Anatomy of special sense organs

Question 1

Development of the eye

Answer 1

• First; optic vesicle is formed from an outgrowth of the neural tubule
• optic vesicle contacts the ectoderm, causing formation of the lens placode
• optic vesicle flattens and folds inward to form optic cup ( becomes the retina)
• lens placode folds inward to become lens vesicle
• lens vesicle induces ectoderm to become the cornea
• optic stalk becomes the optic nerve
• hyaloid vessels eventually regress

Question 2

3 main layers of the eye

Answer 2

• called tunicae
• Outer layer; fibrous tunic. Dense collagen tissue, protects eye and hold its shape
• Middle layer; vascular tunic. Contains blood vessels & structures that hold the lens, regulate pupil size,
  and produce fluid that fills the eyeball
• Inner layer; the retina

Question 3

Fibrous tunic

Answer 3

• sclera
• cornea

Question 4

Vascular tunic

Answer 4

• choroid; highly vascular nourishes the outer layers of the retina. Supplied by ophthalmic artery
  May have tapetum lucidum, reflective for better low light vision
• ciliary body
• iris

Question 5

Nervous tunic

Answer 5

• retina
• neural layer
• pigmented layer

Question 6

Chambers of the eyeball

Answer 6

• Anterior chamber (in-front of lens)
• Posterior chamber (in-front of lens)
• Vitreous chamber

Question 7

Bony orbit

Answer 7

• eye sits within the bony orbit of the skull
• complete in humans, incomplete in most companion species

Question 8

Extra-ocular muscles

Answer 8

• move the eye in orbit
• 4x rectus muscles; move left right up and down
• 2x oblique muscles; rotate around visual axis
• 1x retractor bulbi; retracts eyeball into head

Question 9

Third eyelid

Answer 9

• only present in some species
• contains t-shaped cartilage
• has extra tear gland; accessory lacrimal gland
• kept retracted by smooth muscle under sympathetic control

Question 10

Tear Glands

Answer 10

• washes eye of debris
• lipid top layer; prevent evaporation
• watery middle layer; moistens cornea
• mucus bottom layer; sticks tear film to eye

Question 11

Species differences (eye)

Answer 11

• Birds have fewer retinal blood vessels; don’t block the retina as much better vision; pecten instead
• Many species have bony rings called sclerotic rings to hold shape and attach to muscles
• Snakes have no eyelids

Question 12

Development of the ear

Answer 12

• Inner ear forms first
• ectoderm thickens to form otic placodes
• roll inward to form otic pit
• pinch of to form otic vesicle
• otic vesicle forms membranous labyrinth
• middle ear forms from an outgrowth of the 1st pharyngeal pouch ( endoderm)
• external ear forms from 1st pharyngeal groove (ectoderm)
• eardrum forms where the pouch and groove meet
• bones form endochondrally from neural crest cells

Question 13

Outer ear

Answer 13

• pinna; skin and cartilage
• auricular muscles
• ear canal (external auditory meatus)
• eardrum (tympanic membrane)

Question 14

Pinna & auricular muscles

Answer 14

• funnel-shaped to collect sound waves
• pinna can be moved by auricular muscles to locate sound
• used for communication
• innervated by CN VII (facial nerve)

Question 15

External auditory meatus

Answer 15

• contains sebaceous and modified apocrine (ceruminous) glands
• cerumen catches objects that may damage eardrum
• ## in carnivores and pigs there is a bend

Question 16

Tympanic membrane

Answer 16

• separates the outer and middle ear
• stretched over tympanic ring
• first ossicle (malleus) attaches to it
• sound waves cause the membrane to vibrate

Question 17

Middle ear

Answer 17

• Inside the temporal bone of the skull
• lined with mucous membrane
• communicates with pharynx via auditory tube; pressure equalisation and drainage

Question 18

Guttural pouches in horses

Answer 18

• outpouchings of the auditory tube
• may function in brain cooling
• 2x compartments divided by stylohyoid bone; lateral and medial
• blood vessels & CN IX, X, XI, XII nearby

Question 19

Ossicles

Answer 19

• Malleus; hammer shaped
• Incus; anvil shaped
• Stapes; stirrup shape
• transmit vibrations and amplify them

Question 20

Inner ear

Answer 20

Consists of;
- membranous labyrinth
- osseous labyrinth
- perilymph fills space between labyrinths
Functions;
- hearing via the cochlear
- balance via vestibular system

Question 21

3 parts of the cochlea

Answer 21

• the cochlear duct
• the scala vestibuli
• the scala tympani
• the two scala communicate at the cochlea’s spiral

Question 22

The cochlea and hearing

Answer 22

• stapes vibrate compressing perilymph in the scala vestibuli.
• transmitted to endolymph in cochlear duct via Reissner’s membrane
• movement of endolymph causes pressure on the tectorial membrane
• tectorial membrane puts pressure on the hair cells which send impulse along cochlear nerve to the brain

Question 23

Vestibular system and balance

Answer 23

• 3x semicircular canals & enlargements of membranous labyrinth called saccule and utricle
• semicircular canal senses head rotation
• saccule and utricle sense accelerations
• Information is carried by the vestibular part of the
  vestibulocochlear nerve to the brain

Question 24

Semicircular canals

Answer 24

• Three canals roughly perpendicular to each other
• Can sense movement in 3x directions
• Pitch, roll and yaw
• Rotations cause an endolymph current moving hair cells in the ampulla

Question 25

Saccule and Utricle

Answer 25

• contain hair cells in thickened patches (maculae)
• covered in gel-like layer containing otolith crystals
• endolymph movement bends hairs transmits impulse down vestibulocochlear nerve

Question 26

Species differences (ear)

Answer 26

• only mammals have an external pinna
• reptiles and birds have only one ossicle called columella

Question 27

Olfactory organ

Answer 27

Olfactory mucosa in dorsocaudal nasal cavity
- covers the lateral wall and ethmoidal conchae
- distinguished histologically by presence of olfactory neuronal cells

Question 28

Olfactory cells

Answer 28

• olfactory neuronal dendrites reach the epithelium; present cilia into the cavity
• axons combine to form the fascicles of the olfactory nerve; pass through cribriform plate to connect directly to olfactory bulb on the brain
• Bowman’s glands below epithelium; wash away odour particles, humidify epithelium & bind odourants

Question 29

Vomeronasal Organ

Answer 29

• The ‘sexual’ nose; detects pheromones
• Flehmen reaction; detection of oestrus by males
• 2 narrow parallel ducts embedded in hard palate
• part-lined by olfactory mucosa