Book - The Embryology Anatomy and Histology of the Eye 2
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Brown EJ. The embryology anatomy and histology of the eye. (1906) Chicago: Hazlitt & Walker.
Having hurriedly described the development of the eye ball, we will now go over the adult eye, giving the gross and leaving the minute anatomy until we have advanced farther with the subject. The adult eye ball is 24.5 mm across, 24 mm from front to back, 23.5 from top to bottom, weighs a fraction less than one-quarter ounce and is composed of the segments of two spheres ; the anterior portion, or the cornea, A, Fig. 23 (meaning hor^ilike), being the segment of a much smaller sphere than the posterior or scleral portion, the cornea comprising one-sixth of the outer surface, while the sclerotic (hard or tough), shown at B, makes up the other five-sixths. The cornea is transparent and thus forms the window through which the light is admitted to the eye ball and this transparency allows us to see the iris (rainbow), E, the structure lying directly behind the cornea. The iris is a circular structure pierced at the center by the opening known as the pupil. It contains two muscles, the one surrounding the pupil, which is a narrow band of circular fibers known as the sphincter pupillae muscle (meaning the bijider muscle), K. This muscle closes the pupil, to protect the delicate tissues at the back of the eyeball from bright or intense light, then the dilator pupillae muscle, the fibers of which extend from the base of the iris to the sphincter pupillae. This muscle enlarges the pupil when more light is required to form a denser picture on the retina. The lens, F, lies just back of the pupil but can only be seen after it has lost its transparency. Continuing backward from the base of the iris, will be seen the ciliary body, I, and between this structure and the sclerotic is found the ciliary muscle, H. In front of the ciliary muscle and at the base of the iris, is seen the pectinate ligament (comblike ligament), Q and J. This is made up of many small bundles of connective tissue, running from the periphery of the cornea to the base of the iris, across the angle formed by the junction of the cornea and the iris. This angle is known as the filtration angle, for the aqueous fluid, which fills the anterior and posterior chambers, leaves the eyeball, at this point. It passes into the spaces of fontana (fountain spaces), the spaces of fontana simply being the space between the bundles of fibers forming the pectinate ligament, and from these spaces the aqueous fluid, or nutrient lymph, as it is sometimes called, passes through the tissues to the canal of Schlemm, which is seen in Fig. 23 in the cornea just outside of the spaces of fontana. The canal of Schlemm is a circular channel within the corneal tissue, extending clear around the periphery of the cornea and the fluids pass from the canal of Schlemm to the anterior ciliary veins. Extending backward from the ciliary bodies and continuous with them, are the ciliary processes. These end near the ora serratta (saw tooth mouth), X, of the retina. Running from the ora serratta forward to the lens, imbedded in the outer layer of the hyaloid membrane and bound down firmly to the inner surface of the ciliary processes and bodies is the suspensory ligament or Zonule (belt) of Zinn, as Dr. Zinn first described it, G. The ligament proper is made up of very elastic fibers, which, as before stated, are imbedded in the outer layer of the hyaloid membrane. The hyaloid membra^ie surrounds the vitreous body and these fibers, the writer believes, to be elongated fibers of Mueller, which became attached to the lens during foetal life when the fornix (arch) of the primary and secondary optic vesicles were in apposition (touching) to the equator of the lens and as the globe enlarged they elongated. See Figs. 7 to 20.
Fig. 23. Cross section of the Eye, showing its construction.
This ligament leaves the ciliary bodies and passes across the space between them and the le,ns, a part of the fibers passing a little anterior of the equator and the rest a little posterior to the equator of the lens and are attached to the capsule of the lens. The triangular space formed by this separation of the suspensory ligament fibers is known as the Canal of Petit, shown at R. The lens, F, is a transparent body and occupies the space just back of the iris and between the circle of inward projecting ciliary bodies. It is round, and flattened from before backward, its anterior and posterior surfaces being convex, the posterior surface having the shorter radius of curvature. It lies in a depression m the anterior surface of the vitreous body. This depression is known as the fossae Patellaris (dishlike depression) and is supported by this and the suspensory ligament. The lens is surrounded by a dense transparent membrane known as the capsule. The space in front of the ciliary bodies, suspensory ligament and lens, and back of the iris, is known as the posterior chamber, T, and the space in front of the iris and lens at the pupilary space and behind the cornea, is known as the anterior chamber, S.
The sclerotic coat (tough coat), B, continues backward from the cornea by continuity (continuation of tissue by blending one into another) of tissue over the posterior five-sixths of the eyeball to the optic nerve, where it divides, the inner portion forming the lamina cribrosa (sieve layer), M, whilst the outer portion passes into the sheath of the optic nerve Y. It is pierced by the ciliary arteries P; by nerves which enter the eyeball in a circle surrounding the nerve; by the vena vortacosa, four or five of which leave the eyeball just back of the equator; and by the anterior ciliary arteries and veins which enter the eyeball at the attachments of the extrinsic muscles, just back of the cornea. The region where the cornea ends and the sclerotic begins is known as the limbus (seam), W, and the angle or depression formed by the difference in the radius of curvature of the two spheres, represented in the formation of the eyeball in the corneal and scleral portion Z, is known as the sclero corneal sulsus (furrow). This angle makes the eyeball stronger and more firm at this point and it is just inside, Opposite lliis angle that the cihary muscle, II, is attached anteriorly, whilst posteriorly the longitudinal fibers are attached to the outer surface of the choroid, in the region of the ciliary processes and bodies, as this muscle is interposed between the sclerotic and choroid in this region. The ciliar)' muscle, H, is made up of two sets of muscular fibers, the longitudinal nuining antero-posteriorly which are placed farthest out, next to the sclerotic, and the circular fibers which lie farthest inward, just outside of the ciliary bodies. These last named fibers take a circular course ajid form a band of circular fibers extending entirely around the ciliary ring.
Just inside of the ciliary muscle and sclerotic is found a very vascular pigmented layer, C, knowii as the choroid (meaning membrane). This is loosely attached to the sclerotic by the exchange of bundles of tissue called trabeculae and this space so formed is known as the supra choroidal space. The choroid is the middle tunic, or coat, of the three grand tunics of the eyeball. It is extremely vascular and it is analogous to the pia mater of the brain. The choroid, ciliary processes, ciliary bodies and the iris constitute what is known as the uveal coat (grape skin coat) , and the three combined line all the scleral portion and compose the iris or curtain in front of the lens. Posteriorly the choroid is pierced by the optic nerve and this opening is known as the choroidal fissure (choroidal opening). As before stated, the posterior ciliary arteries ajid nerves pass through the sclerotic to reach the choroid. Here the short, posterior ciliary arteries, P, from twelve to twenty in number, divide, one branch running toward the optic nerve ; the others run anteriorly and begin to subdivide as they run forward supplying the choroid, and some branch to the sclerotic. Two of the interjial branches may be seen near the optic nerve in Fig. 23, the final destination of the anterior branches being the ciliary bodies, where they form capillary loops and turn backward as venous capillaries.
These capillaries keep joining with others and forming constantly larger veins, till finally there are great whorls formed in the region of the equator, where great numbers join to form the vena vortacosa which leave the eyeball just back of the equator to empty into the ophthalmic vein. Close inspection of this layer in Fig. 23 will reveal minute white spots al) through its expanse and these white spots are cross sections of the arteries and their branches as well as the veins of the whorls from which the vena vortacosa are formed within the tissue. There are two long posterior ciliary arteries which enter the eyeball with the short set of arteries; one enters just i.nside, the other just outside of the optic nerve. These pass forward in the choroid without giving off any branches, until they reach the ciliary region. Here they each divide into branches which take a circular course and form a circle of anastomosis at the base of the iris and form what is known as the circulus major (the largest circle), 2, of the iris. The anterior ciliary arteries also join in this network, forming an anastomosis with them ; then from this outer or larger circle branches pass into the iris and run toward the free margin or pupil, and when these reach the region of the sphincter pupillae muscle, another circle of anastomosis is formed and this is called the circulus m.lnor (smallest circle) of the iris ; from this smaller circle are given off capillaries, which form a circle of loops right at the free margin of the iris. These turn back as capillary loops, run one into another and become larger and larger and finally form veins known as the anterior ciliary veins ajid these veins also receive the aqueous humor from the canal of Schlemm, and therefore drain the anterior chamber. This was proven by injecting coloring matter into the anterior chamber, then after a few moments killing the animal and finding this colored matter in the anterior ciliary veins. The anterior ciliary veins leave the eye ball at the muscular attachments and pass away from the eye ball in the muscles finally reaching the ophthalmic vein from them.
The ciliary nerves, about twenty in number, which arise from the ciliary ganglion (knot), enter the eyeball in a circle just outside of the optic nerve. They run forv/ard in the supra choroidal space, giving off branches. Supplaying this structure, as well as the sclerotic, they run forward and form the ciliary plexus, which lies in the ciliary muscle. From this plexus branches run to the iris and cornea, supplying motor impulses to the sphincter pupillae muscle, dilator pupillae muscle, as well as trophic and sensory functions to the iris proper ; the branches passing to the cornea are trophic and sensory only.
Just outside of the optic nerve, where it pierces the eyeball, is ^und a circle of anastomosis, giving a pretty free blood supply to the sheath at this point and sending branches into the substance of the nerve, to supply nutrition to the sustentacular, or binding tissue, which forms trabeculae (beams) between the nerve bundles. This circle. O, is known as the circulus of Zinn, as he was the first to describe it.
Passing to the inner surface of the wall of the eyeball. we find the third of three grand tunics known as the retina (net), D. This lines the inner wall from the head of the optic nerve, also called the optic disc, or papillae, to the era serratta. It is made up of seven layers of nervous tissue, two layers of connective tissue and one single layer of columnar pigmented cell:^. The nine innermost layers are held together by the sustentacular or binding tissue, which is known as the fibers of Muller. The outer or pigmented columnar layer is intimately attached to the choroid, while the other nine layers are loosely attached to this layer, yet firmly attached to the choroid at the ora serratta, while the arrangement of the uorvc fiber kiycr and the passing of the axis cylinder processes through the choroidal fissure and their continuation into the optic nerve bind the retina down firmly at this point. The retina is the nervous tunic and the most sensitive in the eyeball and is the one v^hich makes possible the sense of sight. Its most sensitive area Hes just outside of the optic nerve and is known as the macula lutea, V (the yellow spot), so named from the fact that if examined after death, it will be seen to have a yellowish hue. Then again the central spot within the macula is known as fovea centralis (or central pit). The retina thins down ajid leaves a cone-shaped pit, there being only two layers at this central spot. The retina receives its blood supply from the arteria centralis retina (central ar^^^y)y 3- Ihis enters the eyeball in the substance of the optic nerve, having become incorporated in the nerve during the folding of the optic stalk and vesicles durijig foetal life. See Figs. 10 and 11. When it passes through the choroidal fissure it divides, one branch passing upward, the other downward. These are known as the superior and inferior branches. Each subdivide, making four branches ; one runnijig upward and toward the nose, another upward and toward the temple, another downward and inward toward the nose, and another outward and downward toward the temple and from the direction taken they are named. The one running upward toward the ^lose is known as the superior nasal branch, whilst the one running downward toward the nose is known as the inferior nasal ; the one running upward toward the temple is known as the superior temporal, the one running downward toward the temple is known as the inferior temporal branch. The farther subdivisions become so small and are so inconstant in their arrangement, that they have never beeji named. These vessels are imbedded in the retina, ramifying in the four innermost layers. They are readily seen with an ophthalmoscope from the fact that the retinal tissue surrounding them is transparent. These vessels keep dividing lill (hey become capillaries and turji back as venous capillaries. These capillaries keep joining and rejoining until the vena centralis retina is formed and this passes out by the side of the arteria centralis retina. These veins are normally about one-third larger than the arteries and as they carry vejious blood, which is loaded with waste products, they are of a darker red color when viewed with an ophthalmoscope.
As before stated the sclerotic coat posteriorly divides into three parts, the outer portion continuing into the sheath of the optic nerve, Y, the middle portion passes to the pial sheath, while the innermost portion breaks up into bundles and bridges across the space just back of the choroidal fissure, passing through the optic nerve and as these fibers come from all points and pass across in all directions, there is formed a sieve-like kyer which is known as the lamina cribrosa (sieve layer). This reinforces the globe at this point, which otherwise would not stand the strain exerted by the normal tension within the eyeball. The optic iierve fibers pass through the meshes in this sieve layer and the optic nerve proper commences just back of this, where the insulation in the form of the myelin (marrow) sheaths begin. The opening through the lamina cribrosa, through which the arteria centralis retina and veins pass, is known as the porus opticus. At the head of the optic nerve, at the inner wall of the eyeball, there is found a shallow, funnel-shaped pit, L, known as the physiological cup (normal cup). This pit is formed owing to the fact that when the axis cylinder processes reach the choroidal fissure and turn backward over the edge of the choroid, they make a gradual symmetrical turn, instead of running out and making a sharp right ajigled turn, so the innermost fibers join at the center, after having bent to a certain extent, thus leaving this normal depression. This depression of course is filled by the vitreous body.
The space surrounded by the retina, ciliary processes, ciliary bodies, suspensory ligament and lens, is filled by the vitreous body, U. This is made up of shapeless cells, more to be compared to an open meshed sponge than anything else, and fluid and the whole body is of the consistency of the white of an egg. It is surrounded by the hyaloid membrane, which lies on the injier limiting membrane of the retina. At the ora serratta, this hyaloid membrane divides. The outermost layer is firmly attached to the inner surface of the ciliary processes and bodies and passes from the ciliary bodies to the lens, and imbedded in it are the fibers of the suspensory ligament. The innermost layer continues over the front of the vitreous body and lines the fossae patillaris (dish-like depression), in which the lens rests. The vitreous body and its surrounding membrane are perfectly transparent. Running forward from the head of the optic nerve to the posterior of the lens, is a lymph space, known as the hyaloid canal, or the canal of Stilling; this was the channel through which the hyaloid artery passed to supply nutrition to the developing vitreous and lens, during foetal life. See Fig. i6. This artery atrophies before birth, and leaves this canal. The cornea, aqueous humor, lens and vitreous, form the refractive media of the eye, from the fact that they are transparejit and are of different densities and different curvatures, so arranged that light entering a normal eye is brought to a focus at the retina.
The eyeball has numerous lymph spaces and channels. The space between the sclerotic and choroid is known as the supra choroidal space. The greater portion of the contents of the eyeball are fluids, which are practically the same as lymph found in other parts of the body; they are furnished by the osmosis (passing out), of the fluids of the blood through the walls of the capillaries in the ciliary bodies. A portion passes into the canal of Petit and back into the vitreous body, while the rest passes into the posterior chamber, part directly from the anterior portion of the ciliary bodies and part from the canal of Petit. The supra choroidal space is filled with fluids ajid is drained by the lymph spaces accompanying the vena vortacosa. Tn healthy eyes all these fluids are constantly being supplied and rapidly passing out, so they do not become stagnant.
The orbits are four sided and pyramidal in form. The base is formed by the brim of the orbit, A, Fig. 24. The apex is at the sphenoidal fissure or opening, shown at B. The opening at the brim of the orbit, transversely, is one and one half inches, while vertically it is but one and one-fourth inches. Its depth, from the brim to the sphenoidal foramen, is one and three-fourths inches. The roof arches somewhat and the floor is slightly depressed, while the outer and inner walls are straight. The walls of the orbit are formed by seven bones. The roof is mainly formed by the orbital plate of the frontal bone, shown at C, and a very small portion at the posterior of the orbit by the lesser wing of the sphenoid, shown at D. The inner wall, from before backward, is formed by the nasal process of the superior maxillary, shown at E, lachrymal F, ethmoid H, orbital process of the superior maxillary G and the orbital portion of the sphenoid I. The floor is formed by the orbital plate of the superior maxillary J, orbital process of the plate K and a small portion of malar L. The outer wall is formed by the greater wing of the sphenoid M, and the orbital process of malar N.
The openings in the walls in the orbital cavity are as follows : On the interior wall, from before backward, the lachrymal canal, leading to the nasal cavity, through which the lachrymal duct passes ; the anterior and posterior ethmoidal foramen (opening), through which the nasal branch of the ophthalmic nerve and artery leave the orbit ; at the apex the sphenoidal fissure, through which the third, fourth, sixth and ophthalmic branches of the fifth nerve enter the orbit and the ophthalmic vein leaves it; above and to the inner side of the sphenoidal fissure is found the optic foramen O. It is through this opening that the second or optic nerve and the ophthalmic artery enter the orbit. At the lower, outer side, is found the spheno maxillary fissure P. It is through this
Fig. 24. The Human Skull.
Opening that the upper branch of the superior maxillary or middle division of the fifth or trifacial nerve enters the orbit. It lies in a groove in the floor of the orbit at Q, and leaves the orbit with the infra orbital artery through the infra orbital foramen R.
Above the orbit, at its brim, is found a small opening, known as the supra orbital foramen, shown at S, through which the supra orbital nerve and artery leave the orbit. Sometimes this fails to fill in with bone at the brim and then only forms a notch, as shown at T. The inner walls are practically straight, from before backward, while the outer walls run obliquely backward and inward. Thus it will be seen that the axial poles of the two orbits diverge something like thirty degrees. The two eyeballs occupy the anterior central portion of the orbits. The rest of the orbit is filled with the orbital fat and the structures necessary for the performance of ocular functions and protection to the eyeball.
Covering the front, or base of the orbit and in front of the eyeball, are found the two lids, the upper and the lower, known as the palpebral and shown at G and H, Fig. 25. The opening between the two lids, through which the eyeball is seen is known as the palpebral fissure and where the two lids join, at the outer and inner sides of the eyeball, is called the outer and inner canthus, as shown at A and B. Near the inner canthus, the two lids approach one another, then separate again slightly, before coming together, and this little circular portion of the palpebral fissure is known as the lakus (meaning small lake, and is so called because the tears flow into it before leaving the palpebral fissure). Lying within the lakus is a small, red body, formed of mucous tissue and of some few very fine hairs, also the remains of the schneiderian gland, which is. found in those lower animals which have a third eyelid or nictitating membrane. This body is called the caruncle (small growth of flesh), shown at C, and just outside of the caruncle is found a fold of the conjunctiva (which membrane lines the lids and covers all the portion of the eyeball which is exposed when the lids are parted, except the corneal portion). This fold is the remains of the mem1)rana nictatans and is called the plica semilunaris (half moon fold), and is shown at F. All along the free margin of the lids, there is a row of hairs, which extend forward, with a slight turning upward at the outer ends on the upper lid and downward on the lower lid. These are the cilia (hairs) or eyelashes.
As before stated, when the lids approach, near the inner canthus, they arch away from each other, to form the lakus and on the free margin of the lids at this angle, is found a small, slightly raised pdint, known as the lachrymal papillae (tear pimples), shown at I, from the fact that in the center of each one is found a little opening, called the lachrymal puncta (minute opening), so named from the fact that the tears pass out of the palpebral fissure through these two openings. At the anterior central portion of the eyeball is seen a round, dark area, shown at D, with a central, smaller, round and darker area, shown at E. Tlie outer, lighter portion, is the iris, and the smaller, darker portion is the opening through its center, known as the pupil. These are seen through the transparent cornea, M, and all the opaque, or white portion of the eyeball, seen from ni front, is the sclerotic, L, which is seen through the transparent conjunctiva. When the lids are separated, there is seen above the palpebral fissure, a fold of skin, J, which is caused by a bundle of fibers from the muscle which raises the upper lid passing outward and being attached to the skin, which draws the lower part of the skin, covering the lid, upward and allowing the skin covering the upper part of the lid to drop down, forming the fold, and in this way nature has provided against this loose skin dropping over the edge of the lid and obscuring vision, when the Hd is raised and the skin slackened.
Fig. 26. Showing Tendo Oculi.
Above the orbit, and covering a ridge, is a growth of hairs called the supra cilia (the hairs above) or eyebrows, K. This ridge is known as the supra ciliary ridge and is caused by a ridge of bone and a muscle underlying the skin. If the skin were dissected away, immediately beneath it would be found the superficial facia covering the deeper structure of the lids and stretching across the orbit. This is a thin, fibrous sheet, which is found immediately beneath the skin and areolar tissue in all portions of the body. At the outer and inner sides of the palpebral fissure, running from the canthi to the orbital walls, is seen the external and internal angular or palpebral ligaments, also called the orbicular ligaments (shown at A and B, Fig. 26), and just above the orbit would be seen the corrugator supra ciliary muscle (supra ciliary wrinkler) shown at C. It arises from the frontal bone near the median line and along the supra ciliary ridge, and is attached to the upper and outer fibers of the orbicularis muscle. It is the contraction of this muscle which causes the vertical wrinkles in the skin at the lower central portion of the forehead. Its nerve supply comes from the facial nerve, yet there seems to be a reflex action between this muscle and those of accommodation, for we see this corrugation or wrinkling most frequently in those who are hyperopic.
Fig. 27. Showing Orbicularis Muscle.
If we dissect away the superficial facia, immediately beneath it will be found the orbicularis palpebrarum muscle (circular muscle of the lids) shown at D, Fig. 27. It arises from the bony walls of the orbit at the brim. The bundles of fibers pass inward and take a circular course and surround the palpebral fissure C, being continuous around the two canthi, A and B. This muscle is supplied by the facial nerve, and its action is to close the palpebral fissure and bring the free margins of the lids into apposition (touching), thus hiding the eyeball. If the dissection is continued deeper, the deep facia would be exposed and in the region of the eye it is quite dense and fibrous and is called the ligament of Lockwood. It is shown at A, Fig. 28. In it are embedded the tarsal (lid) cartilages, and above will be found the levator palpebrae superioris muscle (the lifter of the upper lid), shown at B. This muscle arises from the ligament of Zinn, which surrounds the optic foramen; it runs forward and upward and its tendon spreads out fan-shaped and is attached to the upper edge of the tarsal cartilage; a few fibers pass out and are attached to the skin. Its nerve supply is from the third, or motor oculi.
Fig. 28. Showing Ligament of Lockwood.
At the upper, inner side of the orbit, is seen the trochlea (pulley), shown at C, and passing through it and turning outward and downward, to be attached to the eyeball, is seen the superior oblique muscle D. It arises also from the ligament of Zinn, passes forward, upward and inward through the orhit, then becomes tendonous and passes through the trochlea, then runs outward, downward and backward, and is attached to the eyeball underucath and outside of the superior rectus muscle, just back of the equator. This muscle receives its nerve supply from the fourth or patheticus nerve. At the upper, outer side of the orbit is seen the lachrymal gland (tear gland), shown at E.
Fig. 29. Showing Arteries of the Lids.
This is a compound racemose gland (resembling a bundle of grapes), and its ducts empty into the conjunctival sac, at the fornix conjunctiva (arch of the conjunctiva), at the upper, outer angle. This gland secretes the tears which are poured into the conjunctival sac, when the eye is irritated, to wash away any foreign substance which may be the cause of the irritation. This gland is especially supplied with sensory nerves from the branch of the ophthalmic nerve, which is named after the gland. At the outer and inner cantlii are again seen the angular ligaments F, and beneath the internal angular ligament, is found the tensor tarsi muscle, which is supplied by the facial nerve. If the structures of the lids were dissected away, leaving only the arteries, their arrangement would be about as seen in Fig. 29. A is the angular artery, the terminal branch of the facial, and it is through this branch that collateral circulation to the brain is established, if the internal carotid is occluded, for it forms an anastomosis with the frontal artery G, this being the terminal branch of the ophthalmic artery.
Fig. 30. Showing Veins of the Lids.
B is the infra orbital artery which comes to the surface from the orbit, through the infra orbital foramen. D is the supra orbital which comes from the orbit to the face through the supra orbital foramen. H is the lachrymal branch of the ophthalmic artery, after piercing the lid. I shows a branch of the anterior temporal artery as it comes to the region of the eye. This branch is of importance, from the fact that in acute inflammations of the orbit, or its contents, leeching is resorted to on the temple, and it is the blood from this artery that is taken. E shows a branch from the transverse facial artery. Running across the lids, just above and below the opening, are seen two arterial trunks, F and J. They are divided into four arteries, the superior internal palpebral, the superior external palpebral, the inferior internal palpebral and the inferior external palpebral. It will be seen that the lids are well supplied with blood and that there is a free anastomosis of these vessels in and around the eyelids.
Fig. 31. Showing Nerves of the Lids,
Should all the structures of the lids be dissected away, leavuig only the veins. Fig. 30 would be a fair representation. The names of these veins are the same as those of the arteries. A is the angular ; B the infra orbital ; C shows the veins draining the palpebral margins, which are supplied by the four palpebral arteries; D shows the frontal, which forms the anastomosis and is the branch through which all parts of the orbit are drained, if there is occlusion of the ophthalmic vein, near the cavernous sinus, at the back of the orbit. E points out the infra orbital and F the anterior Iciiiporal. Thus it is seen that the drainage from the lids is abundant and this explains why it is that inflammatory conditions in this region are so easily controlled with hot or cold compresses.
If all other structures of the lids were dissected away, leaving the nerves only, Fig. 31 would give a fair idea of their arrangement. At A is seen the supra orbital nerve, after having emerged through the supra orbital foramen. At B, just outside of it, is seen the lachrymal nerve, after having pierced the lid, and at C are seen four branches coming from the facial nerve to supply the orbicularis palpebrarum. These are the only motor nerves shown in Fig. 31. The rest are all sensory nerves and are branches from the first and second divisions of the trifacial or fifth nerve. At D is seen the infra orbital nerve after emerging from the infra orbital foramen. It is the upper branch of the middle division of the trifacial nerve. At E are seen two branches emerging, the upper one passes above the trochlea and is known as the supra trochlear, while the lower passes below the trochlea and is called the infra trochlear nerve. The aggregation of small branches near the free margins of the upper and lower lids at F, is known as the plexus of Mises. It is thus seen that the lids are not wanting in sensory nerves.
If the lower portion of the nose were cut away and the deeper structures exposed between the palpebral fissure and the nosC; we would find the lachrymal (tear) conducting apparatus, A, Fig. 32, which shows the canaliculi (minute canals) above and below the lakus (small lake), B. These empty into the lachrymal sac (tear sack) C, which becomes smaller as it extends downward toward the nasal cavity and is known as the lachrymal or nasal duct, D. This empties into the nasal cavity below the inferior turbinate, E, into the space known as the inferior meatus, F. At G is shown the middle turbinate and H shows the nasal cavity proper. At I will be seen the tendo oculi or palpebral ligament cut short. The lachrymal sack occupies a triangular 8i)aec behind this structure, and in front of the tensor tarsi or Horners' muscle, and when these two structures are made taut, as is the case when the eye is closed, this arrangement causes a pulling forward and outward of the anterior portion of the lachrymal sac by the palpebral ligament, while at the same time the tensor tarsi muscle pulls the posterior portion outward and backward, thus distending the sac.
Fig. 32. Showing Canaliculi and Lachrymal Sac and canal emptying into the nasal canal.
Below the lachrymal sac there are valves in the lachrymal duct leading to the nasal cavity. These open downward and close the duct when there is suction from above, as is the case when the sac is distending, and the closing of the lids (which has distended the sac) has turned the lachrymal papilla, I, Fig. 25, so that their tips, where the lachrymal puncta are located, are pressed into the lakus, B, Fig. 32, and C, Fig. 25. As the lachrymal duct is closed there is produced a suction at these openings so that any of the lachrymal fluid (tear fluid) which may be in the lakus is drawn into the canaliculi and onward into the lachrymal sac. When the eye is opened and the lachrymal sac collapses the valves in the lachrymal ducts open and the fluid is given free passage into the nose.
Fig. 33. Showing Conjunctival Surface of the Lids.
So it is seen that we have here a truly mechanical pumping apparatus to carry the tears from the eye. At J is seen the corrugator supracilia muscle. Should we separate the lids from their attachments and leave only the attachments between them and the nose and swing them around forward, to clear the orbit, and look at the posterior or conjunctival surface of the lids, we would behold about the picture as seen in Fig. 33.
At A is seen the lachrymal gland and at B the openings through the conjunctiva where its ducts empty into the conjunctival sac at the fornix. C shows the conjunctival tissue, dissected from the back of the Hds, exposing the tarsal cartilages in which are imbedded the meibomian glands, shown at D, and their ducts opening onto the free margin of the lids, E. These glands secrete a sebaceous (oily) material which helps to lubricate the lids as they glide over the eyeball and also prevents the lids from sticking together when we sleep. Another function is that as the margins of the lids are kept oiled all the time, the tears do not flow over them so readily and as the two lids come into apposition at the outer angle first and then gradually close the palpebral fissure from without inward toward the nose, the lachrymal fluid flows inward toward the lakus instead of over the margin of the lid and on to the cheek, as it would do if it were not for this sebaceous material being so freely distributed along the free margin of the lid. This oily substance also mixes with the tears and helps to prevent friction between the eye ball and lids, as well as keeping the cornea oiled so it does not dry so quickly as it otherwise would.
Fig. 34, Showing the Anterior Attachment to the Eye Ball of the Recti Muscles.
F shows the location of the canaliculi and G the lachrymal sac ; H shows the tensor tarsi, or Homers' muscle, cut away ; I shows the corrugator supracilli ; J shows the levator labii superioris et aliqua nasi muscle (the lifter of the upper lip and the wing of the nose). This muscle arises just below the inner side of the orbit.
K shows the frontal sinus and L shows the maxillary sinus. These two sinuses sometimes become diseased and affect the eye on account of their nearness to it.
Should the lids be severed throughout their extent except at the inner side and swung out across the nose and all the tissue of the anterior part of the orbit dissected away, except the globe and recti muscles, as shown in Fig. 34, we could see the anterior portions and the attachments of the four straight recti muscles. A, B, C and D, the tendon E and pulley F, of the superior oblique and almost the whole of the inferior oblique muscle G as it arises from the floor of the orbit well forward and runs outward and slightly backward passing below the inferior rectus and is attached to the lower posterior quadrant of the eyeball. H shows the ocular conjunctiva, cut in a circle just outside of the cornea.
Should we make a horizontal cross section through the orbit and its contents, dissecting away all structures except the ligaments, fascias, etc., we would find the arrangement about as shown in Fig. 35. At A is shown the lid with the orbicularis palpebrarum muscle B, and the tarsal cartilage C, with the conjunctiva D, lining the conjunctival sac E, in which lies the plica semilunaris Q. At either side, in front, running from the Hd to the brim of the orbital bones, is seen the orbito tarsal ligamiCnt or tendo oculi F, and just back of it, at the internal side, is found the tensor tarsi muscle or Horner's muscle H. Just next to the wall of the orbit and placed between the tendo oculi and the tensor tarsi muscle is found the lachrymal sac I. At either side of the globe, running forward from the internal recti muscle K and the external recti muscle L, is seen the check ligaments of these muscles G. These are bands of fascia from the muscle sheaths, which run forward and blend with the deep fascia or ligament of Lockwood, which stretches across the front or base of the orbit within the lids, above and below the palpebral fissure. These check ligaments prevent extreme action of the muscles, which otherwise might do harmto the optic nerve, by rotating the eyeball too greatly.
Fig. 35. Cross Section of Orbit and Contents.
Just outside of the posterior portion of the eyeball is seen the space of Tenon N, which is a lymph space, and outside of it Tenon's sheath or capsule. Tenon's space is crossed by loose bundles of connective tissue, running from the sclera to Tenon's capsule and vice versa. These are known as trabeculae (fibrous bands). These are very loose and of sufficient length to allow free movements of the eyeball in the socket formed by Tenon's capsule. When the rectimuscles come near to the eyeball, the sheaths of the muscles blend with the capsule of Tenon, as shown at J, and it must be borne in mind that this connection greatly modifies the action of the recti muscles. Posteriorly is seen the optic nerve O, surrounded by the intra vaginal space P, and surrounding this space is found the sheath of the optic nerve, which is continuous with the sclerotic, and outside of the
Fig. 36. Vertical Cross Section of Orbit.
Optic nerve sheath is found the supra vaginal space S, which is continuous with the space of Tenon. This is surrounded by Tenon's capsule, filling in the spaces between the eyeball and the posterior or apex of the orbit, and between the muscles and other structures, is found the orbital fat T. This acts as a cushion for the eyeball as well as filling the spaces between the structures of the orbit.
Fig. 36 shows a vertical cross section of the orbit ; above and in front is seen the upper lid A and below in front is the lower lid B, the slit between them, the palpebral fissure C. Back of the lids, and in front of the cornea, is the conjunctival sac and above and below is seen the fornices (folds) D, where the conjunctiva leaves the lid (palpebral conjunctiva) and folds on itself, forming the fornix and then covering the anterior of the eyeball (ocular conjunctiva), ceasing at the edge of the cornea. At E, Fig. 36, is found the check ligaments of the levator palpebral Â«"n
Fig. 37. Showing the Muscles of the Orbit.
perioris J, and the iriferior rectus L, and at F is seen a band of tissue running from the upper side of the superior rectus muscle K to the lower side of the levator palpebrae. This band of tissue forms the check ligament of the superior rectus riiuscle. At H is seen the deep fascia or ligament of Lockwood. At G is seen the inferior oblique muscle with its sheath and the intimate relation of its sheath with the sheath of the inferior rectus I, and the capsule of Tenon. This is of importance from the fact of the modification of the action of the inferior obHque which it causes. At M is seen the orbital fat.
Should the roof of the orbit be cut away and all the structures of the orbit dissected away except the muscles, eyeball and the lachrymal gland, we would see about such a picture as shown by Fig. 37.
Fig. 38. Showing Vessels of Orbit.
The levator palpebrae superioris A, which occupies the uppermost portion of the orbit, is cut and thrown forward and exposes the superior rectus B, which lies just below it. At the inner side and above is shown the superior oblique C, running through the trochlea or pulley D, then its tendon E running obliquely outward and backward to its attachment to the globe F, beneath the superior rectus. Just beneath and outside of tlie superior obHque, is seen the internal rectus muscle K. At A IS seen the external rectus muscle and between it and the eyeball is seen the attachment of the inferior oblique muscle H. At the floor of the orbit, just back of the eyeball, is shown a small portion of the inferior rectus muscle J. All these muscles, except the inferior oblique, arise from the ligament of Zinn, which surrounds the optic foramen at the apex of the orbit. In the upper anterior portion of the orbit is shown the lachrymal gland I,
Should the roof of the orbit be cut away and all the structures of the orbit dissected away, except the arteries, veins, eyeball and lachrymal gland, we would see a picture about as portrayed in Fig. 38. Coming from the internal carotid artery, comes off the ophthalmic artery A, which enters the orbit through the optic foramen with the optic nerve. It first gives off the lachrymal branch D, which takes a course outward and upward to the position of the gland I, which it supplies, and after giving off branches to the gland, it pierces the lid and supplies the superficial structures of the lid, at the upper outer side of the orbit. The next branches given off are the several short posterior and long posterior ciliary arteries B, which run forward and pass into the eyeball in a circle around the optic nerve and run forward in the choroid. Shortly after these branches are given off, the arteria centralis retina is given off. This artery passes into the optic nerve ten or twelve millimeters back of the eyeball and passes through the choroidal fissure and gives the blood supply to the retina. There are also muscular branches given off which pass into the muscles and run forward in them to their attachments to the eyeball. These arteries pierce the sclerotic and enter the eyeball and are then known as the anterior ciliary arteries C. Then the supra orbital branch is given off, which runs upward and forward and passes out of the orbit through the supra orbital foramen and supplies the structures just above the orbit. The posterior H, and anterior E, ctliemoid branches, are then given off. These pass through the posterior and anterior ethmoidal foramen, which are found in the upper posterior portion of the internal bony wall of the orbit. They first pass into the cranial cavity, then run downward through the cribriform plate of the ethmoid bone to supply the internal and anterior portion of the nose. Anteriorly the ophthalmic artery gives off the frontal artery. These two then pierce the lids and one or the other forms an anastomosis with the angular artery, which is the terminal branch of the facial artery. This is of importance, from the fact that if the internal carotid artery or the posterior portion of the ophthalmic artery should be occluded (stopped up), collateral circulation would be established by this route. Accompanying all the larger arteries are found the veins, which carry the return flow of blood, and these veins are known by the sam.e name as the artery which they accompany. However, there are no veins leaving the eyeball with the posterior ciliary arteries, but the drainage from the choroid is by the Vena Vorticosse, J. These leave the eyeball just back of the equator and there are usually about five in number. All these veins join to form the ophthalmic vein L, which passes backward through the sphenoidal fissure and empties into the cavernous sinus. As shown at B, Fig. 38, the ophthalmic artery gives off several small branches which enter the eyeball in a circle around the optic nerve. There are some twelve to twenty of these, which are known as the short posterior ciliary arteries and two known as the long posterior ciliary arteries. Should we enucleate the eyeball and dissect away all the tissues down to the choroid and leave only the long and short ciliary arteries. Fig. 39 would give us a fair representation of their distribution. The short posterior ciliary arteries. A, from twelve to twenty in number, enter the eyeball by piercing the sclerotic in a circle just outside of the optic nerve. Immediately after entering the sclerotic, they divide, the main portion running forward (See P, Fig. 23) and enter the choroid, breaking up into smaller vessels and lay in three strata or layers, the layer of large blood vessels, the layer of small blood vessels, which is immediately beneath it, and the chorio capillaris or capillary layer, which is the innermost layer and is just beneath the retina.
Fig. 39. Showing Ciliary Arteries.
The larger vessels run forward in the choroid and ciliary processes to the ciliary bodies, which are just inside of the ciliary muscles B, where they end in capillary loops and turn back as venous capillaries, while the branches given off in their course form the layer of smaller blood vessels and these again break up into the chorio capillaris. The branches that turn toward the optic nerve, just after the short posterior ciliary arteries enter the sclerotic (See Fig. 23) form ii circle of anastomosis around the optic nerve, known as circulus of Zinn, as shown at O, Fig. 23. This circle furnishes a copious blood supply to the head of the optic nerve as well as furnishing a path for the establishment of collateral circulation, when there is trouble with the branches which supply the nerve sheath with which they also connect or anastomose.
There are two long posterior ciliary arteries, C, which enter the eyeball a little farther out than the short posterior ciliary arteries, one to the outer side of the nerve and one to the inner side. These run forward clear to the ciliary region, before they branch, and then when they do branch they join with, or anastomose with the anterior ciliary arteries, which enter the eyeball at the attachments of the recti muscles D, and these then form what is known as the circulus major (larger) of the Iris E, Fig. 39, and 2, Fig. 23. From this circle is given off the vessels for the iris, which run radially in toward the pupil G, and when these come near to the free margin of the iris another circle of anastomosis is formed, which is known as the circulus minor F, Fig. 39, and E, Fig. 23. Inside of this, toward the pupil, are given off arterial capillaries which turn back as veins, which are drained by the anterior ciliary veins, which leave the eyeball at the muscular attachments D. At H is seen the vena vorticosa (whirlpool) and at I is seen the optic nerve.
Should the eyeball be enucleated and the sclerotic and the tissues dissected oft', leaving only the veins of the posterior four-fifths of the eyeball, we would find practically the arrangement as seen in Fig. 40. The smaller veins pass back from the ciliary bodies at A from underneath the ciliary muscle F. These veins constantly join or anastomose with others and form four or five whirls, B, finally join to form the four or five vena vorticosae (whirlpool veins) C, which leave the eyeball just back of the equator and empty into the ophthalmic vein. See J. Fig. 38.
Fig. 40. Veins of the Eyeball
As previously mentioned the ophthalmic artery gives off one branch, which enters the optic nerve at its under surface and about ten to tv^elve millimeters back of the eyeball> which is known as the arteria centralis retina (central artery of the retina), from the fact that it enters the eye ball at the optic disc and spreads out to supply the retina (See 3, Fig. 23), and if we should take an eyeball and make a coronal cut down through it at the equator, then hold it up and look at the inner surface of the globe, we would see the picture as portrayed in Fig. 41. At the disc A are seen the arteries emerging from the head of optic nerve or disc and the veins leaving. The artery first breaks up into two branches, one running upward, the other downward. These are known as the upper, B, and lower, C, branches. These in turn each divide into two branches. Each of these four branches runs obliquely outward from the disc, the upper one running inward toward the nose is called the superior nasal, D, and the one running upward and outward and toward the temple is called the superior temporal, E, while the one below, running inward toward the nose is known as the inferior nasal, F, and the one running downward and outward toward the temple is called the inferior temporal, G.
Fig. 41. Arteries of the Retina
The farther divisions of these arteries are unnamed. However, there are usually one or two small arteries, which run from the disc toward the maculae, which when present arc called the macular arteries, H. These arteries and veins lie in the retina, I, and the arteria centralis retina is what is known as a terminal artery, or in other words, it forms no anastomosis with any other set of arteries, consequently when it breaks up into capillaries, these turn back as veins. These keep joining together and get larger and larger until there are large veins formed, which are named the same as the arteries which they accompany. As there is usually a vein accompanying each artery, these join at the disc and form the vena centralis retina which leaves the eyeball within the optic nerve and lies within it for some ten or twelve millimeters. It then leaves the nerve and empties into the ophthalmic vein (See Fig. 23). The fact of the arteria centralis retina being a terminal artery in the retina having no collateral loops or anastomosis, as is the case in almost all other portions of the body, makes this of especial clinical significance, for if it becomes occluded, the nourishment is cut off from the retina and sight is lost and the retina atrophies in an exceedingly short period. Just to the temporal side of the disc is seen the macula (spot) and at its center the fovea centralis (central spot) J. It is so named from the fact that it is the thinnest spot in the whole retina and turns yellow after death. It is not seen as a yellow spot during life, with an ophthalmoscope, as some inexperienced ones think, but as a dark area devoid of visible blood vessels and the yellow appearance which we see in examining the posterior inner surface of the eyeball after death is a post mortem (after death) change. K shows the choroid and L the scleral coat of the eyeball.
Should we cut away the roof of the orbit and dissect away all the tissues except the nerves, eyeball, recti muscles, levator superioris and the lachrymal gland, Fig. 42 would be a fair representation of what we would observe. At A we see the sixth cranial or abduceus nerve, which innervat-es the external rectus muscle J, and at B is seen the third cranial nerve or the motor oculi, which furnishes nerve impulse to the levator palpebrae superioris K, the superior rectus L, the internal rectus M, the inferior rectus N, and the inferior oblique O, besides giving branches to the ciliary or lenticular ganglion Q. At C is shown the fourth cranial or patheticus (cry) nerve which supplies the superior oblique muscle I. At D is shown the fifth cranial, trigeminus or trifacial nerve, and E the gasserian ganglion on the fifth nerve, and at F the upper or ophthalmic branch of the fifth nerve which supplies sensation to the orbit, eyeball and its structures as well as the lids, and G the superior maxillary nerve or the middle branch of the trifacial or fifth nerve, and H the lower branch or the inferior maxillary nerve. However, we are only particularly interested in the first, upper or ophthalmic branch, and just slightly interested in the second, or superior maxillary branch, for the ophthalmic nerve gives off first the nasal branch, R, which
Fig. 42. The Nerves of the Orbit from above.
runs upward and inward through the orbit, giving a branch or root S to the lenticular ganglion Q, then passes out of the orbit to re-enter the cranial cavity through the ethmoidal foramen, then it leaves the cranial cavity again through the cribiform plate of the ethmoid bone and supplies sensation to the anterior portion and the tip of the nose, and it is this branch which accounts for the reflexes between the nose and the eye. Then the ophthalmic gives off the lachrymal branch, T, which runs upward and outward to the lachrymal gland, I J, and after supplying the gland it pierces the lid and supplies sensation to the upper outer part of the lid (See B, Fig. 31). The ophthalmic then gives one or two branches or roots to the lenticular ganglion direct and continues upward and forward. The main portion of the nerve leaves the orbit through the supra orbital foramen and is known as the supraorbital nerve (See A, Fig. 31). However, just before leaving the orbit it gives oflf a branch which divides, and one branch pierces the lid above the pulley or troclea, V, of the superior oblique muscle.
Fig. 43. The Nerves of the Orbit from the side.
This branch is known as the supra trochlear (See E, Fig. 31) ; the other one pierces the lid just below the trochlea and is known as the infra trochlear branch (See E, Fig. 31). Should we make a vertical section of the walls of the orbit and dissect away the tissues, leaving only the eyeball, nerves and lachrymal gland, we would have the appearance as shown in Fig. 43. At A is shown the sixth or abduceus nerve, which is cut and thrown up at I, and at B is seen the third cranial or motor oculi nerve, and at J its branches or roots to the lenticular ganglion, K, and at D is shown the fifth cranial nerve, and at E the gasserian ganglion. At F is shown the first division, which is known as the ophthalmic nerve, and at L is shown its branches to the lenticular ganglion. At G is shown the second division or superior maxillary nerve, but in the study of the eye we are only interested in two of its branches ; first the one shown at M, known as the orbital nerve, which goes to the lower outer side of the eyeball, forming an anastomosis with the lachrymal nerve, T, and the terminal branch runs forward and passes out onto the face through the infra orbital foramen (See D, Fig. 31) and supplies the sensation to the lower lid and region just below the eye. This branch is known as the infra orbital nerve.
The lenticular ganglion, K, is of vast importance to the eyeball. It is a small pinkish body about the size of a pinhead and is situated some seven to ten millimeters back of the eyeball. On the outer side of the optic nerve, between it and the ophthalmic artery, it receives filaments, or roots, J, from the motor oculi nerve,, which are motor from the nasal nerve, L, as well as from the ophthalmic nerves which are sensory. It also receives filaments or roots from the sympathetic nervous system, which comes from the carotid plexus. Thus it is seen, there are motor, sensory and sympathetic filaments received by it. Then from this ganglion is given oflf the posterior ciliary nerves, N. These are mixed nerves and carry motor, sensory and sympathetic fibers. These nerves, from twelve to twenty in number, enter the eyeball posteriorly with the posterior ciliary arteries (See A, Fig. 39, and A, Fig. 44). These pierce the sclerotic just outside of the optic nerve in a circle and pass forward mostly in the supra choroidal space, and if we should enucleate an eyeball and dissect away the sclerotic and all other structures except the nerves, we would have a picture as shown in Fig. 44. The posterior ciliary nerves, run forward in the supra choroidal space and give numerous branches to the choroid, C, in their course. They then break up into small branches, D, and these form a plexus in the ciliary muscle, E, and from thif plexus is given off branches to the ciliary muscle which are motor to the ciliary bodies which are sympathetic and sensory, then other branches to the iris, F, which are sensory, motor and sympathetic, the motor for the spincter pupillae (See K, Fig. 23), sympathetic, for the dilator pupillae muscle. Other branches go from the ciliary plexus to the cornea which are entirely sensory. Thus it will be seen that the nerve supply to the eye is abundant and of all three varieties, motor, sensory and sympathetic. Having covered the gross anatomy of the eye pretty thoroughly, we will now pass to the more minute anatomy or Histology and in so doing it is well for the reader to be familiar with the gross anatomy, in order to be familiar with the relation of parts.
Fig. 44. Showing Ciliary Nerves.
- Next: Histology
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Cite this page: Hill, M.A. (2020, July 3) Embryology Book - The Embryology Anatomy and Histology of the Eye 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Embryology_Anatomy_and_Histology_of_the_Eye_2
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