Describe the structure of the animal sensory organs, including the ear, eye and nose,
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Answer:
Eye
Eyes are structures that detect the light reflected by objects, transduce it into electrical information that, after a local processing, is sent to other parts of the encephalon for combining with other neuronal information that altogether eventually change the animal behavior. Eyes are rounded and polarized structures. From the anterior to the posterior part, they are made up of cornea, anterior chamber, iris, ciliary muscles, crystalline, vitreous body, retina, choroid, sclerotic or sclera and optic nerve. These components are distributed in three concentric layers or tunics (excepting the crystalline): fibrous tunic, vascular tunic, and internal nervous tunic.
Eye
Main components of the eye of mammals.
Cornea is the most external part of the eye so it is in contact with the air. It is a transparent structure that focuses the light and protects the eye surface. The optic properties of the cornea is a consequence of the arrangement and type of the collagen fibers it contains. There are no blood vessels in the cornea, and that is why it is relatively easy to transplant this part of the eye in surgeries.
Cornea
Layers of the cornea.
The cornea is a sheet of tissue made up of five layers: corneal epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium. Corneal epithelium is the most outer layer, it is stratified squamous epithelium, contains many nervous fibers, and can be easily auto-repaired. Corneal epithelium is laterally continuous with the epithelium of the conjunctiva. The Bowman's layer is located inmediatelly under the corneal epithelium. This layer is not found in all mammal species. Bowman's layer contains collagen fibers, but not elastic fibers. The stroma, found under the Bowman's layer, is the thicker layer of the cornea accounting for about 90 % of the thickness of the cornea. The stroma is connective tissue with collagen fibers, mainly types I and IV, arranged in layers with different spatial orientation between each other. There are also proteoglycans like chondroitin sulfate and keratan sulfate. Some cells are also present, such as fibroblasts and errant lymphocytes. Descemet's membrane lines the inner part of the stroma, and, actually, it is the basal lamina of the endothelium. The endothelium is the inner part of the cornea, and forms the anterior wall of the anterior chamber of the eye. The cornea is laterally connected with the sclera through a region known as corneal limbus.
The ciliary body is found behind the iris and performs two main functions: releasing vitreous humor and changing the shape of the crystalline to focus the light on the retina. It links the ora serrata of the choroid to the root of the iris, and is connected to the crystalline by a ligament. The ciliary body shows ring shape, and, in transverse sections, looks like a triangle. It is divided in two components: pars plicata and pars plana. Pars plicata is located close to the crystalline lens and is organized in finger like structures known as ciliary processes, whereas pars plana shows a flattened form. The smooth muscle of the ciliary body is known as ciliary muscle, which controls accommodation by changing the shape of the crystalline lens to focus the light on the retina. The internal part of the ciliary body is dense connective tissue with abundant elastic fibers and blood vessels. The ciliary epithelium is made up of two layers, the most internal one is pigmented and releases aqueous humor. The ciliary body, together with the iris, constitutes the uvea.
Ciliary body
Ciliary body.
The iris is the structure or the eye that separates the anterior chamber from the posterior chamber, and is attached to the ciliary body through its periphery part. In the central area of the iris, there is an opening, known as pupil, through which the light can reach the crystalline lens. The pupil area of the iris is the closest part to the pupil and the periphery part is known as ciliary area. The iris is mostly highly vascularized loose connective tissue. No matter the diameter, blood vessels show the same organization. They do not contain muscle layer. The posterior part, the deepest one, of the iris is a highly pigmented two layered epithelium, which gives color to the eyes depending on the amount and arrangement of the pigment. The iris works as a adjustable diaphragm thanks to the activity of two muscles. One of them is the sphincter of the pupil that sets the diameter of the pupil. It is smooth muscle innervated by parasympathetic fibers of the ciliary ganglion and is arranged circularly. The other muscle is the dilator muscle that increase the pupil diameter (pupil dilation). It is composed of radially oriented myoepithelial muscles, which are innervated by sympathetic neurons of the superior cervical ganglion.