Brain

The brain is a jelly-like substance, which in adults weighs about three pounds. It is divided into three parts: the brain stem, which is an extension of the spinal cord, the forebrain (which consists mainly of the cerebruim) and the cerebellum. The forebrain and cerebellum are divided into two hemispheres which are linked by a thick band of nerve fibers and these hemispheres have areas, called "lobes," which perform specific functions. The brain's surface lies in rather ugly, wrinkled folds. Traditionally referred to as one's "gray matter," it does, indeed, contain gray nerve cell bodies which surround a smaller mass of white nerve fibers. The brain, like the heart, is protected by a buffer zone. This, in the form of fluid, may be the source of "water on the brain," but it is very necessary to our survival. Only these pools of fluid and the skull protect the brain from the bumps and grinds of daily living which would damage this fragile organ. With them, we are able to think, reason, love, forgive, create and remember, as well as to survive through automatic processes such as breathing and digesting, and we have reflexes which signal in case of "fight or flight" emergencies. Just think of it!

Optic Nerves (II)

The "optic nerves" are the second pair of the cranial nerves, and lead from the eyes to the brain. The sensory cell bodies of the nerve fibers occur in "ganglia" within the eyes. Their axons (conductors of impulses away from their point of origin) pass through the orbits into the brain. The optic nerves hold the number "II" in the order of cranial nerves.

The Eye

The eyes are the two organs of sight. They are located in the front upper part of the skull and consist of structures that focus an image onto the retina at the back of the eye which is a network of nerves that convert this image into electrical impulses to be recorded in a region of the brain. The eyeball lies in pads of fat within the orbit, a bony socket that provides protection from injury. Each eyeball is moved by six delicate muscles which are activated and coordinated by nerves in the brain stem. The eyeball has a tough, outer coat called the "sclera," or white part of the eye. The front, circular part is the "cornea" and is transparent. The cornea is the main lens of the eye and performs most of the focusing. Behind the cornea is a shallow chamber full of watery fluid, at the back of which is the "iris" (colored part) with the "pupil" (center). The pupil is black and its diameter is changed by light intensity to control the amount of light which enters the eye. Immediately behind the iris, and in contact with it is the crystalline lens, which contracts to alter its shape and allow focusing power. Behind the lens is the main cavity of the eye, filled with a clear gel. On the inside of the back of the eye is the retina, a structure of nerve tissue on which the image formed by the cornea and the crystalline lens forms. The retina needs a constant supply of oxygen and sugar, and the need is supplied by a thin network of branching blood vessels which lie just under it called the choroid plexus. The eyeball is sealed off from the outside by a flexible membrane called the "conjunctiva," which is attached to the skin at the corners of the eye and forms the inner lining of the lids and contains many tiny tear-secreting and mucus-forming glands that protect the eyes from damage due to dryness. A "black eye" is really not black at all. A "shiner" is a mixture of purple, red and yellow hues caused by hundreds of tiny, broken blood vessels.

Visual Nerve Pathways

The axons and ganglion cells in the retina leave the eyes to form the "optic nerves." Just in front of the pituitary gland, these nerves form the X-shaped "optic chiasma," and within the chiasma some of the fibers cross over. The fibers from the nasal half of each retina cross over, but those from the temporal sides do not. Specifically, fibers from the nasal half of the left eye and the temporal half of the right eye form the right "optic tract;" and the fibers from the nasal half of the right eye and the temporal half of the left form the left optic tract. The nerve fibers then continue in the optic tracts. Just before they reach the thalamus, a few of them leave to enter nuclei that function in various visual reflexes. Most of the fibers, however, enter the thalamus and form a junction (synapse) in the back of it. From this region the visual impulses enter nerve pathways called "optic radiations," which lead to the visual cortex of the occipital lobes of the brain.

The Ear

The ear is an organ for hearing and balance. It consists of three parts: the outer ear, the middle ear, and the inner ear. The outer and middle ear mostly collect and transmit sound. The inner ear analyzes sound waves and contains an apparatus that maintains the body's balance. The outer ear is the part which is visible and is made of folds of skin and cartilage. It leads into the ear canal, which is about one inch long in adults and is closed at the inner end by the eardrum. The eardrum is a thin, fibrous, circular membrane covered with a thin layer of skin. It vibrates in response to changes in the air pressure that constitute sound. The eardrum separates the outer ear from the middle ear. The middle ear is a small cavity which conducts sound to the inner ear by means of three tiny, linked, movable bones called "ossicles." These are the smallest bones in the human body and are named for their shape. The hammer (malleus) joins the inside of the eardrum. The anvil (incus) has a broad joint with the hammer and a very delicate joint to the stirrup (stapes). The base of the stirrup fills the oval window which leads to the inner ear. The inner ear is a very delicate series of structures deep within the bones of the skull. It consists of a maze of winding passages, called the "labyrinth". The front (see cochlea) is a tube resembling a snail's shell and is concerned with hearing. The rear part is concerned with balance. "Keeping one's ear to the ground" means to keep up on current trends. The phrase dates back to early 20th century politicians and comes from frontier lore of both pioneers and Indians, who listened for the sound of approaching hoofbeats

Nasal Passages

Smell is often considered to be the least important of all the senses, but it may be one of the oldest, and probably acts on the subconscious more than the other senses. There is little doubt that scents have important roles in human behavior. The body is provided with glands to produce specific odors, many of which appear to be associated with sexual attraction and excitement, and others that have considerable significance as well. The bond between a baby and its mother is thought to be tightened by a form of "scent imprinting." In it, a baby which is suckling at the mother's breast pushes his or her face into a bank of scent organs that surround the nipple. A further sign of the importance of the sense of smell is the way it becomes a major source of information when other senses are not working, especially sight. Only a small part of the nose and nasal cavity is taken up by the organs of smell; the rest of it is mainly concerned with processing the airflow on its way through to the lungs. The walls of the nasal cavity, and particularly the flaplike middle and inferior conchae, are coated with respiratory mucous membranes which incorporate a vast number of tiny hairlike cells which act to move waves of mucus toward the throat. Dust, bacteria, and chemical particles which are inhaled from the air are trapped by the mucus, carried back and swallowed; they are then taken care of by gastric juices to nullify any potential harm. The sense organs themselves are made up of two yellowish-gray patches of tissue, called the olfactory membranes, each about the size of a postage stamp. They are located in a pair of clefts just under the bridge of the nose and at the top of the nasal cavity. The reasons for the coloration are not completely clear, but it seems to be necessary for the membrane to work. During normal breathing, most of the air flows through the nose, with only a small part reaching the olfactory clefts, but this is enough to get a response to a new smell. When a person "sniffs the air" to detect smells, the air moves through the nose much faster, increasing the flow that makes its way to the olfactory clefts and so carrying more odor to those sensors. If you "follow your nose," you are taking a route that lies straight ahead and is obvious (as the nose on your face), or else you are going ahead without a plan, that is, following wherever instinct leads.

Tongue

Anchored to the floor of the mouth and slung at the rear from muscles attached to a spiky outgrowth at the base of the skull, the tongue is a strong muscle that is covered by the lingual membrane, which has special areas which detect the flavor of food. The tongue is made up of muscles covered by mucous membranes. These muscles are attached to the lower jaw and to the hyoid bone (a small, U-shaped bone, which lies deep in the muscles at the back of the tongue) above the larynx. There are very small nodules, called papillae, from the top surface of the tongue, which give it its rough texture. Between the papillae at the sides and base of the tongue are small, bulblike structures that are sensory organs, called "taste buds," which enable us to enjoy the sensations of flavor and warn us when food is unfit to eat. The muscle fibers are heavily supplied with nerves, so it can manipulate food in the mouth and place it between the teeth for chewing - without being bitten in the process. Babies have many more taste buds than an adult, and they have these almost everywhere in the mouth, including the cheeks. Nevertheless, adults enjoy more flavors than babies, who dislike bitter tastes and prefer bland food. The tongue also aids in the formation of sounds of speech and coordinates its movements to aid in swallowing. It is especially helpful when we are forced to "eat our words." Enjoy!

Skin

Skin is the outside covering of body tissue, which protects inner cells and organs from the outside environment. The skin is the largest organ of the body, and its cells are continuously replaced as they are lost to normal wear and tear. The skin totals between twelve and twenty square feet in area and accounts for 12%% of body weight. It is composed of three integrated layers: the epidermis, the dermis and the subcutis. The thickness of the epidermis and the dermis varies over different parts of the body. It is thickest on the palms of the hands and feet, where friction is needed for gripping, and it is thinnest on the eyelids, which must be light and flexible. The epidermis also grows into fingernails, toenails and hair. The dermis, or true skin, is thick, sturdy, rich in nerves and blood vessels and in sweat glands. It shields and repairs injured tissue. This layer consists mostly of collagen, which originates from cells called fibroblasts and is one of the strongest proteins found in nature. It gives skin durability and resilence. The subcutis, joined to the bottom of the dermis, is the deepest layer of the skin. It contains "lipocytes," which produce lipids for the subcutaneous tissue to make a fatty layer which cushions muscles, bones and inner organs against shocks, and acts as an insulator and source of energy during lean times. The skin registers sensation constantly and supports a teeming, unseen population of tiny organisms. Not only does the skin harden from use, but it molds into varied shapes, and it responds to the most delicate touch, becoming an organ of communication - sometimes more eloquent than words. So tough and durable is the skin that when a 2,000-year-old Egyptian mummy was fingerprinted, the ridges were perfectly preserved.

Blood Vessels/Nerves in The Dermis

Blood vessels in the dermis supply nutrients to the deep living layers of the epidermis, as well as to dermis cells. These vessels also play an important role in the regulation of body temperature. There are numerous nerve fibers scattered throughout the dermis. Some of them (motor fibers) carry impulses to dermal muscles and glands, causing these structures to react. Others (sensory fibers) carry impulses away from specialized sensory receptors located within the dermis. One set of dermal receptors (Pacinian corpuscles) is stimulated by heavy pressure, while another set (Meissner's corpuscles) is sensitive to light touch. Still other receptors are stimulated by temperature changes or by factors that can damage tissues.

The Heart

The heart is a pumping system which intakes deoxygenated blood through the veins, delivering it to the lungs for oxygenation and then pumping it into the various arteries to be transmitted to where it is needed throughout the body for energy. The heart is about the size of a fist but delivers a more powerful punch. Luckily for us, it contains a buffer zone to decrease its force or we would be shaken by every beat. This buffer zone also protects the heart from outside injury and keeps it from scraping against the chest wall. In some instances, nightmares can seem so real that the heart will pound in fear. In one study, the heart rate of the sleeper was timed at 150 beats per minute. Myth has it that the heart is the seat of the emotions, but it is, instead, a pump to circulate the blood throughout the body and only contributes to the emotions by sending oxygenated blood to our brain cells; so, if you want to gain someone's affection, you may have to ask Cupid to shoot them through the head rather than the heart. An arrow through the heart (or through the head, for that matter) would stop all bodily functions. The Medical Dictionary reports that the heart beats more than 2.5 billion times in an average lifetime. Isn't that "thumping"?

Pulmonary Artery/Vein

When the muscular wall of the right ventricle contacts, the blood inside the heart chamber is put under more pressure, and the tricuspid valve closes. As a result, the only exit is through the "pulmonary trunk," which divides to form the right and left "pulmonary arteries." At the base of this trunk is a "pulmonary semilunar valve" that is made up of three leaflets or cusps. This valve opens when the right ventricle contracts. When the right ventricular muscles relax, blood starts back up the pulmonary trunk, causing the valve to close to prevent the flow from returning into the ventricular chamber. The pulmonary vein travels parallel to the pulmonary artery as it carries the blood back up to the heart.

Brachiocephalic Artery/Vein

The brachiocephalic artery supplies blood to the tissues of the brain and the head. It is the first branch of the aortic arch and rises up to a point near the junction of the sternum (shoulder blade) and the right clavicle (collarbone). At this point, it divides, giving rise to the "common carotid artery," which carries blood to the right side of the neck and head, and the right "subclavian artery," which leads to the right arm. Branches of the subclavian artery supply blood to parts of the shoulder, neck and head. The brachiocephalic vein takes blood from these sites back to the heart from the subclavian vein.

A Vertebra

A typical vertebra has a drum-shaped "body" (centrum) that forms a thick, anterior portion of the bone. A longitudinal row of the bodies supports the weight of the head and trunk. The intervertebral disks, which separate joining vertebrae, are fastened to the roughened upper and lower surfaces of the bodies. These disks cushion and soften the forces created by walking and jumping, which might otherwise fracture the vertebrae or jar the brain. Each intervertebral disk is composed of a band of fibrous fibrocartilage (anulus fibrosus) that surrounds a gelatinous core, called the "nucleus pulposus." The bodies of adjacent vertebrae are joined on the front surfaces by "anterior ligaments" and on the back by "posterior ligaments." Projecting from the back of each body are two short stalks called "pedicles." They form the sides of the "vertebral foramen." Two plates (laminae) arise from the pedicles and fuse in the back to become "spinous process." The pedicles, laminae, and spinous process together complete a bony vertebral arch around the vertebral opening, through which the spinal cord passes. Between the pedicles and laminae of a typical vertebra is a "transverse process" that projects laterally and toward the back. Various ligaments and muscles are attached to the spinal process and the transverse process. Projecting upward and downward from each vertebral arch are "superior" and "inferior articulating processes." These processes bear cartilage-covered facets by which each vertebra is joined to the one above and the one below it. On the surfaces of the vertebral pedicles are notches that align to create openings, called "intervertebral foramina." These openings provide passageways for spinal nerves that proceed between joining vertebrae and connect to the spinal cord.

Spinal Nerves

Thirty-one pairs of spinal nerves originate from the spinal cord. They are all mixed nerves, and they provide a two-way communication system between the spinal cord and parts of the arms, legs, neck and trunk of the body. Although spinal nerves do not have individual names, they are grouped according to the level from which they stem, and each nerve is numbered in sequence. Hence, there are eight pairs of "cervical nerves" (numbered C1 - C8), twelve pairs of "thoracic nerves" (T1 - T12), five pairs of "lumbar nerves" (L1 - L5), five pairs of "sacral nerves" (S1 - S5), and one pair of "coccygeal nerves". The nerves coming from the upper part of the spinal cord pass outward nearly horizontally, while those from the lower regions descend at sharp angles. This is derived from the consequence of growth. In early life, the spinal cord extends the entire length of the vertebral column, but with age, the column grows faster than the cord. As a result, the adult spinal cord ends at the level between the first and second lumbar vertebrae, so the lumbar, sacral, and coccygeal nerves descend to their exits beyond the end of the cord.