EYE — 3 Tunics:
• Fibrous: sclera (white CT) + cornea conventionally described without a distinct Bowman's layer in dog, cat and horse (epithelium + stroma + Descemet's membrane + endothelium); ruminant descriptions require species/source qualification.
• Vascular (Uvea): choroid (with tapetum), ciliary body, iris.
• Nervous: retina (rods + cones + bipolar + ganglion + horizontal + amacrine cells).

EAR:
• Cochlea: organ of Corti (hair cells + tectorial membrane).
• Vestibule + canals: maculae + cristae ampullares (hair cells in gel).

SKIN (Integument):
• Epidermis (stratified squamous keratinised — 5 strata).
• Dermis (papillary + reticular CT).
• Hypodermis (fat).
• Appendages: hair, sebaceous glands, sweat glands, hoof, horn, claw.

SENSE ORGANS & INTEGUMENT — HISTOLOGY

I. EYE:

A. Fibrous Tunic:
• Sclera: dense irregular CT; opaque white.
• Cornea (4 conventional layers in domestic animals, anterior to posterior):
1. Anterior Epithelium — stratified squamous non-keratinised.
2. Substantia Propria (Stroma) — dense regular CT (orthogonal collagen lamellae) → transparent.
3. Descemet's Membrane — thick basement membrane.
4. Posterior Endothelium — simple squamous; pumps Na⁺ → keeps cornea dehydrated/clear.
• Avascular → nourished by aqueous humour & tears.

B. Vascular Tunic (Uvea):
• Choroid: vascular pigmented layer with tapetum lucidum (reflective layer in dorsal fundus). Tapetum cellular (carnivores) or fibrous (herbivores).
• Ciliary Body: ciliary processes (project into posterior chamber, secrete aqueous humour) + ciliary muscle (smooth muscle, accommodation).
• Iris: pigmented diaphragm with central pupil. Sphincter pupillae (parasympathetic) + dilator pupillae (sympathetic).

C. Nervous Tunic (Retina, 10 layers):
1. Pigmented epithelium.
2. Photoreceptor layer (rods + cones).
3. External limiting membrane.
4. Outer nuclear layer (rod/cone nuclei).
5. Outer plexiform layer (synapses).
6. Inner nuclear layer (bipolar + horizontal + amacrine + Müller cells).
7. Inner plexiform layer.
8. Ganglion cell layer.
9. Nerve fibre layer.
10. Internal limiting membrane.
• Optic disc = where ganglion axons exit as optic nerve; no photoreceptors = blind spot.
• Area centralis and visual streak are distinct retinal specializations; a visual streak is prominent in horse and cattle = high cone density region.

D. Lens, Aqueous, Vitreous:
• Lens — transparent biconvex; concentric layers of lens fibres; held by zonules from ciliary body.
• Aqueous in anterior + posterior chambers; secreted by ciliary body, drained at iridocorneal angle.
• Vitreous body — gel filling the vitreous chamber behind the lens.

II. EAR:

A. External + Middle Ear:
• External canal lined by skin (with sebaceous + ceruminous glands → cerumen).
• Tympanic membrane = 3 layers (epidermis + thin CT + simple cuboidal mucosa).
• Middle ear cavity lined by simple cuboidal/squamous epithelium.
• Ossicles = bone (malleus, incus, stapes) connected by tiny synovial joints.

B. Internal Ear:

1. Cochlea (Hearing):
• Bony spiral with inner membranous duct (cochlear duct/scala media).
• 3 fluid-filled spaces: scala vestibuli (perilymph), scala media (endolymph), scala tympani (perilymph).
• Organ of Corti on basilar membrane:
– Inner + outer hair cells (mechanosensory).
– Stereocilia bend in tectorial membrane → depolarise hair cells.
– Innervation by cochlear branch of CN VIII.

2. Vestibular Apparatus (Balance):
• Maculae (in utricle + saccule) → linear acceleration. Hair-cell stereocilia project into an otolithic membrane containing calcium-carbonate otoconia.
• Cristae ampullares in 3 semicircular canals → angular acceleration. Hair cells project into cupula (gelatinous mass).
• Innervation: vestibular branch of CN VIII.

III. SKIN (INTEGUMENT) — Histology:

A. Epidermis:
Stratified squamous keratinised epithelium with 5 layers (most clear in thick skin):
1. Stratum basale — single layer of cuboidal cells on basement membrane; mitotic; contains melanocytes + Merkel cells.
2. Stratum spinosum — polyhedral cells with desmosomes ('prickle cells'); Langerhans cells (dendritic immune cells).
3. Stratum granulosum — flattened cells with keratohyalin granules.
4. Stratum lucidum — present in appropriately thick specialized skin, especially footpads; occurrence at nasal and labial sites is species- and region-dependent; thin clear layer.
5. Stratum corneum — flat, dead, anucleate cornified cells.

B. Dermis:
• Papillary dermis (superficial; loose CT with papillae projecting into epidermis).
• Reticular dermis (deep; dense irregular CT with collagen + elastic fibres).
• Vessels, nerves, glands, hair follicles.

C. Hypodermis (Subcutis):
• Loose CT + adipose; not part of skin proper but anchors skin to body.

D. Skin Appendages:
• Hair Follicles: epithelial root sheath surrounding hair shaft; bulb at base contains matrix cells. Arrector pili muscle (smooth) connects follicle to dermis (hair stand-up = piloerection).
• Sebaceous Glands: holocrine; secrete sebum into hair follicle.
• Sweat Glands:
– Apocrine sweat glands — most domestic mammals; in haired skin; open into follicle.
– Epitrichial (apocrine) sweat glands — abundant in equine skin and responsible for sweat cooling; eccrine/atrichial glands occur mainly at specialized sites such as carnivore footpads.
• Hoof / Horn / Claw: heavily keratinised modifications of epidermis around tip of digit (or skull bone for horn).
• Mammary glands = modified apocrine sweat glands.