LECTURE No. 1. The structure of the eye (part I)

The eye as an integral part of the so-called opto-vegetative (OVS) or photoenergetic (FES) system of the body is involved in the adaptation of the internal environment of the body to external conditions. The vast majority of information about the world around comes to the child through the organ of vision. The eye is in a figurative and literal sense a part of the brain, placed on the periphery.

1. The structure of the orbit

When studying the anatomy of a child, it must be remembered that the orbit in children under the age of one year approaches a trihedral prism in shape. Later it takes the form of a truncated tetrahedral pyramid with rounded edges. The base of the pyramid faces outwards and anteriorly, the apex inwards and backwards. In newborns and children of the first year of life, the angle between the axes of the orbits is sharper, which creates the illusion of convergent strabismus. However, this imaginary strabismus gradually disappears, as the angle between the axes of the orbits increases. The upper wall of the orbit borders the cranial cavity and is formed in front by the orbital part of the frontal bone, and behind by the lesser wing of the sphenoid bone. In the outer corner of the wall, a recess for the lacrimal gland is revealed, and at the place where the upper wall passes into the inner wall, a notch (or hole) for the superior orbital vein and artery is determined. There is also a spike block through which the tendon of the superior oblique muscle is thrown. In the process of comparing the orbits in terms of age, it is revealed that in children the upper wall of the orbit is thin, there is no pronounced superciliary tubercle.

When studying the outer wall of the orbit, it is noted that it borders on the temporal cranial fossa. The orbital process of the zygomatic bone separates the orbit from the maxillary sinus, and the sphenoid bone of the inner wall separates the contents of the orbit from the ethmoid sinus. The fact that the upper wall of the orbit is simultaneously the lower wall of the frontal sinus, the lower upper wall of the maxillary sinus, and the inner side wall of the ethmoidal labyrinth, explains the relatively unhindered and rapid transition of the disease from the paranasal sinuses to the contents of the orbit and vice versa.

At the top of the orbit in the lesser wing of the sphenoid bone, a round opening for the optic nerve and ophthalmic artery is defined. The superior orbital fissure is located outside and below this opening between the large and small wings of the sphenoid bone and connects the orbit with the middle cranial fossa. All the motor branches of the cranial nerves pass through this gap, as well as the superior ophthalmic vein and the first branch of the trigeminal nerve, the ophthalmic nerve.

The inferior orbital fissure connects the orbit to the inferior temporal and pterygoid fossae. The maxillary and zygomatic nerves pass through it.

The entire orbit is lined with periosteum; in front of the bony edge of the orbit to the cartilage of the eyelids is the tarsoorbital fascia. With closed eyelids, the entrance to the orbit is closed. Tenon's capsule divides the orbit into two sections: the eyeball is located in the anterior section, and the vessels, nerves, muscles, and orbital tissue are located in the posterior section.

2. Muscles and soft tissues of the eye

The superior, inferior, external and internal rectus and superior and inferior oblique muscles related to the oculomotor muscles, as well as the muscle that lifts the upper eyelid, and the orbital are located in the orbit. Muscles (except for the inferior oblique and orbital) start from the connective tissue ring that surrounds the visual opening, and the inferior oblique muscle from the inner corner of the orbit. The muscles are separated from the limbus by an average of 5,58,0 mm. In newborns, this value is 4,05,0 mm, and in children aged fourteen years, 5,07,5 mm. The superior and inferior oblique muscles are attached to the sclera 16 mm from the limbus, the external rectus turns the eye outward, the internal inwards, the superior moves upwards and inwards, the inferior downwards and inwards.

The eyelids cover the front of the eye socket. The connection of the free edges of the lower and upper eyelids with each other occurs through external and internal adhesions. There is a variation in the width and shape of the palpebral fissure. Normally, the edge of the lower eyelid should be 0,51,0 mm below the corneal limbus, and the edge of the upper eyelid should cover the cornea by 2 mm. In newborns, the palpebral fissure is narrow, its vertical size is 4,0 mm, horizontal 16,5 mm. The skin of the eyelids is thin, delicate, poor in fatty tissue, loosely connected with the underlying parts, the underlying vessels shine through it.

The muscles of the eyelids are poorly developed. The muscular layer of the eyelids is represented by a circular muscle, innervated by the facial nerve and providing closure of the eyelids. Under the muscle is cartilage, in the thickness of which the meibomian glands are located, translucent in the form of yellowish radial stripes. The back surface of the eyelids is covered with a connective sheath. On the front edge of the eyelids there are eyelashes, near the root of each eyelash there are sebaceous and modified sweat glands. The lifting of the upper eyelid is carried out with the help of the muscle of the same name, which is innervated by the branches of the oculomotor nerve.

The blood supply to the eyelids is carried out by the external branches of the lacrimal artery, the internal arteries of the eyelids and the anterior ethmoid artery. The outflow of blood occurs through the veins of the same name and further into the veins of the face and orbit.

Lymphatic vessels located on both sides of the cartilage flow into the anterior and submandibular lymph nodes. Sensitive innervation of the eyelids is carried out by the first and second branches of the trigeminal nerve, motor by the third and seventh pair of cranial nerves and the sympathetic nerve.

3. Connective sheath of the eye

The connective sheath, the conjunctiva, covers the eyelids from the inside, passes to the sclera and continues in an altered form to the cornea. There are three sections of the conjunctiva: cartilage (or eyelids), transitional fold (or fornix) and the eyeball. All three sections of the conjunctiva with closed eyelids form a closed slit cavity, the conjunctival sac.

The blood supply of the conjunctiva is carried out by the arterial system of the eyelids and the anterior ciliary arteries. The veins of the conjunctiva accompany the arteries, the outflow of blood occurs in the system of facial veins and through the anterior ciliary veins of the orbit. The conjunctiva of the eye has a well-developed lymphatic system. Lymph enters the anterior and submandibular lymph nodes. The conjunctiva receives sensory nerves in large quantities from the first and second branches of the trigeminal nerve.

The conjunctiva in young children has a number of features. It is thin and tender, somewhat dry due to insufficient development of the mucous and lacrimal glands, subconjunctival tissue is poorly developed.

The sensitivity of the conjunctiva in a child of the first year of life is reduced. The conjunctiva performs mainly protective, nourishing and absorption functions.

4. Lacrimal organs

The lacrimal organs consist of a tear-producing and tear-removing apparatus. The tear-producing apparatus includes the lacrimal gland and Krause's glands. The lacrimal gland is located in the bony cavity of the upper outer part of the orbit. Twenty or more (up to thirty) excretory ducts of the gland open into the lateral part of the superior conjunctival fornix. Tear production is carried out mainly from the second month of a child's life. Krause's lacrimal glands are located in the conjunctiva of the upper and lower fornix and secrete tear fluid constantly.

The lacrimal ducts include the lacrimal puncta, lacrimal canaliculi, lacrimal sac, and lacrimal canal. The lacrimal openings normally gape, facing the eyeball and immersed in the lacrimal lake. They lead to the superior and inferior lacrimal ducts, which empty into the lacrimal sac. The wall of the lacrimal sac consists of a mucous membrane covered with a bilayer columnar epithelium and submucosal tissue. The lower section of the lacrimal sac passes into the lacrimal canal, which opens under the inferior nasal concha at the border of its anterior and middle thirds. In 5% of cases at birth, the lacrimal canal is closed with a gelatinous film. If it does not resolve, the outflow of tears stops, its stagnation is formed, resulting in dacryocystitis of newborns.

Author: Shilnikov L.V.

>> Forward: Structure of the eye (part II) (Structure of the eyeball. Cornea and sclera. Vascular tract of the eye. Retina and optic nerve. Lens and vitreous body. Blood supply and innervation of the eye)

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