EYE – ACCESSORY ORGANS

Dr N.S.SunilkumarAssistant Professor,Dept of Veterinary Anatomy and Histology,College of Veterinary and Animal Sciences, mannuthy,Kerala.

AESTHESIOLOGY

Aesthesiology is the study of organs of special sense. They receive external stimuli and conduct impulses to the brain which result in sensations of sight, hearing, taste, smell and touch. These organs consist of specialized neuroepithelial cells and a conduction path.

EYE (LATIN – OCULUS; GREEK – OPHTHALMOS)

The eye is the organ of vision. Vertebrate eye is an optical instrument of

living tissue simulating an electronic camera.

It comprises of the eyeball or globe, optic nerve and certain accessory organs associated with it.

ACCESSORY ORGANS

1. Bony orbit

2. Eyelids (palpebrae)

3. Conjunctiva

4. Lacrimal apparatus

5. Extraoccular muscles

6. Orbital fasciae

In domestic animals, the position of eyes on head depends on the feeding habits and the behaviour of the animal.

In hunting animals eyes are set forwards to give a concentrated, binocular and acute vision.

Here the field of vision is less.

In herbivores, eyes are placed on the sides or on the lateral aspect of the skull so that the vision is not much sharp but the field of vision is more.

Angle formed by the optic axes in cat and dog - 20º ,pig - 35º, horse - 40º sheep & ox - 50º.

ACCESSORY OCULAR ORGANS / ADNEXA1. BONY ORBIT Bony orbit in horse is formed by bones such as

frontal, lacrimal, zygomatic, temporal sphenoid, palatine and maxilla.

In ox, all the above bones except the temporal are form the orbit.

ACCESSORY OCULAR ORGANS / ADNEXA1. BONY ORBIT

In dog, cat pig and elephant, orbital rim is incomplete and is completed by the orbital ligament.

2. EYELIDS (PALPEBRAE) The eyelids are two

movable folds of skin situated in front of the eyeball and when closed, they cover the entrance into the orbit.

The two ends of this fissure are the medial and lateral canthi or angles.

The upper eyelid is more extensive and movable then the lower eyelid.

The interval between the upper and lower eyelids is the palpebral fissure angle is rounded but the medial one is narrow and drawn into V-shaped recess, the lacrimal recess or lake.

In this lake, there is a round pigmented body called the caruncula lacrimalis or lacrimal caruncle.

The edge of each lid is pierced near its medial angle by a small opening on either side of the caruncle- called puncta lacrimalis which are the entrances into the lacrimal ducts.

STRUCTURE OF EYELID

a. orbicularis oculi the

sphincter muscle of eye lids; b. Levator palpebrae

superioris for raising upper eye lid;

c. malaris for lower eye lid; d. frontalis for movement of

upper eye lid and eye brows; e. levator anguli oculi

medialis, well developed in carnivores and absent in ox

1) Skin

2) subcutis with no fat 3) palpebral muscles

The tarsal or Meibomian glands are arranged in parallel rows between tarus and conjunctiva and secrete the palpebral sebum. Each gland has an independent duct and opening above the posterior margin of the free border of the eye lids.

4. Palpebral ligament

5. Fibrous layer constituting the tarsus

6. Palpebral conjunctiva

3. CONJUNCTIVA Is the mucous membrane,

which lines the internal surface of the eyelids (palpebral conjunctiva).

It is light pink in colour and is reflected on the anterior part of the eyeball as bulbar conjunctiva

the line of reflection being the fornixconjunctivae.

space between the two parts of the conjunctiva constitutes the conjunctival sac.

Pale conjunctiva may be due to anemia, shock or internal haemorrhage

CHEMOSIS

The third eyelid or nictitating membraneis a semilunar fold of conjunctiva that covers the medial canthus and encloses a curved plate of hyaline cartilage.

The cartilage is leaf or shovel or T- shaped in outline.

made of hyaline cartilage in ruminants and dog and elastic cartilage in horse, pig and cat.

Cat is the only domestic animal to have muscle fibres in the third eye lid and plays a part in retraction of the third eye lid.

HARDARIAN GLAND The deep part of the cartilage is surrounded by a

mixed gland called the gland of the third eyelid or Harderian gland or accessory lacrimal gland, which resembles lacrimal gland in structure.

Well developed in pig, rabbit, some wild ruminants and birds, but not in terrestrial carnivores and non-human primates.

CHERRY EYE IN DOGS

HARDARIAN GLAND The lipid-rich secretion of the harderian gland in

chicken contains a large number of antibody producing plasma cells (Ocular vaccination).

In water birds, the Harderian gland plays a role in sodium -chloride balance.

Birds can move their third eye lid with the help of two well developed muscles (pyramidalis & Quadratus). This movement helps in spreading the lacrimal fluid and cleaning the surface

4. LACRIMAL APPARATUS

1. Lacrimal gland or tear gland,

2. Excretory ducts of the gland,

3. Two lacrimal ducts or canaliculi,

4.Lacrimal sac 5.Nasolacrimal duct

Is situated on the dorsolateral surface of the eyeball under the supraorbital process.

It is flattened oval and its superficial face is convex and distinctly lobulated.

It resembles the parotid gland in structure in ox and horse (serous).

In pig, mucous type in sheep, goat and dog, it is

seromucous. The excretory ducts are 6 to

8 large ones and several small ones and these open into the dorso-lateral part of the fornix conjunctivae.

By the movement of palpebrae, the fluid gets distributed to the eye ball uniformly.

The fluid temporarily gets collected near the medial canthus and gets drained to lacrimal ducts through puncta lacrimalia

The puncta lacrimalia are the entrances into the lacrimal ducts which drains in to the lacrimal sac that is placed in the lacrimal fossa of the lacrimal bone.

Lacrimal sac drains through nasolacrimal duct that opens into the nasal cavity.

The orifice is on internal aspect of the fold of the ventral turbinate

5. PERIORBITA Is a conical fibrous sheath

that encloses the eyeball with muscles, vessels and nerves.

Its apex is attached around the optic and orbital foramina and its base is in part attached to the bony rim of the orbit, in part continues with the fibrous layer of the lids.

The fat cushion around periorbita and within it is the intra-orbital adipose tissue, which fills the interstices between the eyeball, muscles, etc.

Just beneath the periorbita, there are three layers of orbital fascia, the outer layer, intermediate layer and the deep layer called the Tenon’s capsule.

6. EXTRA-OCULAR MUSCLES OR EXTRINSIC MUSCLES OF EYE BALL

Striated muscles, 7 in number, 5 straight and 2 oblique

1.Superior or dorsal rectus 2.Inferior or ventral rectus 3.Medial or internal rectus 4.Lateral or external rectus 5.Posterior rectus or retractor bulbi 6.Inferior oblique 7.Superior oblique

Recti muscles arise around the optic foramen and the first four diverge towards the respective aspect of the eye ball and inserted the four corresponding parts of the sclera.

Contraction of these muscles results in movement of eye ball to the respective directions.

These four muscles when contract together will retract the eye ball

The posterior rectus surrounds the optic nerve and is inserted to the sclera at its posterior part.

This muscle helps to retract the eye ball inwards.

It is absent in man and birds because perhaps we do not need the additional protection provided to the more protruding eyes of animals.

Superior oblique is the longest and narrower of the ocular muscles,and arises near the ethmoid foramen.

This muscle is being hooked by a cartilaginous pulley or trochlea (that is why the nerve is called trochlear nerve), then it comes forward, passes under superior rectus and is inserted to the sclera between superior and lateral recti muscles.

Trochlear nerve is also called as pathetic nerve because, trochlear nerve when stimulated, causes contraction of superior oblique muscle causing a pathetic look to the face.

Inferior oblique arises near the fossa for the lacrimal sac and is inserted to the sclera between lateral and inferior recti muscles.

Both oblique muscles help in rotation of eye ball.

An additional striated muscle within the orbit is the levator palpebrae superioris.

It does not attach to the eye ball but insert and elevate the upper eye lid.

Extraocular muscles are striated but they differ from most skeletal muscles in having greater ratio of nerve fibres approximately 1:10 (in other muscles, it ranges from 1:200 to 1:2000), and in having richer blood supply thus permitting more rapid movements of the eye and greater resistance to fatigue.

Blood supply: Ophthalmic artery Nerve supply:

SO – Fourth, trochlear (SOFT)PR & ER – Abducent, Sixth (PEAS)IO, IR, SR & MR – Oculomotor, Third