Human Anatomy: Part 2 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2008

Special Part
Sense organs, organa sensuum
Organ of vision - Accessory structures of the eye

Class="center">Muscles of the Eyeball

The muscles located in the Orbit are divided into straight (recti) and oblique muscles according to their appearance and topography (Fig. 217). All muscles, except the inferior oblique, originate from a common tendinous ring, anulus tendineus communis, which is anchored deep within the orbit to the inner opening of the canalis opticus and the medial part of the fissura orbitalis superior. Extending forward, they form the so-called muscular cone. The Base of the cone is situated anterior to the equator of the eyeball, where the Muscle tendons attach to the sclera. Upon contraction, these muscles rotate the eye, thereby adjusting the positions of the pupil and the visual axis. The superior rectus muscle, m. rectus superior, passes above the Optic nerve to the upper part of the eyeball, rotating it upward and slightly laterally. The inferior rectus muscle, m. rectus inferior, passes beneath the optic nerve to the lower part of the eyeball, rotating it downward and slightly medially. The medial rectus muscle, m. rectus medialis, runs along the medial side of the optic nerve and turns the eyeball inward. The lateral rectus muscle, m. rectus lateralis, courses along the lateral side of the optic nerve and abducts the eyeball laterally. The superior oblique muscle, m. obliquus superior, runs between the superior and medial recti, transitioning into a tendon that loops through the trochlea, changing its direction: it proceeds at an acute angle backward and laterally, passes under the superior rectus muscle, and attaches to the superior-lateral surface of the eyeball posterior to the equator; it rotates the pupil downward and laterally. The inferior oblique muscle, m. obliquus inferior, originates from the orbital surface of the Maxilla (lateral to the fossa sacci lacrimalis), passes between the inferior rectus muscle and the inferior wall of the orbit laterally and upward, and attaches to the lateral surface of the eyeball posterior to the equator; it rotates the pupil upward and laterally.

Fig. 217. Muscles of the eyeball

There are two additional muscles within the orbit. The levator palpebrae superioris muscle, m. levator palpebrae superioris, originates near the internal opening of the optic canal, extends forward, and attaches to the upper border, the tarsus superior. The orbital muscle, m. orbitalis, is located posterior to the eyeball. Unlike the previous seven muscles, which consist of striated muscle fibers, the orbital muscle is composed of Cytology/cytology/32.html">Smooth Muscle tissue. It is anchored to the Connective Tissue membrane covering the fissura orbitalis inferior. This muscle influences THE POSITION OF the eyeball in the orbit and regulates venous Blood Circulation.

The Orbital Fasciae, fasciae orbitales, comprise a set of structures that ensure the stable positioning of the eyeball and accessory Organs within the orbit. Clinically, it is significant that these fasciae divide the orbit into distinct compartments, thereby acting as barriers against the spread of purulent-inflammatory processes.

METABOLISM/2.html">THE CONCEPT OF orbital fasciae also includes its periosteum, the periorbita. In the region of the canalis opticus and fissura orbitalis superior, it is tightly fused with the dura mater. In other areas, the periorbita is easily separated from the cranial bones.

The orbital septum, septum orbitale, closes the orbit anteriorly. It is a connective tissue sheet attached to the margins of the orbit and connected to the tarsus superior et inferior. Near the medial and lateral walls of the orbit, the orbital septum sends off extensions that form fascial sheaths for the lacrimal sac and lacrimal gland. This fascia separates the eyelid region from the orbit proper.

The orbital spaces not occupied by the eyeball, its muscles, and the Lacrimal Apparatus are filled with the orbital fat body, corpus adiposum orbitae. The muscular cone divides it into an internal part (between the extraocular muscles and the eyeball) and an external part (between the extraocular muscles and the orbital walls). Both parts communicate with each other through the gaps between the muscles. The fat body acts as a soft, elastic cushion for the eyeball. It is separated from the eyeball by the Sheath of the eyeball, Vagina bulbi, which is a dense fibrous membrane. The sheath is fairly tightly connected to the fat body, whereas its surface facing the eyeball is attached to it only by separate connective tissue strands that do not restrict eye movement. The surfaces of the sclera and the sheath enclose a slit-like space—the episcleral space, spatium episclerale.

The lacrimal apparatus, apparatus lacrimalis, includes the lacrimal gland and the lacrimal drainage system (Fig. 218).

The lacrimal gland, glandula lacrimalis, secretes a clear fluid containing Water, the enzyme Lysozyme, and minor amounts of salts and Proteins. The larger upper part of the gland lies in a fossa on the superior orbital wall and is called the orbital part, pars orbitalis (while the smaller lower part is the palpebral part, pars palpebralis). The tendon of the levator palpebrae superioris muscle passes between these two tubulo-alveolar parts. The excretory ducts of the gland (10–12 in number) open into the lateral part of the conjunctival sac. The tear fluid flows through the capillary cleft formed by the palpebral conjunctiva, the bulbar conjunctiva, and the cornea along the margins of the upper and lower eyelids toward the medial angle of the eye, draining into the lacrimal canaliculi.

The superior and inferior lacrimal canaliculi, canaliculus lacrimalis superior et inferior, begin with the lacrimal puncta, puncta lacrimalia, at the apex of the lacrimal papillae, papillae lacrimales, in the medial angle of the eye. Their diameter is about 0.5 mm, and their length is 8–10 mm. Initially, both run vertically—the upper one upward, the lower one downward—and then turn horizontally, opening into the lacrimal sac either separately or via a common trunk.

The lacrimal sac, saccus lacrimalis, is lodged in a fossa on the medial wall of the orbit. Its narrowed blind end, the fornix of the lacrimal sac, fornix sacci lacrimalis, is directed upward. Inferiorly, it transitions (without a sharp demarcation) into the nasolacrimal duct, ductus nasolacrimalis (Fig. 218). The latter is housed within the nasolacrimal canal and opens into the inferior nasal meatus. The total length of the lacrimal sac and duct is 5 cm.

The walls of the lacrimal sac and duct consist of Dense connective tissue lined with stratified squamous epithelium. The medial palpebral ligament, lig. palpebrale mediale, passes anterior to the lacrimal sac, and the lacrimal part of the m. orbicularis oculi attaches to its walls.

The upper and lower eyelids, palpebrae superior et inferior, are Skin folds situated in front of the eyeball that completely cover it when closed. Each eyelid has an anterior (convex) and a posterior (concave) surface, facies anterior et posterior, and an anterior and posterior margin, limbus palpebralis anterior et posterior. The margins enclose the palpebral fissure, rima palpebralis, and the lateral junctions of the upper and lower eyelid margins form commissures that close the corners of the eye. Thus, there are the medial and lateral palpebral commissures, commissura palpebralis medialis et lateralis, and the medial and lateral angles of the eye, angulus oculi medialis et lateralis. A small recess, the lacrimal lake, lacus lacrimalis, is formed in the region of the medial angle. This area also contains a vestige of the nictitating membrane found in lower vertebrates—the semilunar fold of the conjunctiva, plica semilunaris conjunctivae.

Fig. 218. Lacrimal apparatus of the left eye

Eyelashes, cilia, grow along the anterior margin of the eyelids and are more numerous and longer on the upper eyelid. Eyebrows, supercilia, are located at the boundary between the upper eyelid and the forehead skin.

Within the thickness of the eyelids lie plates of extremely dense connective tissue resembling Cartilage, which is why they are called the tarsal plates, tarsi. Near the medial angle of the eye, both tarsal plates connect to the medial palpebral ligament, lig. palpebrale mediale, which runs subcutaneously from the medial angle of the eye to the frontal process of the maxilla. Bundles of the palpebral part of the m. orbicularis oculi run across the anterior surface of the tarsus. Embedded within the tarsal plates are the tarsal (modified sebaceous) glands, glandulae tarsales, which secrete an oily substance, sebum palpebrale. Their long excretory ducts open near the posterior margin of the eyelid. The outer surface of the eyelids is covered with thin skin (devoid of subcutaneous tissue). The inner surface is lined with the conjunctiva, tunica conjunctiva. It resembles a mucous membrane—pink in color, supported by a connective tissue framework, and containing small mucous glands and lymphoid follicles. The palpebral conjunctiva, tunica conjunctiva palpebralis, reflects onto the eyeball as the bulbar conjunctiva, tunica conjunctiva bulbaris. It is loosely attached to the sclera by loose connective tissue and terminates at the corneoscleral junction. The transitional zones between the palpebral and bulbar conjunctiva are called the superior and inferior conjunctival fornices, fornix conjunctivae superior et inferior. Together, the conjunctiva encloses the conjunctival sac, saccus conjunctivalis.

The eyeball and Accessory structures of the eye receive their blood supply from the ophthalmic artery, a. ophthalmica. The retina and optic nerve, n. opticus, are vascularized by the central retinal artery, a. centralis retinae, while the vascular tunic (uvea) is supplied by the long and short posterior ciliary Arteries, aa. ciliares posteriores longi et breves, and anterior ciliary arteries, aa. ciliares anteriores, which anastomose to form two arterial rings within the iris: the Major and minor iridial arterial circles, circulus arteriosus iridis major et minor. Venous blood drains via the ophthalmic Veins, vv. ophthalmicae, into the cavernous sinus, sinus cavernosus.

Sensory Innervation of the orbital contents is provided by the ophthalmic nerve, n. ophthalmicus, whereas the striated muscles of the eye are innervated by the oculomotor, trochlear, and abducens nerves (n. oculomotorius, n. trochlearis, et n. abducens). The sympathetic and parasympathetic innervation of the smooth muscles and the lacrimal gland was discussed above.