The<unk>  has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the<unk>  causes an involuntary reflex to close the eyelid. Because transparency is of prime importance, the healthy<unk>  does not have or need blood vessels within it. Instead, oxygen dissolves in tears and then diffuses throughout the<unk>  to keep it healthy. Similarly, nutrients are transported via diffusion from the tear fluid through the outside surface and the aqueous humour through the inside surface. Nutrients also come via neurotrophins supplied by the nerves of the<unk> . In humans, the<unk>  has a diameter of about 11.5 mm and a thickness of 0.5–0.6 mm in the center and 0.6–0.8 mm at the periphery. Transparency, avascularity, the presence of immature resident immune cells, and immunologic privilege makes the<unk>  a very special tissue. The most abundant soluble protein in mammalian<unk>  is albumin. The human<unk>  borders with the sclera at the<unk> l limbus. In lampreys, the<unk>  is solely an extension of the sclera, and is separate from the skin above it, but in more advanced vertebrates it is always fused with the skin to form a single structure, albeit one composed of multiple layers. In fish, and aquatic vertebrates in general, the<unk>  plays no role in focusing light, since it has virtually the same refractive index as water. Microanatomy Epithelium. 2: Anterior elastic lamina. 3: substantia propria. 4: Posterior elastic lamina (Descemet's membrane). 5: Endothelium of the anterior chamber. a: Oblique fibers in the anterior layer of the substantia propria. b: Lamellæ the fibers of which are cut across, producing a dotted appearance. c:<unk> l corpuscles appearing fusiform in section. d: Lamellæ the fibers of which are cut longitudinally. e: Transition to the sclera, with more distinct fibrillation, and surmounted by a thicker epithelium. f: Small blood vessels cut across near the margin of the<unk> . SD-OCT The human<unk>  has five layers (possibly six, if the Dua's layer is included).<unk> s of other primates have five known layers. The<unk> s of cats, dogs, wolves, and other carnivores only have four. From the anterior to posterior the layers of the human<unk>  are:<unk> l epithelium: an exceedingly thin multicellular epithelial tissue layer (non-keratinized stratified squamous epithelium) of fast-growing and easily regenerated cells, kept moist with tears. Irregularity or edema of the<unk> l epithelium disrupts the smoothness of the air/tear-film interface, the most significant component of the total refractive power of the eye, thereby reducing visual acuity.<unk> l epithelium is continuous with the conjunctival epithelium, and is composed of about 6 layers of cells which are shed constantly on the exposed layer and are regenerated by multiplication in the basal layer. #Bowman's layer (also known as the anterior limiting membrane): when discussed in lieu of a subepithelial basement membrane, Bowman's Layer is a tough layer composed of collagen (mainly type I collagen fibrils), laminin, nidogen, perlecan and other HSPGs that protects the<unk> l stroma. When discussed as a separate entity from the subepithelial basement membrane, Bowman's Layer can be described as an acellular, condensed region of the apical stroma, composed primarily of randomly organized yet tightly woven collagen fibrils. These fibrils interact with and attach onto each other. This layer is eight to 14 micrometres (μm) thick and is absent or very thin in non-primates.<unk> l stroma (also substantia propria): a thick, transparent middle layer, consisting of regularly arranged collagen fibers along with sparsely distributed interconnected keratocytes, which are the cells for general repair and maintenance. They are parallel and are superimposed like book pages. The<unk> l stroma consists of approximately 200 layers of mainly type I collagen fibrils. Each layer is 1.5-2.5 μm. Up to 90% of the<unk> l thickness is composed of stroma. There are 2 theories of how transparency in the<unk>  comes about: ## The lattice arrangements of the collagen fibrils in the stroma. The light scatter by individual fibrils is cancelled by destructive interference from the scattered light from other individual fibrils. ## The spacing of the neighboring collagen fibrils in the stroma must be < 200 nm for there to be transparency. (Goldman and Benedek) #Descemet's membrane (also posterior limiting membrane): a thin acellular layer that serves as the modified basement membrane of the<unk> l endothelium, from which the cells are derived. This layer is composed mainly of collagen type IV fibrils, less rigid than collagen type I fibrils, and is around 5-20 μm thick, depending on the subject's age. Just anterior to Descemet's membrane, a very thin and strong layer, Dua's layer, 15 microns thick and able to withstand 1.5 to 2 bars of pressure.<unk> l endothelium: a simple squamous or low cuboidal monolayer, approx 5 μm thick, of mitochondria-rich cells. These cells are responsible for regulating fluid and solute transport between the aqueous and<unk> l stromal compartments. (The term endothelium is a misnomer here. The<unk> l endothelium is bathed by aqueous humor, not by blood or lymph, and has a very different origin, function, and appearance from vascular endothelia.) Unlike the<unk> l epithelium, the cells of the endothelium do not regenerate. Instead, they stretch to compensate for dead cells which reduces the overall cell density of the endothelium, which affects fluid regulation. If the endothelium can no longer maintain a proper fluid balance, stromal swelling due to excess fluids and subsequent loss of transparency will occur and this may cause<unk> l edema and interference with the transparency of the<unk>  and thus impairing the image formed. Iris pigment cells deposited on the<unk> l endothelium can sometimes be washed into a distinct vertical pattern by the aqueous currents - this is known as Krukenberg's Spindle. Nerve supply The<unk>  is one of the most sensitive tissues of the body, as it is densely innervated with sensory nerve fibres via the ophthalmic division of the trigeminal nerve by way of 70–80 long ciliary nerves. Research suggests the density of pain receptors in the<unk>  is 300–600 times greater than skin and 20–40 times greater than dental pulp, making any injury to the structure excruciatingly painful. The ciliary nerves run under the endothelium and exit the eye through holes in the sclera apart from the optic nerve (which transmits only optic signals). The nerves enter the<unk>  via three levels; scleral, episcleral and conjunctival. Most of the bundles give rise by subdivision to a network in the stroma, from which fibres supply the different regions. The three networks are, midstromal, subepithelial/sub-basal, and epithelial. The receptive fields of each nerve ending are very large, and may overlap.<unk> l nerves of the subepithelial layer terminate near the superficial epithelial layer of the<unk>  in a logarithmic spiral pattern. The density of epithelial nerves decreases with age, especially after the seventh decade.
Cornea
Apnea
Hyperpnea
Bronchospasm
Orthopnea
Tachypnea
Hypopnea
Cyclospasm