Aesthesiology

Author Dr.R.Menaka

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Aesthesiology deals with the description of special sense organs, which receives external stimuli and convert it in to a electrical signal and they conveyed to the brain by receptors. Receptors are specialized structural modifications of neurons, which contains photosensitive molecules that are chemically transformed by light impulses and react with neural
activity of surrounding cells. Sense organs are
1. Eye-Vision
2. Ear - Hearing/Listening
3. Tongue - Taste
4. Nose- Smell
5. Skin -Touch, pain temperature etc.

Organ of Vision- EYE
- Vision is based on a very complex system.
-It involves all parts of the eye, its accessory structures as well as various parts of the
  brain (visual pathways-Afferent pathways of the sense organs.
- It comprises of
1. Eye ball- Fibrous layer/ or fibrous tunics; Vascular layer/ or vascular tunics; and Nervous layer / or nervous tunics
2. Accesory Structures
a. Eyelids
b. Conjunctiva
c. Lacrimal Apparatus
d. Occular muscles
3. Optic nerve
4. Visual area of the cerebral cortex

- The eye ball is situated in the anterior part of the orbit.
- It is protected inform by the eyelids.
EyeLids (Palpebrae)
- The eyelids are musculo-fibrous folds and it can be drawn over the anterior surface of the eyeball.
- It protects the cornea and occlude light and it assist in keeping the cornea moist.
- There are three eyelids in the domestic mammals.
a. Upper eyelid
b. Lower eyelid and
c. Third eyelid
- The gap between the upper and lower eyelids are known as palpebral fissure.
-The upper eyelid is more extensive than the lower eyelid.
-It is an oblique slit and the ends are the medial canthus and lateral canthus.
- The lateral canthus is rounded.
- The medial canthus is narrow and drawn into "V" shaped recess, which is
  termed as Lacrimal recess or lake. In this recess, there is a round pigmented   body which is known as
  Caruncula lacrimalis (a mucosal prominence)
- There are stiff hairs situated at the edge of the eyelids, known as Cilia or   eyelashes.
-The eyelids contain several glands.
a. Sebaceous glands- opens into the cilia.
b. Ciliary glands- apocrine sweat glands, which secretes into hair follicles.
c. Tarsal glands- present in both eyelids. The tarsal glands are specially   modified sebaceous glands, which produce the oily superficial layer of the tear film. and it provides lubrication, prevents overflow of tears   from the lid margin and retards evaporation of the underlying   aqueous layer.

Third Eyelid (Nictitating membrane)

-Third eyelid is situated at the medial canthus of eye.
- It is supported by a "T" shaped piece of cartilage, which is covered by a fold   of conjunctiva.
- The free end is pigmented and partially moves over the antero-medial aspect of   the eyeball.
- Numerous (many) lymphatic nodules are found within the third eyelid.
-The third eyelid consists of hardarian gland, which provided local immunity of   the eyeball.
Note: "T"- shaped piece of cartilage is elastic cartilage in horse, pig and cat.   In ruminants and dog is hyaline cartilage.
Conjunctiva
- It is a light pink coloured mucous membrane, which indicates one of the normal   health sign of an individual animals.
- The part of the conjunctiva, which reflected on the anterior surface of the   eyeball, known as bulbar conjunctiva.
- The part of the conjunctiva, which lines the posterior surface of the eyelids   known as palberal conjunctiva.
- The point of reflection is the conjunctival fornix.
- When the two eyelids are in opposition, the capillary space forms between the   bulbar conjunctiva
 and palpebral conjunctiva and it constitutes the conjunctival sac.
- Conjunctival sac contains mucus and tears.
Lacrimal Apparatus
- Lacrimal apparatus includes,
a. Lacrimal gland
b. Excretory ducts - (Approx. 12 in numbers)
c. Two lacrimal ducts and canals
d. Lacrimal sac
e. Puncta lacrimalis
f. Caruncula lacrimalis
g. Naso-lacrimal duct

- Lacrimal apparatus are responsible for the production, dispersal and
  disposal of the tears.
- The lacrimal gland is situated on the dorso-lateral surface of the eyeball and   below the supra orbital process.
- The lacrimal apparatus and its secretion is serous in all domestic mammals,   but in pig, its secretion is mucoid.
- The lacrimal gland is situated beneath the orbital ligament in carnivores   whereas in the horse, the lacrimal gland occupies the lacrimal fossa.
- The secretion is conveyed through excretory ducts (many minute ducts) i.e.   approximately 12 in number to the superior
  conjunctival fornix- Conjunctival sac.
- Blinking movements distributes the tear fluid over the anterior surface of the   eye.
- There is a triangular space at the medial angle (i.e. medial canthus) of the   eye, known as Lacus Lacrimalis.
- Caruncula lacrimalis is a small rounded pigmented body which is situated in   Lacus lacrimalis and it helps in the flow of tears in proper direction.
- Puncta lacrimalis are minute orifices which are lateral close to the caruncula   lacrimalis
  at hte medial canthus of the eye.
- The drainage of lacrimal secretion begins with the puncta lacrimalis
  which leads to short narrow canaliculus (i.e. two lacrimal canal)and opens in   the lacrimal sac.
- The lacrimal sac accomudates in the lacrimal fossa of the lacrimal bone.
- The lacrimal sac is connected with the naso-lacrimal duct, which directly   opens into the nasal cavity.
- Lacrimal drainage varies among the species and between individuals.

Muscles of the eyeball

- The eyeball muscles constitute three groups.
A. The intrinsic muscles-which regulates the pupillary diameter and the shape of   the lens.
B. The extrinsic muscles- which are concerned with the movement of the eyeball.
C. The palpebral muscles- which regulates the shape and position of the   palpebral fissure.

The extrinsic group of muscles are Seven. This group includes,
a. Four recti musclesb. Two oblique muscles
c. One retractor occuli
- The four recti muscles are
i. Superior rectus
ii. Inferior rectus
iii. Medial rectus
iv. Lateral rectus

- The recti muscles originates from optic foramen and close proximity of orbital   fissure.
- They are helps in the axial movement of the eyeball.
- The two oblique muscles are
i. Dorsal oblique
ii. Ventral oblique
- These two oblique muscles are responsible for rotatory movement of the   eyeball.
- The dorsal oblique muscle runs along the angle between medial and dorsal wall   of the orbit
  and it lies above the medial rectus muscle.
- The dorsal oblique muscle rotates the visual axis downward.
- The ventral oblique muscle runs postero-laterally, below the inferior rectus   muscle and it rotates the visual axis upward.
- The one retractor occuli forms a complete muscular cone around optic nerve.
  This retractor occuli is not present in man.

Structure of the eyeball

- The wall of the eyeball is formed by three concentric layers, which encloses   the interior of the eyeball.
- The interior of the eyeball is divided into three chambers
i. Anterior chamber - between the cornea and iris.
ii. Posterior chamber - between iris, ciliary body and lens.
iii. Vitreous chamber - behind the lens, surrounded by retina.

The layers of the eyeball,

A. Fibrous layer of eyeball- Sclera and cornea
B. Vascular layer of eyeball- Choroid, ciliary body and iris
C. Nervous layer of eyeball- retina and optic part of retina

A. Fibrous layer/ or Fibrous tunics / or fibrous coat:

- This is the outer protective and hard coat of the eyeball.
- The fibrous layer is responsible for the shape of the eyeball.
- It composed of two parts,
- The transparent cornea, which covers the anterior part of the eyeball (globe).
- The opaque, whitish sclera, which encloses the posterior 3/4th part of the   eyeball.
- The two components meet at the corneo-scleral junction, it also referred as   corneal limbus.

Cornea:

- Cornea is a transparent membrane.
- It is one of the refractive medium of the eye.
- The anterior surface is convex and the posterior surfcae is concave.
- Cornea forms the boundary of anterior chamber of eyeball.
- The cornea is composed of 5 layers.
1. Anterior epithelium
2. Anterior limiting membrane
3. Substantia propria
4. Posterior limiting membrane
5. Posterior epihtelium

* in carnivores, the cornea is rounded.

Sclera:

- The sclera is composed of white fibrous tissue intermixed with elastic fibers.
- Externally, the sclera is white in colour and infront covered by conjunctival
  mucous membrane.
- The internal surface is brown in color and has grooves.
- Posteriorly, the sclera is pierced by optic nerve, ciliary vessels and ciliary   nerves.
- The optic nerve pierce the sclera just medial to the posterior pole at the   optic disc or blind spot of the eye.
- This area has an apparance of a cribriform plate known as lamina cribrosa   sclera. This lamina transmits central retinal
  vessels along with nerve budles.
- Suprachoroid lamina, a delicate layer occupies the perichondrial space between
  the sclera and choroid.
- Within the sclero- corneal junction an encircling canal or sinus is situated,   known as canal of Schlemm or sinus venosus sclera.
- The scleral venous plexus, through which the aquous humor drains is loacted   between these two structures
  and is important for the regulation of the occular pressure.
- Internally, there is a projected rim of scleral tissue behind the canal known  as scleral spur.

B. Vascular layer of the eyeball:

- The vascular layer remains between the sclera and retina.
- The vascular layer consists of three parts,
i. Choroid- Tapetum lucidum
ii. Ciliary body
iii. Iris
- Vascular layer has numerous functions.
a. Blood supply
b. Suspension and regulation of the shape of the lens
c. Regulation of the size of the pupil and
d. Production of the aquous humor.

Choroid:

- The choroid is a pigmented layer and highly vascular layer that envelops
  the posterior part of the eyeball.
- The pigment layer is a half moon shaped area that has an additional reflective   layer,
  the tapetum lucidum.
- The tapetum lucidum is present in all domestic mammals other than the pig  (Pigment free).
- The tapedal cells contains crystalline rods that are highly reflective and it   aids vision
 during dawn and night.
- The tapetum lucidum has a disctinctive color in the differentspecies and  breeds
 and accounts for the riidescent appearance of an animal's eye.
For example-
 Cat- Yellow in color
 Dog- Green in color
 Ox and Horse : Blue- green in color
 (This tapedal cells, which gives aglare at night).

ii. Ciliary Body:

- The ciliary body is the thickened middle segment of the vascular tunic between  the choroid and the iris.
- This middle thickening is due to the presence of 70 to 8 raised ridges- the  ciliary processes.
- Group of fine fibers extends from the ciliary
 processes to the capsule of the lens, forms suspensory ligamnets of lens.
 These ligaments maintain the position of the lens.
- The involuntary ciliary muscle lies in the depth of the ciliary body and are  attached to the scleral spur.
 Contraction of these muscle fibers loosen the suspensory ligaments
 and aloow the lens to become more convex and helps in accomudation.

iii. Iris:

- The iris is the anterior continuation of the vascular tunics.
- This iris remain suspended in the aqueous humor (behind the cornea
 and just front of the lens).
- The central aperture of iris is known as pupil.
- The iris separates the anterior chamber and posterior chamber of the eye,
 which communicates through pupil.
- The iris is composed of fibrous stroma, circular muscle fibers, pigment cells,  blood vessels etc.
- The size of the pupil is regulated by the contraction of these muscle fibers.
- The pigmented cells of the iris contain melatonin, which protects the retina  from intense light.
- The number and size of the pigment determines the color of the eye which is  genetically determined by several co dominant genes.

Note:
* If the fibers are densely packed and there is a paucity of pigmented cells  within the iris stroma the eye appears to be blue or grey in goat and pig.
* If there are many pigmented cells within a thin network of collagen fibers the  iris is coloured
 dark brown in ox and horse.
* Fewer pigmented cells results in a lighter, yellow colour of the iris in dog,  pig and small ruminants.
* In albinos, the iris is completely devoid of pigment. Their eyes appears red  due to the blood supply in this area, which is not obscured by pigment.

C. Nervous layer of the eyeball:

Retina:

- Retina is the inner most layer of the eyeball.
- Actually, retina is a membranous expansion of the optic nerve elements.
- The optical part of the retina ends behind the ciliary body in a serrated  border, known as ora serrata.
- If the centre of the posterior pole of the eyeball the retina presents an oval  yellow spot termed macula lutea.
- At the middle of the macula lutea there is a depressed area named fovea  centralis, in which the sense of vision is most acute.
- The retina is composed of nervous elements blood vessels, pigment cells and  modified connective tissue. These are arranged in 10 conventional layers.
- However, in these layers, the retina contain many chains of three microscopic   neurons are situated.
a. Rods and cones
b. Bipolar cells
c. Ganglion cells

- The photosensitive receptor cells of the retina are the rods and cones.
- The rods contain membraneous discs filled with rhodopsin, which is responsible   for the transduction of light energy into chemical energy.
  New discs are continuously produced and moved to the end of the photoreceptor   segments, where they undergo phagocytosis
  by the pigmented cells.
- The rods are highly sensitive receptors for black and white (night), while the   cones are concerned with color vision (day).
- The number of rods are three times more than cones.
- Cones have a similar architecture, but donot contain rhodopsin as   photosensitive pigment (but idopsin present).
- Cats appear to be abled to differentiate between colors green and blue but   have otherwise a poor color vision.
- Ruminants and horse are not able to see the colors red and blue.
- Pigs have a color spectrum that is similar to those of humans.
- The dog has a dichromate vision, in that it is able to distinguish between   stimuli
  having predominantly short and long wavelengths.

Lens

-Lens is the chief refractory medium of the eyeball.
- Lens is a biconvex transparent disc and entire lens is covered by the capsule.
- The lamellar architecture of the lens is essential for transparency. Loss of   transparency develops cataract.
- Lens is situated behind the pupil and infront of vitreous body and held in   Position by fibers of the
  suspensory ligament.
- The posterior surface is usually more convex than the anterior. During   accommodation the convexity of the lens changes.
- Structures of the lens are;
1. Lens capsule
2. Lens Epithelium
3.Lens fibers
- Anteriorly there is a depression for the accommodation of lens, termed hyaloid fossa.

Chambers of the eyeball:

Aqueous Humor:

- The anterior chamber and the posterior chambers are filled with aqueous humor.
- Aqueous humor is produced by an active secretory process from the epithelium   of the ciliary body.
- Aqueous humor is a clear and colorless, containing several electrolytes,   glucose, aminoacids and ascorbic acid.
- It is important for the nutrition of the avascular structures of the eye   (Cornea and lens).
- The anterior chamber is the space surrounded by the posterior surface of the   cornea and the anterior surface of the iris and lens.
- Posterior chamber is the space formed between the iris and ciliary body   anteriorly; posteriorly by the lens capsule and the vitreous.
- The anterior chamber is an direct communication with the posterior chamber   through the aperture of the pupil.
- The aqueous humor flows from it site of production into the posterior chamber,   from here it passes through the pupil into the
  anterior chamber and drains through the spaces of the irido-corneal angle to the   scelral venosus plexus.
- In the healthy eye the rate of production, balances the rate of drainage, thus   keeping intraoccular pressure constant.
- Impairment of outflow results in an increase in intraocular pressure known as   glaucoma which leads to retinal atrophy and blindness.

Vitreous body:

- The vitreous chamber is surrounded anteriorly by the lens and the ciliary   body,
  the retina encloses the remainder.
- The vitreous chamber is occupied by the vitreous body, which is soft, clear   gel that conforms to the space of   its surroundings.
- The vitreous body is covered by hyaloid membrane.
- The hyaloid canal traverses the vitreous from the posterior surface of the   lens to the optic disc.
  It is the remnant of the hyaloid artery, a branch of the retinal arteries, which   supply the lens during        embryological development.
  This artery usually degenerates after birth and the lens is then nourished by   diffusion.
- The hyaloid canal is present in the ox, pig and in carnivores.

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