comprises: (1) the lacrimal gland, which secretes the clear lacrimal fluid; (2) the excretory ducts of the gland; (3) the two lacrimal ducts or canaliculi, lacrimal sac, and naso-lacrimal duct, which receive the fluid and convey it to the nostril.

The lacrimal gland (Glandula lacrimalis) is situated between the supraorbital process and the dorso-lateral surface of the eyeball (Fig. 438). It is flattened, oval in outline, and measures about two inches (ca. 5 cm.) transversely and an inch or more (2.5 to 3 cm.) in the sagittal direction. Its upper face is convex and is related to the concave lower surface of the supraorbital process. The lower face is concave in adaptation to the eyeball, from which it is separated by the periorbita. The excretory ducts (Ductuli excretorii) are very small and are twelve to sixteen in number; they open into the outer part of the conjunctival sac along a line a little in front of the fornix conjunctivæ superior. In appearance and structure the gland resembles the parotid. It receives its blood-supply chiefly from the lacrimal artery. The sensory nerve is the lacrimal, and the secretory fibers are derived from the sympathetic.

The puncta lacrimalia are the entrances to the two lacrimal ducts. Each is a fine slit-like opening (about 2 mm. long), situated close behind the free edge of the lid and about a third of an inch (ca. 8 mm.) from the internal canthus. The lacrimal ducts (Ductus lacrimales), upper and lower, begin at the puncta and converge at the inner commissure to open into the lacrimal sac. The latter (Saccus lacrimalis) may be regarded as the dilated origin of the naso-lacrimal duct. It occupies the funnel-like origin of the bony lacrimal canal, and leads to the naso-lacrimal duct (Ductus naso-lacrimalis), which passes forward and a little downward along the outer wall of the frontal sinus and the nasal cavity and opens near the lower commissure of the nostril. Its length is about ten to twelve inches (ca. 25 to 30 cm.). In the first part of its course it is inclosed in the osseous lacrimal canal; further forward it lies in the lacrimal groove of the maxilla, covered at first by a plate of cartilage and then by the mucous membrane of the middle meatus. The terminal part lies in the inferior turbinal fold and opens on the skin of the floor of the nostril near the transition to mucous membrane. Accessory openings may occur a little further back.

The first part of the duct, about 6 to 7 mm. in diameter, extends in a gentle curve, convex dorsally, from the internal commissure toward a point just above the level of the infraorbital foramen. The second part (isthmus) is narrower (ca. 3 to 4 mm.); it extends forward and a little ventrally about to a transverse plane through the first cheek tooth and lies in the groove above the inferior turbinal crest. Beyond this the duct inclines upward and widens very considerably, crosses the nasal process of the premaxilla obliquely, and contracts at its termination. The mucous membrane may present valvular folds, the most distinct of which is situated at the origin.

THE PERIORBITA

The periorbita or ocular sheath is a conical fibrous membrane which incloses the eyeball with its muscles, vessels, nerves, etc. Its apex is attached around the optic and lacerated foramina, and its base is in part attached to the bony rim of the orbit, in part continuous with the fibrous layer of the lids. Its inner part, which is in contact with the orbital wall, is thin; incorporated with it beneath the root of the supraorbital process is the bar of cartilage around which the superior oblique muscle is reflected. The outer part is thicker, and is strengthened by an elastic band which is attached to the pterygoid crest and furnishes origin to the thin unstriped orbital muscle. A quantity of fat (Corpus adiposum extraorbitale) lies about the periorbita, and within it is the intraorbital adipose tissue (Corpus adiposum intraorbitale) which fills the interstices between the eyeball, muscles, etc.

THE ORBITAL FASCIÆ AND OCULAR MUSCLES (Figs. 438, 439, 555)

The straight muscles of the eyeball and the oblique muscles in part are inclosed in fibrous sheaths (Fasciæ musculares), formed by superficial and deep layers of fascia, which are united by intermuscular septa in the interstices between the muscles. The superficial fascia is thin; it blends in front with the fibrous layer of the eyelids and is attached behind around the optic foramen. The deep fascia consists anteriorly of two layers, one of which is continuous with the fibrous tissue of the lids, while the other is attached at the corneo-scleral junction.

The posterior part of the eyeball is covered by the bulbar fascia or capsule of Tenon (Fascia bulbi), so that between them a lymph space (Spatium interfasciale) is inclosed which communicates with the subdural space along the course of the optic nerve.

The levator palpebræ superioris muscle^[217] is a thin band about half an inch in width which lies above the rectus superior. It is narrow at its origin above and behind the ethmoidal foramen and ends by an expanded tendon in the upper lid. Its action is to raise the upper lid.

The recti oculi (Mm. recti bulbi) are four in number and are designated according to their positions as superior, inferior, internal, and external. They are all band-like, arise close together around the optic foramen, and diverge as they pass forward to the eyeball. On reaching the latter they end in thin tendons which are inserted into the sclera in front of the equator of the eyeball.

The retractor oculi (M. retractor bulbi) surrounds the optic nerve, and is incompletely divided into four parts which alternate with the recti. They arise around the optic nerve and are inserted into the sclera behind the recti.

The obliquus oculi superior (M. obliquus bulbi dorsalis s. superior) is the longest and narrowest of the ocular muscles. It arises near the ethmoidal foramen and passes forward internal to the rectus internus. Under the root of the supraorbital process it is reflected almost at a right angle around a cartilaginous pulley (trochlea), which is attached to the anterior part of the inner wall of the orbit, a bursa being interposed here. The muscle is then directed outward and somewhat forward, and ends in a thin tendon which passes between the rectus superior and the eyeball, and is inserted into the sclera between the superior and external recti, about half an inch behind the margin of the cornea.

The obliquus oculi inferior (M. obliquus bulbi ventralis s. inferior) is wide and much shorter than the recti. It arises from the inner wall of the orbit in the small depression (Fossa muscularis) behind the lacrimal fossa. It curves around the rectus inferior and is inserted into the sclera near and partly beneath the rectus externus.

Actions.—The superior and inferior recti rotate the eyeball about a transverse axis, moving the vertex of the cornea upward and downward respectively. Similarly the internal and external recti rotate the eyeball about a vertical axis, turning the vertex of the cornea inward and outward respectively. The oblique muscles rotate the eyeball about a longitudinal axis; the superior oblique raises the outer end of the pupil, while the inferior oblique lowers it. The retractor as a whole draws the eyeball backward, and its parts may separately reinforce the corresponding recti. Also the four recti acting together will retract the eyeball.

The actual movements of the eyeball are by no means so simple as might be inferred from the foregoing general statements. Practically all movements are produced by the coördinated actions of several muscles, involving combinations which are quite complex and difficult to analyze accurately. Further complication is caused by the fact that the recti are not inserted at equal distances from the equator and the axes of rotation of the oblique muscles do not correspond to the longitudinal axis of the eyeball.

Nerve-supply.—The oculomotor nerve supplies the foregoing muscles, with the exception of the rectus externus and obliquus superior, which are innervated by the abducens and trochlearis respectively.

THE EYEBALL

The eyeball (Bulbus oculi) is situated in the anterior part of the orbital cavity, protected in front by the eyelids and conjunctiva, in its middle by the complete orbital ring, and related behind to the bulbar fascia, fat, and ocular muscles.

It has the form approximately of an oblate spheroid, but is composed of the segments of two spheres of different sizes. The anterior, transparent, corneal segment has a radius of curvature of about 17 mm., and the posterior, opaque, scleral segment a radius of about 25 mm. The anterior segment therefore projects more strongly, and the junction of the two segments is marked externally by a broad, shallow groove, the sulcus scleræ. The central points of the anterior and posterior curvatures of the eyeball are termed respectively the anterior and posterior poles (Polus anterior, posterior), and the line connecting the poles is the optic axis (Axis oculi externa). The angle of divergence of the optic axes is about 137 degrees. The equator (Æquator) is an imaginary line drawn around the eyeball midway between its poles, and meridians (Meridiani) are lines drawn around it through the poles.

The average transverse diameter of the eyeball is about 5 cm., the vertical about 4.5 cm., and the axial about 4.25 cm. The distance from the anterior pole to the point of entrance of the optic nerve is about 3 cm.

The eyeball consists of three concentric tunics or coats, within which three refractive media are inclosed.

The Fibrous Tunic

The fibrous tunic (Tunica fibrosa oculi) is the external coat and is composed of a posterior opaque part, the sclera, and a transparent anterior part, the cornea.

The sclera, popularly termed the “white of the eye,” is a dense fibrous membrane which forms about four-fifths of the fibrous tunic. Thickest in the vicinity of the posterior pole (ca. 2 mm.), it thins at the equator (ca. 0.4 mm.), and increases in thickness toward the junction with the cornea (ca. 1.3 mm.). It is in general white, but may have a bluish tinge in its thinnest parts. Its external surface furnishes insertion to the ocular muscles and is covered by the conjunctiva scleræ in its anterior part. The episcleral tissue, which is richly supplied with vessels and nerves, attaches the conjunctiva to the sclera; it is abundant and loosely meshed except at the junction with the cornea. The inner surface is attached to the chorioid coat by a layer of delicate, pigmented, connective tissue, the lamina fusca. The anterior border is oval, the long axis being transverse, and is continuous with the cornea. The transition from the opaque scleral tissue to the transparent corneal substance occurs in such manner that the sclera appears to form a groove (Rima cornealis), into which the cornea fits somewhat as a watch-glass in the case. Near the corneo-scleral junction there is a circular venous plexus, the plexus s. sinus venosus scleræ, formerly called the canal of Schlemm. The optic nerve passes through the posterior part of the sclera a little below and external to the posterior pole. The opening for the nerve is crossed by interlacing fibrous strands, forming the lamina cribrosa scleræ.

The sclera consists of interlacing bundles of white fibrous tissue, associated with which there are a few elastic fibers. The bundles are arranged chiefly in meridional and equatorial layers. The very limited blood-supply is derived from the ciliary arteries, and the veins open into the venæ vorticosæ and ciliary veins. The lymphatics are represented by intercommunicating cell spaces. The nerves are derived from the ciliary nerves.

The cornea forms the anterior fifth of the fibrous tunic. It is transparent, colorless, and non-vascular. Viewed from in front it is oval in outline, the long axis being transverse and the broad end internal; it appears more nearly circular when viewed from behind. Its anterior surface (Facies anterior) is convex and is more strongly curved than the sclera; its central part is termed the vertex corneæ. The posterior surface (Facies posterior) is concave; it forms the anterior boundary of the anterior chamber, and is in contact with the aqueous humor. The margin (limbus corneæ) joins the sclera; the latter overlaps the cornea more in front than behind, and more above and below than at the sides, thus explaining the difference in outline of the two surfaces. The cornea is thinnest at the vertex.

The cornea consists, from before backward, of the following layers: (1) The epithelium corneæ is continuous with that of the conjunctiva scleræ, and is of the stratified squamous type. (2) The lamina limitans anterior is merely a condensation of the next layer. (3) The substantia propria forms the bulk of the cornea and is composed of interlacing bundles of connective tissue, arranged in part in lamellæ disposed parallel with the surface. In the amorphous cement substance between the lamellæ are flattened connective-tissue cells, the corneal corpuscles. These have branching processes which unite with those of other cells, thus forming a protoplasmic network.^[218] (4) The lamina elastica posterior, also termed the membrane of Descemet or Demours, is a thin and practically homogeneous membrane which is less intimately attached to the substantia propria than the anterior lamina. It is clear, glistening, and elastic. At the periphery the lamina divides into three sets of fibers. The anterior fibers join the sclera, the middle give attachment to the ciliary muscle, while the posterior pass into the iris and form the ligamentum pectinatum iridis. (5) The endothelium (cameræ anterioris) consists of a layer of flattened polygonal cells, and is reflected on to the anterior surface of the iris.

The cornea is without blood-vessels except at its periphery, where the terminal twigs of the vessels of the sclera and conjunctiva terminate in loops. The nerves are derived from the ciliary nerves. They form a plexus around the periphery (Plexus annularis), from which fibers pass into the substantia propria, become non-medullated, and form the fundamental or stroma plexus. From this perforating branches pass through the anterior limiting layer and form a subepithelial plexus, from which filaments ascend between the epithelial cells. Other branches from the plexuses in the substantia propria end as fibrils which are in close relation with the corneal corpuscles.

The Vascular Tunic

The vascular tunic (Tunica vasculosa oculi) lies internal to the fibrous coat; it comprises three parts—the chorioid, the ciliary body, and the iris.

1. The chorioid (Chorioidea) is a thin membrane which lies between the sclera and retina. It is in general rather loosely attached to the sclera by the lamina fusca, but is intimately adherent at the point of entrance of the optic nerve and less closely in places where the ciliary vessels and nerves pass through. The inner surface is in contact with the layer of pigmented cells of the retina, which adhere so closely to the chorioid that they were formerly regarded as a part of the latter. The general color of the chorioid is dark brown, but an extensive semilunar area a little above the level of the optic papilla has a remarkable metallic luster, and is termed the tapetum of the chorioid (Tapetum chorioideæ). The appearance here varies in different individuals, but the prevailing colors in most cases are iridescent blue and green in various nuances shading into yellow. Posteriorly the chorioid is perforated by the optic nerve, and anteriorly it is continuous with the ciliary body.

The chorioid consists of four layers, which from without inward are as follows: (1) The lamina suprachorioidea consists of interlacing fine lamellæ of fibrous tissue, each containing a network of elastic tissue. Among these are large-branched, pigmented, connective-tissue cells. The spaces between the lamellæ are lined with endothelium, and form a system of lymph-clefts which together form the perichorioid space (Spatium perichorioideale). (2) The lamina vasculosa is the outer part of the proper tissue of the chorioid. It contains the larger blood-vessels, which are supported by connective-tissue and elastic fibers. (3) The lamina choriocapillaris consists of an extremely rich network of capillaries embedded in an almost homogeneous matrix. Between it and the lamina vasculosa is a layer of fibro-elastic tissue, the tapetum fibrosum, which causes the metallic luster mentioned above. (4) The lamina basalis is very thin and transparent. It is composed of an inner homogeneous part and an outer elastic part.

2. The ciliary body (Corpus ciliare), the middle part of the vascular coat, connects the chorioid with the periphery of the iris. In meridional section it has the form of a narrow triangle, the base of which is next to the iris. On its inner side are the ciliary processes and on its outer side the ciliary muscle. It consists of three parts—the ciliary ring, ciliary processes, and ciliary muscle. The ciliary ring (Orbicularis ciliaris) is the posterior zone, which is distinguished from the chorioid mainly by its greater thickness and the absence of the choriocapillaris. Its inner face presents numerous fine meridional ridges, by the union of which the ciliary processes are formed. The ciliary processes (Processus ciliares), more than a hundred in number, form a circle of radial folds which surround the lens and furnish attachment to the zonula ciliaris or suspensory ligament of the latter. They are small at their origin on the ciliary ring and become much thicker and higher toward their inner ends. The width of the circle formed by them is narrower at the inner side than elsewhere. Their bases extend forward to the periphery of the iris, and their inner ends are close to the margin of the lens. They bear numerous secondary folds. Their inner surface is covered by a continuation of the lamina basalis of the chorioid, on which there are two layers of epithelial cells which constitute the pars ciliaris retinæ. They consist of a rich network of tortuous vessels supported in pigmented connective tissue. The ciliary muscle (M. ciliaris) (Figs. 552, 555, 559) constitutes the outer part of the ciliary body, and lies between the sclera and the ciliary processes. It forms a circular band of unstriped muscle, the fibers of which are for the most part directed meridionally. They arise from the inner surface of the sclera and from the ligamentum pectinatum iridis close to the corneo-scleral junction, and run backward along the sclera to be inserted into the ciliary processes and ring. When the muscle contracts, it pulls the processes and ring forward, thus slackening the ciliary zone or suspensory ligament of the lens, and allowing the latter to become more convex. This is the mechanism of accommodation for near objects.

In man the muscle has the form of a prismatic ring which is triangular in meridional section, the base being directed toward the periphery of the iris. It consists chiefly of meridional fibers, but a ring of circular fibers forms the inner angle of its base. In the horse the muscle is much less developed, and has the form of a flat band; it does not contain circular fibers, but the arrangement is rendered more or less plexiform by the existence of oblique and equatorial fibers.

3. The iris (Figs. 552, 557, 559) is a muscular diaphragm placed in front of the lens, and is visible through the cornea. It is pierced centrally by an elliptical opening, the pupil (Pupilla), which varies in size during life and determines the amount of light admitted to act on the retina. In strong light the vertical diameter of the pupil is very short, but the opening is almost circular when the pupil is fully dilated. The ciliary border (Margo ciliaris) or periphery of the iris is continuous with the ciliary body and is connected with the corneo-scleral junction by the ligamentum pectinatum iridis. The ligament consists of strands of connective tissue which are attached externally to the corneo-scleral junction. Its bundles interlace and inclose spaces (Spatia anguli iridis) which are lined with endothelium and communicate with the anterior chamber. The pupillary border (Margo pupillaris) surrounds the pupil. Its upper part bears in its middle several black masses of variable size, termed the granula iridis or corpora nigra; similar, but much smaller, projections may be seen on the lower margin of the pupil. The anterior surface (Facies anterior) determines the color of the eye, which is dark brown usually. It is marked by delicate concentric lines (Plicæ iridis), which fade out near the pupil. The smooth, narrow, central part is termed the pupillary zone or annulus iridis minor, while the much broader plicated part is the ciliary zone or annulus iridis major. The posterior surface (Facies posterior) is usually black. It bears numerous fine radial lines except at the pupillary margin. Its central part is in contact with the anterior surface of the lens, but peripherally the two are separated by a narrow space termed the posterior chamber. The iris consists chiefly of the stroma iridis, a delicate framework of connective tissue, which supports numerous blood-vessels, and contains branched pigmented cells. The muscular tissue (unstriped) consists of a sphincter and a dilator of the pupil. The sphincter pupillæ lies in the posterior part around the pupil, with which the fibers are largely concentric. The dilatator pupillæ consists of fibers which radiate outward from the sphincter to the ciliary border. The anterior surface of the iris is covered by a continuation of the endothelium of the cornea. Beneath this is a condensation of the stroma, in which the cells are close together and are full of pigmented granules.^[219] There appear to be minute clefts here by which the lymph-spaces of the stroma communicate with the anterior chamber. The posterior surface is covered by a pigmented epithelium, the stratum pigmenti iridis, which is a continuation of that of the ciliary body.

The arteries of the vascular tunic come from the ciliary branches of the ophthalmic artery. The arteries of the chorioidea are derived chiefly from the short posterior ciliary arteries. These perforate the sclera around the posterior pole, run forward in the lamina vasculosa, and form the rich capillary network of the choriocapillaris. The two long ciliary arteries perforate the sclera obliquely near the optic nerve; they run forward in the lamina suprachorioidea in the horizontal meridian, one on the inner, the other on the outer, side of the eyeball. On reaching the ciliary body each divides into diverging branches; the subdivisions of these unite with each other and with twigs of the anterior ciliary arteries to form a circular anastomosis, the circulus arteriosus major. From this branches go to the ciliary muscle and processes and to the iris. The two anterior ciliary arteries, dorsal and ventral, form an episcleral plexus around the corneo-scleral junction, and give off branches which perforate the sclera. These supply twigs to the ciliary muscle and recurrent branches to the chorioid, and assist in forming the circulus arteriosus major.

The blood is carried away from the vascular tunic chiefly by four or five venous trunks, the venæ vorticosæ, which are formed by the convergence in whorls of numerous veins, coming not only from the chorioid, but also from the ciliary body and iris. The venæ vorticosæ perforate the sclera about at the equator and join the veins of the ocular muscles.

The nerves come from the long and short ciliary nerves. They form a plexus in the lamina suprachorioidea, which contains ganglion cells, and sends numerous non-medullated fibers chiefly to the blood-vessels of the chorioid. At the ciliary muscle a second plexus (P. gangliosus ciliaris) is formed, which supplies the muscle and sends fibers to the iris. The sphincter pupillæ is supplied by fibers derived from the oculomotor nerve, while the dilatator pupillæ is innervated by the sympathetic.

THE RETINA

The retina or nervous tunic of the eyeball is a delicate membrane which extends from the entrance of the optic nerve to the margin of the pupil. It consists of three parts. The large posterior part, which alone contains the nervous elements, including the special neuro-epithelium and the optic nerve-fibers, is termed the pars optica retinæ. It extends forward to the ciliary body, where it terminates at an irregular line called the ora serrata. Here the retina rapidly loses its nervous elements, becomes much thinner, and is continued over the ciliary body and the posterior surface of the iris by two layers of epithelial cells. In the pars ciliaris retinæ the inner stratum is non-pigmented, while the outer layer is a direct continuation of the stratum pigmenti of the pars optica. In the pars iridica retinæ both are pigmented. In the dead subject the pars optica is an opaque, gray, soft membrane which can be stripped of the chorioid, leaving most of its outer pigmented layer on the latter. During life it is transparent, except as to its pigmented epithelium, and the reddish appearance of the fundus as viewed by the ophthalmoscope is caused by the blood in the network of the choriocapillaris. The entrance of the optic nerve forms a sharply defined, oval, light area, the optic papilla (Papilla nervi optici), situated about 15 mm. below the horizontal meridian and 3 to 4 mm. external to the vertical meridian. The central part of the papilla is slightly depressed (Excavatio papillæ n. optici).

The transverse diameter of the papilla is about 6 to 7 mm., and the vertical about 4 to 5 mm. It is commonly situated a little below the margin of the tapetum, but the latter may extend down somewhat on either side of the papilla. The lower margin is often indented a little. In inspection of the fundus with the ophthalmoscope numerous fine branches of the arteria centralis retinæ are seen radiating from the periphery of the papilla.

The optic nerve-fibers converge from all parts of the pars optica to the papilla, where they collect into bundles which traverse the lamina cribrosa of the chorioidea and sclera, and constitute the optic nerve. The area centralis retinæ is a round spot, 2 to 3 mm. in diameter, situated above and external to the optic papilla; it corresponds to the macula lutea of man, which is histologically more highly differentiated than the rest of the retina and is the area of most acute vision.

The structure of the retina is very complex. It consists of nervous elements which are supported in a peculiar sustentacular tissue, and are covered externally by a layer of pigmented epithelium (Stratum pigmenti retinæ). The nervous elements comprise a highly specialized neuro-epithelium, the rods and cones; ganglion cells, the axones of which form the optic nerve; and intermediate neurones. Ten layers may be recognized in sections microscopically.^[220]

The arteries of the retina are derived from the arteria centralis retinæ and anastomotic branches from the short ciliary arteries. The arteria centralis enters the optic nerve a short distance behind the eyeball and runs in the axis of the nerve. The artery divides 2 to 3 mm. before reaching the papilla, and gives off thirty to forty branches which radiate in the posterior part of the retina and divide dichotomously into end-arteries. The anterior part of the retina is non-vascular. The veins accompany the arteries except in the capillary plexuses; their walls consist merely of a layer of endothelial cells, around which are a lymph-channel and sheath.

CHAMBERS OF THE EYE AND AQUEOUS HUMOR

The anterior chamber of the eye (Camera oculi anterior) is inclosed in front by the cornea and behind by the iris and lens (Figs. 552, 555, 557). It communicates through the pupil with the posterior chamber (Camera oculi posterior); this is a small annular space, triangular in cross-section, which is bounded in front by the iris, behind by the peripheral part of the lens and its ligament, and externally by the ciliary processes. The chambers are filled by the aqueous humor (Humor aqueus), a clear fluid which consists of about 98 per cent. of water, with a little sodium chlorid and traces of albumin and extractives. It is carried off chiefly through the spaces (of Fontana) in the zonula ciliaris or suspensory ligament of the lens into the plexus venosus scleræ.

REFRACTIVE MEDIA OF THE EYEBALL

The vitreous body (Corpus vitreum) is a semifluid transparent substance situated between the crystalline lens and the retina. In front it presents a deep cavity, the fossa hyaloidea, which receives the posterior surface of the lens. It consists of a framework of delicate fibrils, the stroma vitreum, the meshes of which are filled by the fluid humor vitreus. The surface is covered by a condensation of the stroma known as the membrana hyaloidea.

The crystalline lens (Lens crystallina) is a biconvex, transparent body, situated in front of the vitreous body and in partial contact with the posterior surface of the iris. Its periphery, the æquator lentis, is almost circular and is closely surrounded by the ciliary processes. The anterior surface (Facies anterior) is convex; it is bathed by the aqueous humor and is in contact with the iris to an extent which varies with the state of the pupil. The posterior surface (Facies posterior) is much more strongly curved than the anterior. It rests in the fossa of the vitreous body. The central points of the surfaces are the anterior and posterior poles (Polus anterior et posterior lentis), and the line which connects them is the axis of the lens (Axis lentis).

The transverse diameter of the lens is about 2 cm., the vertical diameter is slightly smaller, and the axis measures about 13 mm. The radius of curvature of the anterior surface is 13.5 mm., and of the posterior surface 9.5 to 10 mm. But the curvatures of its surfaces—especially that of the anterior—vary during life according as the eye is accommodated for near or far vision.

The zonula ciliaris (of Zinn) or suspensory ligament of the lens (Fig. 552) consists of delicate fibers (Fibræ zonulares) which pass in a meridional direction from the ciliary processes to the capsule of the equator of the lens. Many fibers cross each other, and the spaces between the fibers (Spatia zonularia) are filled with aqueous humor; they communicate with each other and with the posterior chamber.

The substance of the lens (Substantia lentis) is inclosed by a structureless, highly elastic membrane, the capsule of the lens (Capsula lentis), and consists of a softer substantia corticalis, and a dense central part, the nucleus lentis. The capsule is thickest on the anterior surface, and here it is lined by a layer of flat polygonal cells, the epithelium of the lens capsule. The lens substance, when hardened, is seen to consist of concentric laminæ arranged somewhat like the layers of an onion, and united by an amorphous cement substance. The laminæ consist of lens fibers, hexagonal in section, and of very different lengths. Faint lines radiate from the poles and indicate the edges of layers of cement substance which unite the groups of lens fibers. These lines, the radii lentis, are three in number in the fœtus and new-born, and form with each other angles of 120 degrees. On the anterior surface one is directed upward from the pole and the other two diverge downward; on the posterior surface one is directed downward and the others diverge upward. The developed lens has neither vessels nor nerves.

In the fœtus the lens is nearly globular, and is soft and pinkish in color. During part of fœtal life it is surrounded by a vascular network, the tunica vasculosa lentis. This is derived chiefly from a temporary vessel, the hyaloid artery, which is a continuation forward of the arteria centralis retinæ through the hyaloid canal that traverses the vitreous body. In old age the lens tends to lose its elasticity and transparency; it also becomes flatter and the nucleus especially grows denser.

The Ear

The ear or organ of hearing (Organon auditus) consists of three natural divisions—external, middle, and internal.

THE EXTERNAL EAR

The external ear (Auris externa) comprises (1) the auricula, a funnel-like organ which collects the sound waves, together with its muscles, and (2) the external auditory canal, which conveys these waves to the tympanic membrane, the partition which separates the canal from the cavity of the middle ear.

The auricula or pinna is attached by its base around the external auditory canal in such a manner as to be freely movable. In the following description it will be assumed that the opening is directed outward and that the long axis is practically vertical. It has two surfaces, two borders, a base, and an apex. The convex surface or dorsum (Dorsum auriculæ) faces inward and is widest in its middle part; its lower part is almost circular in curvature, while above it narrows and flattens. The concave surface (Scapha) is the reverse of the dorsum; it presents several ridges which subside toward the apex. The anterior border is sinuous; it is largely convex, but becomes concave near the apex. It divides below into two diverging parts (crura helicis). The posterior border is convex. The apex is flattened, pointed, and curved a little forward. The base is strongly convex. It is attached to the external auditory process of the petrous temporal bone, and around this there is a quantity of fat. The parotid gland overlaps it below and externally. The structure of the external ear comprises a framework of cartilages (which are chiefly elastic), the integument, and a complicated arrangement of muscles.

The conchal or auricular cartilage (Cartilago auriculæ) determines the shape of the ear; its form can be made out without dissection, except below, where it is concealed by the muscles and the parotid gland. The basal part is coiled to form a tube, which incloses the cavity of the concha (Cavum conchæ). This part is funnel-shaped and curves outward and a little backward. Its internal (medial) surface is strongly convex, forming a prominence termed the eminentia conchæ. The lowest part of the internal margin bears a narrow, pointed prolongation, the styloid process. This process is about an inch long and projects downward externally over the annular cartilage; the guttural pouch is attached to its free end. Behind its base there is a foramen through which the auricular branch of the vagus passes.

The basal part of the posterior border is cut into by a notch, which separates two irregular quadrilateral plates. The upper plate (Tragus) is overlapped by the anterior border, and is separated from the adjacent part of the posterior border (Antitragus) by a notch (Incisura intertragica). The lower plate is curved to form a half ring and partly overlaps the anterior border and the annular cartilage. Behind the notch there is a foramen, which transmits the internal auricular artery and internal auricular branch of the facial nerve.