Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009

CONJUNCTIVA

It is a thin mucous membrane which attaches eyelids to the eyeball. It covers inner surface of the eyelids and reflected to cover anterior part of the sclera upto the limbus (Fig. 2-1).

REGIONS OF CONJUNCTIVA

a.Palpebral

i.Marginal

ii.Tarsal

iii.Orbital.

b.Fornix

c.Bulbar

d.Limbal.

STRUCTURE

It consists of:

Epithelium

Marginal and limbal parts are lined by stratified squamous epithelium. The tarsal and orbital parts are lined by 2 layers of epithelium. The deeper layer is composed of cubical cells and superficial one is of cylindrical cells. At the fornix, often a 3rd layer of polyhedral cells is encountered between the 2 layers. But the tarsal conjunctiva of lower eyelid is composed of 3-5 layers of cells. From the fornix to the limbus more layer of cells are added between the superficial cylindrical and deep cubical cells. Goblet cells, i.e.

Anatomy of Appendages of the Eyeball 33

d.Drainage of tear through lacrimal system by pumping effect of lids on the lacrimal sac during blinking.

e.Emotional reactions.

Fig. 2-2: Surface anatomy of the eyelid

Palpebral aperture or fissure measures about 11–12 mm × 28 mm in elderly and 14–15 mm × 28 mm in adults. The lateral canthus is about 2 mm above the plain of the medial canthus (Fig. 2-2). Caruncle is a small area of skin containing glands which opens into fine hair follicles. Plicae Semilunaris is a crescentic fold, just lateral to the caruncle and represents 3rd eyelid, i.e. nictitating membrane of lower vertebrates. Normal blink rate is 12-20/minute.

LID MARGIN

Both upper and lower eyelid margin is 2 mm broad, has a rounded anterior border and a sharp posterior border. The lacrimal puncta divides it into medial smaller lacrimal part and larger lateral ciliary part. The lacrimal part is devoid of any eyelashes and tarsal glands. Just in front of posterior border lies a row of small openings of tarsal glands. Just in front of these openings, a thin grey line can be seen, where the eyelid can be easily separated into 2 distinct layers (Fig. 2-3).

Anatomy of Appendages of the Eyeball 35

c.The layer of striated muscle: Muscle fibres of orbicularis oculi are arranged concentrically around the palpebral fissure. Muscle of Riolan is a part of this muscle which lies adjacent to whole thickness of the lid margin.

d.The submuscular connective tissue: It is similar to the subcutaneous tissue. It represents the grey line of the eyelid margin, i.e. through this plane the eyelid can be easily split into 2 distinct portions. Fibres of levator palpebrae superioris (present only in the upper eyelid) traverses through this layer. Some of the fibres pass through the orbicularis fibres to be attached to the skin to form the superior palpebral furrow. Residual fibres are attached to the lower-third of the tarsal plate anteriorly.

e.The fibrous layer: It consists of:

i.Tarsal plate: The central thick portion present in both the eyelids forming the skeleton of the eyelids.

ii.Orbital septum: The peripheral thin portion.

f.The layer of non-striated muscle (muscle of Müller): This muscle takes origin from levator palpebrae superioris (in the upper eyelid) and inferior rectus (in the lower eyelid). They are inserted into the attached border of the tarsal plates, i.e. upper border of superior tarsus and lower border of inferior tarsus.

g.The palpebral conjunctiva: This layer is firmly attached to the tarsal plates.

GLANDS OF THE EYELID

Meibomian Glands (or Tarsal Glands)

i.They are long sebaceous glands, located within the substance of tarsal plates and secrete sebum.

ii.They are arranged vertically parallel with each other.

iii.They extend from the attached margin to the free margin. So, they open into the eyelid margin as a single row of orifices (25 for the upper lid and 20 for the lower lid), just anterior to the posterior border (Figs 2-3 and 2-4).

iv.The sebaceous material secreted by these glands constitute the outermost lipid layer of precorneal tearfilm. It prevents overflow of lacrimal secretion and evaporation of underlying tear secretion.

v.It is also responsible for airtight closure of the eyelids. Thus, it also prevents maceration of the skin by the tear.

Anatomy of Appendages of the Eyeball 37

into lateral part of lower fornix. It is a tubuloracemose gland similar to parotid gland structurally.

Accessory lacrimal glands of Krause are located in the subconjunctival areolar tissue of the fornices, mostly in the superior. Accessory lacrimal glands of Wolfring are located in the region of the upper border of the tarsus of the upper eyelid.

DRAINAGE SYSTEM (LACRIMAL PUNCTA,

CANALICULUS, COMMON CANALICULUS,

LACRIMAL SAC AND NASOLACRIMAL DUCT – FIG. 2-5)

i.Lacrimal puncta are small oval openings situated on small elevations, one in each lid, 6 mm from the medial canthus at the junction of ciliary and lacrimal part of the lid margin.

ii.Lacrimal canaliculus, 2 in number, upper and lower, connects lacrimal sac with the lacrimal punctum. It consists of a vertical (2 mm) and horizontal portion (6–8 mm). Usually, each canaliculus opens separately into the lacrimal sac (2.5 mm below the apex). Occasionally, they join together to form common canaliculus before opening into the lacrimal sac. A mucosal fold present at this point to prevent reflux of tear is called “valve of Rosenmüller.”

iii.Lacrimal sac is lodged in the lacrimal fossa, formed by the lacrimal bone and the frontal process of the maxilla. Portion of the lacrimal sac above the medial palpebral ligament is called fundus. Distended lacrimal sac measures 15 mm in length and 5–6 mm in width. It is enclosed by lacrimal fascia. In the between lacrimal sac and the fascia lies a fine venus plexus.

iv.Nasolacrimal duct is the downward continuation of the lacrimal sac. It is 12–24 mm in length and 4 mm in diameter. It is lodged in a bony canal and passes backwards, laterally and downwards to open into the inferior meatus of the rose. Upper end of nasolacrimal duct is the narrowest portion. Several mucosal valves are described within the nasolacrimal duct. All of them are simply mucous membrane fold without any valvular function except the valve of Hasner. “Valve of Hasner” is located at the lower end of the nasolacrimal duct (Fig. 2-5).

Anatomy of Appendages of the Eyeball 39

•Superior oblique

•Inferior oblique.

ORIGIN

All the extrinsic muscles except inferior oblique have a common origin from the annular tendon of Zinn, around the optic foramen, at the apex of the orbit. Inferior oblique arises from a small depression just behind the inferior orbital margin, near the medial wall, close to the lacrimal fossa.

INSERTION

All the extrinsic muscle tendons are inserted into the sclera (Table 2-1) after piercing the Tenon’s capsule.

Table 2-1: Distance of the insertion of the recti from the limbus

Superior oblique muscle after origin passes forward to reach trochlea which is situated anteriorly between medial wall and roof of the orbit. Here it becomes tendinous and passes through the trochlea (or pulley) and turns backwards, laterally and downwards to pass below the superior rectus. It is inserted in the sclera obliquely above and lateral to the posterior pole.

Inferior oblique muscle after origin runs below the inferior rectus to insert in the sclera below and lateral to the posterior pole. Its insertion lies very close to the macula.

NERVE SUPPLY

All the muscles are supplied by the oculomotor (IIIrd cranial) nerve except the superior oblique which is supplied by the Trochlear (IVth cranial) nerve and lateral rectus which is supplied by the abducens (VIth cranial) nerve.