Substances insoluble in water have no taste; distilled water also has no taste.

The anterior part of the vascular membrane of the eyeball forms an iris, which has a color from light blue to dark brown due to pigment substances. In the center of the iris is the pupil – a round hole that can change its diameter with the help of muscles depending on the intensity of light. Behind the pupil is a transparent biconvex lens surrounded by the ciliary muscle.

The choroid forms the fluid contained in the chambers of the eye – between the cornea and the iris, as well as in the inner part of the eyeball, filled with glass visible body – a transparent gelatinous substance. The optic nerve emerges from the retina, its end comes to the rods and cones on the side of the glass of the visible body. The place of its exit deprived of light of receiving cells (cones and sticks) is a blind spot. On the side of it is a yellow spot, where there are mostly cones. There are both cones and sticks around the macula, and only sticks on the periphery.

Sticks are receptors for twilight vision; they are excited by a weak light and at the same time do not distinguish kings and see indistinctly. Cones are receptors for day vision; they perceive bright light and are able to distinguish colors.

Auxiliary apparatus of the human eye consists of eyebrows (protects the eye from sweat); eyelids and eyelashes (cover the eye from dust and other foreign bodies; the back surface of the eyelids is covered by a shell – the conjunctiva, which partially passes to the eyeball); muscles that move the eye; lacrimal apparatus. The lacrimal gland secretes a fluid that moisturizes the eye, washes away foreign bodies, performs a thermoregulatory function, contains a substance that kills microorganisms – lysozyme.

Vision is used to perceive the shape, size and colors of surrounding objects, to orient among them, to determine the distance to objects and their volume. Thanks to it the visual information is received, the environment is perceived and performance of difficult work is provided.

Receptor devices, the specific stimuli of which are chemicals of different nature, are called nemoreceptors and are the peripheral department of the corresponding analyzers. In the internal organs and tissues there are a large number of nemoreceptors, which belong to the internal analyzers. As for internal analyzers, there are two analyzers, among them there are two analyzers that perceive chemical stimuli – olfactory and gustatory.

The receptor apparatus of the olfactory analyzer is located in the middle of the nose. In the area of ​​the upper nasal passage and the posterior upper part of the nasal septum, the mucous membrane differs in its thickness and yellow-brown color.

The receptor apparatus of the olfactory analyzer consists of three cells: olfactory, resistance, basal, which base the olfactory epithelium. Olfactory cells have the shape of a spindle. The free end of the cell reaches the surface of the olfactory epithelium and ends with a small swelling – the olfactory bubble – with eyelashes. For every 160 olfactory cells there are about 100 resistance. Resistant cells, which have a cylindrical shape, pass through the entire thickness of the epithelium and are basically adjacent to the basal cells. The epithelium is impregnated with excretory ducts of serous glands.

In the olfactory epithelium of the upper nasal passage in humans found olfactory cells of two types: rod- and cone-like; in the external processes of these cells, the distinctiveness of the myoid elements, which allow the olfactory vesicles to rise above the surface of the olfactory epithelium, and deepen into the epithelium, interrupting this contact. The inner end of each olfactory cell continues in the form of a nerve fiber. These fibers combine into thin threads, pass through the opening of the lattice bone in the middle of the skull and connect with the nerve cells of the olfactory bulbs.

Nerve fibers that branch off from the two olfactory bulbs combine into a thick bundle, the olfactory tract, which forms a triangular extension on its way. The human olfactory tract together with the bulb are underdeveloped olfactory folds of macroamatic animals.

From the olfactory triangle the fibers of the olfactory tract go in separate bundles; as a part of a lateral olfactory bunch – in folds of a sea horse; a small number of fibers pass through the anterior calligraphy to the opposite side; part of the fibers goes to the gray matter of the transparent septum, and the second part goes to the substantia perforata anterior. In animals with a highly developed sense of smell, the last part of the brain reaches significant sizes.

During the act of eating, there is irritation of the receptors of the olfactory analyzer also in the air, which we exhale when chewing food through the hoans. In this way, the olfactory analyzer differentiates by smell not only that which comes from the outside through the nose – air, but also food that enters the mouth.

Some olfactory substances are not only adequate irritants of olfactory cells, but irritate such sensitive endings of the trigeminal nerve, which are embedded in the mucous membrane of the middle of the nose. Under the action of such substances, especially at high concentrations, there are a number of reflex changes in the body, the main factor in respiration. Respiratory arrest, which occurs suddenly at the very beginning of ether or chloroform anesthesia, is one of the devices of such reflex reactions.

The sharpness of the sense of smell is characterized by the threshold of sensation, ie the minimum amount of odorous substances that can cause the sensation of smell, to measure the sharpness of the smell of the proposed special devices (olfantometers).

Our clinics and laboratories often use a modified Elberg-Levy device. This device consists of a screen glass with a capacity of about 500 cm3, in which the fragrant substance is placed in a liquid or solid state. Through the rubber tube that covers the throat of the glass, there are two tubes – glass and metal. In a glass tube bent at right angles, one end is deeply recessed into the glass, and the other end has a rubber nozzle with a tap. The short metal tube is connected to the outer end through an outlet valve with a tube that terminates in two oils.

When measuring the acuity of the sense of smell inside the glass is injected with a syringe through a rubber nozzle into a glass tube strictly dosed amount of air, so that the pressure in the middle of the glass increases compared to the outside. After that, the tap is closed, and the oils are injected into the nostrils of the subject. When the outlet valve is opened, the pressure equalizes, and a beam of air enters the nostril from the glass, which contains a pore of odorous substance that acts on the olfactory receptors.

With this method of determining the degree of olfactory acuity – "olfactory factor" – is the minimum, measured in cubic centimeters, the volume of air that is necessary for the air stream to enter the middle of the nose to evoke an olfactory sensation. This method allows you to enter into the nasal cavity pores of odorous substances in a strictly dosed amount. Knowing the atmospheric pressure and temperature, you can calculate the batch pressure and also the concentration of odorous substances in the jar.

The acuity of smell in relation to the same odorous substance varies widely in different people. It also varies in the same person over a wide range depending on many conditions. External factors – humidity, temperature, atmospheric pressure and others – have a significant impact on changes in olfactory thresholds.

The peripheral, or receptor, apparatus of the taste analyzer is located in the oral cavity, ie at the beginning of the digestive system. With the help of a taste analyzer, the food is tested by direct contact with the mucous membrane of the oral cavity. In addition, from the receptor field of the oral cavity with the participation of the taste analyzer reflexively launched a complex mechanism of the digestive system. Suffice it to mention the reflex secretion of saliva of different qualitative composition depending on the chemical properties of the stimulus, which is in the oral cavity, as well as the secretion of gastric juice.

In the mucous membrane of the oral cavity there are special formations – taste buds, which are specific end devices that perceive taste stimuli. In adults, the taste buds are located on the tip of the tongue, on the dock or dorsal surface, as well as on the anterior and posterior surface of the epiglottis, on the posterior wall of the pharynx, on the anterior and soft palate. In children, the area of ​​taste buds is much larger than site that writes lab reports in adults. In old age their number decreases.

Taste bulbs occur in the form of individual inclusions in the epithelium of the mucous membrane, but on the tongue they are part of the papillae. Humans have grooved, leaf-like, mushroom-shaped papillae, which contain taste buds. The taste buds are arranged in such a way that they penetrate the entire thickness of the epidermis, reaching its free surface. They look like a bottle that opens to the outside with a small hole – the taste bud.

Each taste bulb consists of two types of cells: external – supporting and internal – taste. Nerve fibers from the subepithelial plexus enter the middle of the taste bud and end there freely; the second part of the fibers is placed between the individual taste buds.

The innervation of the taste area is very complex. In the spine of animals there are no separate taste nerves, as for olfactory receptors. The taste buds of different areas of the oral mucosa receive nerve fibers from four different nerves. After cutting the taste nerve fibers, degeneration of taste buds is observed with their successive complete reduction.

All taste fibers are part of a single bundle of the medulla oblongata and end in the area of ​​its nucleus. From here the taste fibers go along the spectator mound. This is where the third neuron begins. As for the neutral, or cortical, end of the taste analyzer, so far its location is not precisely established. Irritants of taste receptors are various substances in aqueous solutions. Substances insoluble in water have no taste; distilled water also has no taste. There are three groups of flavors that cause four types of first taste sensations: sour, salty, bitter and sweet.

The different tastes of our food are the result of red sensations, which arise due to the fact that in addition to specific taste buds, on the surface of the tongue there are sensitive endings of the second kind, which perceives temperature, tactile and painful irritations …