Skin

The skin is the outermost covering of the body. It is stretched all over in the form of a layer. There are many structures and glands derived from the skin. The skin together with these derivatives is called the Integumentary System. It is often referred to as the largest organ in human body.

Functions of the Skin

1.    Protection :It is the primary function of skin. It affords protection in four different ways: It protects underlying tissues from mechanical shocks; It holds the body fluids inside & prevents excessive loss of water by evaporation.; It prevents entry of harmful substances or disease causing germs; It protects body against excess UV light which is potentially very harmful.

2.    Sensation : Our skin serves as a sense organ for touch, pain, pressure, heat etc.

3.    Temperature Regulation :Our skin prevents loss of heat in cold weather & facilitates loss of heat in hot weather by sweting.

4.    Storage of food:The skin stores reserve food in the form of a layer of fat contained in special cells.

5.    Excretion:Skin assists in process of excretion (through sweating), eliminating water, salts and to a very limited extant area.

6.    Synthesis of Vitamin D: The skin can synthesize Vit D when exposed to sunshine.

7.    The skin on our fingers and palms is produced into ridges & grooves which provide more efficient grasp.

Structure of the Skin

Skin Proper

Microscopically, the skin is composed of 2 layers – outer Epidermis & Inner Dermis.

Epidermis

The epidermis is that outer thinner part of the skin. It is formed for stratified epithelium filed up layer after layer. At places, the epidermis becomes thick and hard as on the palms, soles and specially on heels. It is devoid of bloodvessels at all places. It shows three regions or sublayers depthwise:

Skin pigmentation

The skin pigment acts like an umbrella to protect inner part of body from the harmful effects of the ultra violet rays of sunlight. Skin also contain DNA repair enzymes which reverse UV damage & people who lack these genes for these enzymes are predisposed to skin cancer. One form predominantly produced UV light is – Malignant Melanoma which is especially invasive & often deadly.Two abnormal conditions of skin pigmentation are:

Leucoderma : In this, skin pigmentation is lost from smaller or larger patches at different regions of the body, exact cause of the disease is not yet known.

Albinism: In this, there is complete loss of pigmentation all over the body including hair. Skin of such persons appears pinkish because of the underlying blood capillaries. Albinism is inherited.

Dermis[3]

The dermis (also called Corium) is made of inner thick layer of connective tissue made of elastic fibres. It is tough & flexible. It contains several other structures – blood vessels, nerve fibres, sensory organs, hair follicles, glands, etc.

The outer region of the dermis which lies next to epidermis is raised into numerous small processes called Papillae which contain blood capillaries and nerve endings. The nerve endings and sense organs here are concerned with sensations of touch and pain.

The layers beneath the dermis contain numerous fat cells (Adipose Tissue). The subcutaneous fat not only serves as a food reserve but also as a heat-insulating layer.

Derivatives of the Skin

The special derivations of human skin include the following:

1.    Hair: A hair consists of three parts. Hair shaft is part which projects from the skin may extend slightly below the surface of the epidermis. Hair Root is the part embedded within the dermis. The lowest part of the hair root is expanded to form a hair bulb which contains a projection of the dermis called Hair Papilla, with capillary blood supply. The hair follicle is a structure enclosing hair root. The hair bulb and hair follicle together are responsible for growth & elongation of hair. The growth of hair occurs by addition of cells at the base, which soon die. The colour of hair is due to varying quantities of melananin. The gray or silvery colour of hair is due to minute air spaces formed in the hair when pigment is lost. Every one has sometimes experience “goose flash” during winter or during some emotions. In this, the hair is lifted and surface of skin presents a somewhat contracted & wrinkled appearance. This is caused by Erector muscle of hair which runs obliquely between hair follicle and outer part of dermis. The contraction of this muscle at one end pulls the otherwise obliquely placed hair-follicle to some what vertical position & at other end depresses the epidermis.

Humans have three different types of hair.

a)    Lanugo: Its unpigmented hair found in foetus.

b)    Vellus hair:  Its unpigmented hair in both sexes most commonly found in children.

c)    Terminal hair: Its fully developed hair, generally longer, coarser & thicker & darker than vellus hair.

2.    Sebaceous Glands: These branched glands usually open into a hair follicle though sometimes even directly to outside. They give out an oily secretion called Sebum which makes hair & outer surface of skin oily and water proof to keep the epidermis supple and to prevent loss of water evaporation.

3.    Sweat Glands: Each Sweat gland is a simple coiled tube consisting of a deeper secretory part and an excretory part which runs upwards to open on the surface. The outer openings are called the Sweat pores. The total number in the body is estimated at about 2 million. Human race belonging to hot countries usually have more numerous sweat pores than those belonging to cold countries. The number of sweat pores varies from 60-80 per sq.cm on back to 400 per sq.cm on palms of hands. More sweat pores on the palms account for sufficient surface tension for grip. The secretory part of sweat gland absorbs fluid from the surrounding cells and blood capillaries of dermis and passes it into the excretory Sweat Duct which pours it out at the surface.  Sweating (or perspiration) goes on at all times in minute quantities. The major function of sweating is to lose body heat by evaporation. Sweat consists of about 99% water, 0.2 to 0.5% satts (esp. Sodium Chloride) & traces of urea (0.08%). The urea lost through sweat is about 1% of total area excreted by the body.    “Cold Sweats” may be due to psychic influences such as fright & nervousness. Sweating may also accompany accuses & severe pain.

4.    Mammary Glands: The mammary (or milk) glands are modified sweat glands. These glands are present both in male and female. But in males they persist only in a rudimentary state where as in females at puberty, they enlarge in the form of a pair of breasts. Each breast carries a central conical projection called the nipple 15-20 milk ducts open on the nipple. Each milk-duct is contained inward in a branching manner to jain cluster of 15 to 20 lobes of glandular tissue. These lobes are mammary glands. The activity of mammary glands in related with the reproductive hormones & pregnancy. The milk secreted by mammary glands is highly nutritious for the new born baby.

5.    Ceremonious Glands : Earwax/Cerumen is a yellowish waxy substance secreted in the ear canal of humans & many other mammals. Its plays vital role in human ear canal, assisting in cleaning & lubrication and also provides protection against foreign bodies excess or Impacted cerumen can press against eardrum and or exclude the External Auditory Canal & impair hearing. Cerumen is a mixture of viscous secretions from sebaceous glands & less viscous secretions from modified sweat glands.

Heat Production and Loss

There are many sources of heat production in our body. Chemical reactions occurring in all body cells, specially the level, generally produce heat. Most of the heat produced in our body comes from the activity of our muscles. Vigorous activity makes us warm on a cold day & overheats us on a hot day. A small amount of heat comes from the ingestion of hot food & beverages.

Heat Loss

The heat is lost from our body through the following channels:

1.    Skin: About 85% of body heat is lost through skin by convection conduction, radiation & through evaporation of water in sweet.

2.    Lungs: Heat is lost in the warm air which is breathed out. This loss in warm air can easily be experienced by gently blowing on the back of your hand keeping the mouth wide open. Some heat is also lost during vaporization of water from lungs.

3.    Urine & Faeces: These substances are eliminated at body temperature.

4.    Foods: Heat is also lost when cold food, water or beverages are taken into the body.

Temperature Regulation

The principal heat regulating centre is located in the Hypothalamus, a portion of fore-brain. This part acts like a thermostat. When the body cools below the normal temperature, it “switches on” or speeds up the heat producing process and when the body tends to get overheated it accelerates the cooling process. Simultaneously, it also slows down or “switches off” the heat losing or heat producing processes as case may be.

• In Cold, When outside temperatures are low, the blood vessels get narrowed (Vaso-Construction). This reduces the blood supply to skin. As a result there is less loss of heat by convection, conduction and radiation, and also less loss of heat through vaporization of sweat because with reduced blood supply, the sweat glands also act slowly. This is a method to conserve heat. When vasoconstriction occurs, it makes a person look pale or bluish because of reduced blood supply to the skin. Simultaneously, the heat production is increased by speeding up the metabolic rate and through increased muscular activity which is sometimes in the form of shivering.
• In heat, When outside temperature is high or when a person is engaged in strenuous exercise which means over-production of heat within the body, the blood supply to the skin is increased by dilation of blood vessels in the skin  (vasodilation). This results in greater loss of heat by radiation in greater loss of heat by radiation etc and also by vaporization of sweat which is now produced over the body help to speed up evaporation of sweat, that is why fans which do not cool the air of the room, have a cooling effect on our body.

Eye

The eyes of various species vary from simple structures that are capable only of differentiating between light and dark to complex organs, such as those of humans and other mammals, that can distinguish minute variations of shape, color, brightness, and distance. The actual process of seeing is performed by the brain rather than by the eye. The function of the eye is to translate the electromagnetic vibrations of light into patterns of nerve impulses that are transmitted to the brain.

The entire eye, often called the eyeball, is a spherical structure approximately 2.5 cm (about 1 in) in diameter with a pronounced bulge on its forward surface. The outer part of the eye is composed of three layers of tissue. The outside layer is the sclera, a protective coating. It covers about five-sixths of the surface of the eye. At the front of the eyeball, it is continuous with the bulging, transparent cornea. The middle layer of the coating of the eye is the choroid, a vascular layer lining the posterior three-fifths of the eyeball. The choroid is continuous with the ciliary body and with the iris, which lies at the front of the eye. The innermost layer is the light-sensitive retina.

The cornea is a tough, five-layered membrane through which light is admitted to the interior of the eye. Behind the cornea is a chamber filled with clear, watery fluid, the aqueous humor, which separates the cornea from the crystalline lens. The lens itself is a flattened sphere constructed of a large number of transparent fibers arranged in layers. It is connected by ligaments to a ringlike muscle, called the ciliary muscle, which surrounds it. The ciliary muscle and its surrounding tissues form the ciliary body. This muscle, by flattening the lens or making it more nearly spherical, changes its focal length.

The pigmented iris hangs behind the cornea in front of the lens, and has a circular opening in its center. The size of its opening, the pupil, is controlled by a muscle around its edge. This muscle contracts or relaxes, making the pupil larger or smaller, to control the amount of light admitted to the eye.

Behind the lens the main body of the eye is filled with a transparent, jellylike substance, the vitreous humor, enclosed in a thin sac, the hyaloid membrane. The pressure of the vitreous humor keeps the eyeball distended.

The retina is a complex layer, composed largely of nerve cells. The light-sensitive receptor cells lie on the outer surface of the retina in front of a pigmented tissue layer. These cells take the form of rods or cones packed closely together like matches in a box. Directly behind the pupil is a small yellow-pigmented spot, the macula lutea, in the center of which is the fovea centralis or Yellow spot, the area of greatest visual acuity of the eye. At the center of the fovea, the sensory layer is composed entirely of cone-shaped cells. Around the fovea both rod-shaped and cone-shaped cells are present, with the cone-shaped cells becoming fewer toward the periphery of the sensitive area. At the outer edges are only rod-shaped cells.

Where the optic nerve enters the eyeball, below and slightly to the inner side of the fovea, a small round area of the retina exists that has no light-sensitive cells. This optic disk forms the blind spot of the eye.

Functioning of the Eye

In general the eyes of all animals resemble simple cameras in that the lens of the eye forms an inverted image of objects in front of it on the sensitive retina, which corresponds to the film in a camera.

Focusing the eye, as mentioned above, is accomplished by a flattening or thickening (rounding) of the lens. The process is known as accommodation. In the normal eye accommodation is not necessary for seeing distant objects. The lens, when flattened by the suspensory ligament, brings such objects to focus on the retina. For nearer objects the lens is increasingly rounded by ciliary muscle contraction, which relaxes the suspensory ligament. In the later years of life most people lose the ability to accommodate their eyes to distances within reading or close working range. This condition, known as presbyopia, can be corrected by the use of special convex lenses for the near range.

Structural differences in the size of the eye cause the defects of hyperopia, or farsightedness, and myopia, or nearsightedness.

As mentioned above, the eye sees with greatest clarity only in the region of the fovea; due to the neural structure of the retina. The cone-shaped cells of the retina are individually connected to other nerve fibers, so that stimuli to each individual cell are reproduced and, as a result, fine details can be distinguished. The rodshaped cells, on the other hand, are connected in groups so that they respond to stimuli over a general area. The rods, therefore, respond to small total light stimuli, but do not have the ability to separate small details of the visual image. The result of these differences in structure is that the visual field of the eye is composed of a small central area of great sharpness surrounded by an area of lesser sharpness. In the latter area, however, the sensitivity of the eye to light is great. As a result, dim objects can be seen at night on the peripheral part of the retina when they are invisible to the central part.

The mechanism of seeing at night involves the sensitization of the rod cells by means of a pigment, called visual purple or rhodopsin, that is formed within the cells. Vitamin A is necessary for the production of visual purple; a deficiency of this vitamin leads to night blindness. Visual purple is bleached by the action of light and must be reformed by the rod cells under conditions of darkness. Hence a person who steps from sunlight into a darkened room cannot see until the pigment begins to form. When the pigment has formed and the eyes are sensitive to low levels of illumination, the eyes are said to be dark-adapted.

A brownish pigment present in the outer layer of the retina serves to protect the cone cells of the retina from overexposure to light. If bright light strikes the retina, granules of this brown pigment migrate to the spaces around the cone cells, sheathing and screening them from the light. This action, called light adaptation, has the opposite effect to that of dark adaptation.

Subjectively, a person is not conscious that the visual field consists of a central zone of sharpness surrounded by an area of increasing fuzziness. The reason is that the eyes are constantly moving, bringing first one part of the visual field and then another to the foveal region as the attention is shifted from one object to another. These motions are accomplished by six muscles that move the eyeball upward, downward, to the left, to the right, and obliquely. The motions of the eye muscles are extremely precise; The muscles of the two eyes, working together, also serve the important function of converging the eyes on any point being observed, so that the images of the two eyes coincide. When convergence is nonexistent or faulty, double vision results. The movement of the eyes and fusion of the images also play a part in the visual estimation of size and distance.

Protective Structures

Several structures, not parts of the eyeball, contribute to the protection of the eye. The most important of these are the eyelids, two folds of skin and tissue, upper and lower, that can be closed by means of muscles to form a protective covering over the eyeball against excessive light and mechanical injury. The eyelashes, a fringe of short hairs growing on the edge of either eyelid, act as a screen to keep dust particles and insects out of the eyes when the eyelids are partly closed. Inside the eyelids is a thin protective membrane, the conjunctiva, which doubles over to cover the visible sclera. Each eye also has a tear gland, or lacrimal organ, situated at the outside corner of the eye. The salty secretion of these glands lubricates the forward part of the eyeball when the eyelids are closed and flushes away any small dust particles or other foreign matter on the surface of the eye. Normally the eyelids of human eyes close by reflex action about every six seconds, but if dust reaches the surface of the eye and is not washed away, the eyelids blink oftener and more tears are produced. On the edges of the eyelids are a number of small glands, the Meibomian glands, which produce a fatty secretion that lubricates the eyelids themselves and the eyelashes. The eyebrows, located above each eye, also have a protective function in soaking up or deflecting perspiration or rain and preventing the moisture from running into the eyes. The hollow socket in the skull in which the eye is set is called the orbit. The bony edges of the orbit, the frontal bone, and the cheekbone protect the eye from mechanical injury by blows or collisions.

Diseases of eyes

Ear

It is composed of three divisions—external, middle, and internal—the greater part of which is enclosed within the temporal bone.

Structure

The external ear is that portion of the auditory apparatus lateral to the eardrum, or tympanic membrane. It comprises the auricle, or pinna (the external flap of the ear), and the external auditory canal, which is 3 cm (1.25 in) in length.

The middle ear, on the inner side of the eardrum, embodies the mechanism for the conduction of sound waves to the internal ear. It is a narrow passage, or cleft, that extends vertically for about 15 mm (about 0.6 in) and for about the same distance horizontally. The middle ear is in direct communication with the back of the nose and throat by way of the eustachian tube, which allows for passage of air into and out of the middle ear. Traversing the middle ear is a chain of three small, movable bones called the ossicles: the malleus, or hammer handle; the incus, or anvil; and the stapes, or stirrup. The ossicles connect the eardrum acoustically to the internal ear.

The internal ear, or labyrinth, is the part of the temporal bone containing the organs of hearing and equilibrium to which the filaments of the auditory nerve are distributed. It is separated from the middle ear by the fenestra ovalis, or oval window. The internal ear consists of membranous canals housed in a dense portion of the temporal bone and is divided into the cochlea (Greek, “snail shell”), the vestibule, and three semicircular canals. All these canals communicate with one another and are filled with a gelatinous fluid called endolymph.

Hearing

Sound waves are carried through the external auditory canal to the eardrum, causing it to vibrate. These vibrations are communicated by the ossicular chain in the middle ear through the oval window to the fluid in the inner ear. The movement of the endolymph stimulates in the cochlea a set of fine hairlike projections called hair cells. Collectively these projections are called the organ of Corti. The hair cells transmit signals directly to the auditory nerve, which carries information to the brain. The overall pattern of response of the hair cells to vibrations of the endolymph encodes information about sound in a way that is interpretable by the brain’s auditory centers. The range of hearing, like that of vision, varies in different persons.

The variation in the sensitivity of the ear to loud sounds causes several important phenomena. Extremely loud tones produce in the ear entirely different tones that are not present in the original tone. These subjective tones are probably caused by imperfections in the natural function of the middle ear. The harshness in tonality caused by greatly increasing sound intensities, as when a radio volume control is adjusted to produce excessively loud sounds, results from subjective tones produced in the ear. The loudness of a pure tone also affects its pitch. High tones may increase as much as a whole musical-scale note; low tones tend to become lower as sound intensity increases. This effect is noticeable only for pure tones. Because most musical tones are complex, hearing is usually not affected to an appreciable degree by this phenomenon. Another phenomenon is known as masking. The production in the ear of harmonics of lower-pitched sounds may deafen the ear to the perception of higher-pitched sounds. Masking is the phenomenon that makes necessary the raising of one’s voice in order to be heard in a noisy place.

Nose

Organ of smell, and also part of the apparatus of respiration and voice. Considered anatomically, it may be divided into an external portion—the visible projection portion, to which the term nose is popularly restricted—and an internal portion, consisting of two principal cavities, or nasal fossae, separated from each other by a vertical septum, and subdivided by spongy or turbinated bones that project from the outer wall into three passages, or meatuses, with which various sinuses in the ethmoid, sphenoid, frontal, and superior maxillary bones communicate by narrow apertures.

The margins of the nostrils are usually lined with a number of stiff hairs (vibrissae) that project across the openings and serve to arrest the passage of foreign substances, such as dust and small insects, which might otherwise be drawn up with the current of air intended for respiration. The skeleton, or framework, of the nose is partly composed of the bones forming the top and sides of the bridge, and partly of cartilages. On either side are an upper lateral and a lower lateral cartilage, to the latter of which are attached three or four small cartilaginous plates, termed sesamoid cartilages. The cartilage of the septum separates the nostrils and, in association posteriorly with the perpendicular plate of the ethmoid and with the vomer, forms a complete partition between the right and left nasal fossae.

The nasal fossae, which constitute the internal part of the nose, are lofty and of considerable depth. They open in front through the nostrils and behind end in a vertical slit on either side of the upper pharynx, above the soft palate, and near the orifices of the Eustachian tubes, leading to the tympanic cavity of the ear.

In the olfactory region of the nose the mucous membrane is very thick and colored by a brown pigment. The olfactory nerve, or nerve of smell, terminates in the nasal cavity in several small branches; these ramify in the soft mucous membrane and end in tiny varicose fibers that in turn terminate in elongated epithelial cells projecting into the free surface of the nose.

Tongue

Tongue is a musculo-sensory organ present on the floor of buccal cavity. The upper surface of tongue is lived by stratified epithelium and has a Median groove and three types

(a)   Lingual Papillae: These are numerous in number, mushroom shaped & red coloured distributed on sides & tip of tongue.

(b)   Filliform Papillae: These are most abundant smallest sized conical-shaped & present an anterior 2/3^rd part of tonge.

(c)   Circumallate Papillae: These are 8-12 in number, largest in size, circular.

Tongue has number of lymphoid nodules called Lingual tonsils.

Sense of Taste

The sense of taste is located in the taste buds of tongue. A taste bud is an ovoid group of sensory & supporting cells. The sensory cells end in hair like processes & have serve fibre extending from their bases. The taste hairs project into outer taste pore located in the surface of epithelium. Substances in solution enter these pores & stimulate sensory hairs. There are four fundamental tastes in man – Sweet, Salt, Bitter & Sour. Each basic taste is confined to a special area of tongue:

Tip of Tongue                       -                               Sweet & Salty

Sides of Tongue                   -                               Sour

Back of Tongue                   -                               Bitter