The Senses

The human body has 5 major senses; sight, hearing, smell, taste and touch, that provide the information about the external environment and transmit stimuli to sensory nerves and on to the brain for processing.

The sense organs allow humans to interact with their surroundings.

Lights, sounds, smells, taste and touch from our surrounding environment are received by the sense organs of the eyes, ears, nose, tongue and skin.

Sensory receptors in each of these organs allow the nervous system to collect stimuli from the environment.

The sense organs that collect light from the environment are the EYES.

The human eye is about 2.5 centimeters in diameter.

Most of the eyeball rest in the bony eye socket of the skull. Only about 1/6th of the eye is exposed.

3 pairs of small muscles attach the eye to the eye socket.

Secretions from the tear glands keep the eye moist.

The CONJUNCTIVA, a protective membrane covers the eye.

External parts of the eye include the eyelids, eye lashes and eyebrows.

The eye is made up of many different parts that help to collect, focus and analyze light energy.

The eyeball is divided into 2 chambers that are separated by a lens.

– The front chamber contains the CORNEA, the IRIS and blood vessels.
– The CORNEA focuses the light that enters the eye from the environment.
– The IRIS is the part of the eye that controls the opening to the inner eye. This opening in the inner eye is called the PUPIL.
– The iris can adjust the size that the pupil opens to let in more or less light.
– The pigment in the iris gives the eye its color.
– The chamber is filled with a clear watery fluid called AQUEOUS HUMOR.

– The LENS is transparent and is made of many layers of protein fibers.
– It is about 8 millimeters in diameter.
– The function of the lens is to focus light on the retina at the back of the eye.
– The lens helps the eye adjust the light coming off near or far-away objects.
– The shape of the lens changes; FLATTENS when focusing on DISTANT OBJECTS and THICKENS when focusing on NEAR OBJECTS.
– The ability to bring objects into focus even though they are located at different distances is called ACCOMMODATION.
– Many vision problems are caused by improper functioning of the lenses of the eyes.

– The posterior chamber contains the RETINA, RODS, CONES, and the OPTIC NERVE.
– Light is projected from the lens of the eye to the RETINA, located in the inner eye.
– There are PHOTORECEPTORS found in the retina that convert light energy into nerve impulses that are sent to the brain by way of the OPTIC NERVE.
– 2 types of photoreceptors, CONES and RODS, control the collection of light energy in the retina.
– RODS are good at collecting light energy, even at low levels. They help the eye accommodate in dim light and aid in night vision.
– There are about 120 million rods.
– CONES also collect light, but they can distinguish the different colors of light. They are responsible for color vision.
– There are about 7 million cones.
– The chamber is filled with a jelly-like substance called the VITREOUS HUMOR.

Light enters the eye through the pupil and passes through the cornea, the aqueous humor, the lens and the vitreous humor.

Light reaches the retina, where the signals are set up. These signals are sent to the optic nerve, which carries them to the visual portions of the brain.

The lens turns the image upside down and reverses it from left to right.

The visual centers in the brain correct the inversions and reversals of the lens to make the image right side up.

The eyes collect, focus and send light energy to the BRAIN. The brain interprets and gives meaning to the light energy.

Colors that we can see are really just different wavelengths (or energy amounts) of visible light. The brain associates color with them.

Sound is the movement of air molecules detected by a listening device. The ears detect movement of sound waves in the air.

Ears collect the stimulus of sound in the process of hearing.

The human ear is made up of 3 sections; the OUTER EAR, the MIDDLE EAR and the INNER EAR.

– The outer ear catches sound waves and transports then to the EARDRUM.
– Waves of air molecules first enter the AUDITORY CANAL.
– At the end of the auditory canal is a thin membrane called the EARDRUM or TYMPANUM. The eardrum separates the outer ear from the middle ear.
– The movement of air molecules causes the ear drum to vibrate.

– The middle ear is a small cavity that is filled with air.
– It lies within the skull bone between the outer and inner ear.
– At the bottom of the middle ear is an opening that leads into a canal.
– The canal, called the EUSTACHIAN TUBE, is a passageway that connects the middle ear to the throat.
– The air movement that caused the eardrum to vibrate now causes 3 tiny bones to vibrate.
– The 3 bones are called the HAMMER, ANVIL and STIRRUP, the smallest bones in the body.
– The vibrations move along these bones until they are transferred from the stirrup to the OVAL WINDOW, a membrane covered opening between the middle and inner ear.

– The inner ear is entirely encased in bone.
– In the inner ear is the CHOCHLEA, a fluid-filled structure. The cochlea has many canals that are lined with hair.
– The vibrations from the stirrup cause waves within the fluid found inside the cochlea. These fluid waves trigger very small, fine HAIR in side the cochlea.
– The stimulation of these hairs in the cochlea produces nerve impulses via the AUDITORY NERVE that are sent to the brain for interpretation.

– The ears also help the human body maintain BALANCE.
– Above the cochlea are 3 structures called the SEMICIRCULAR CANALS. These canals contain fluid and are lined with hairs.
– Depending on the position of the body, especially the head, the fluid in these structures will be found in different parts of the canal.
– The hairs lining the inside of these canals will be triggered depending on where the fluid is at any particular moment.
– These hairs then send nerve impulses to the brain, and the brain interprets the position of the body.

– At the base of the middle ear is a tube, the EUSTACHIAN TUBE, that connects the middle ear with the back of the throat.
– This tube maintains atmospheric pressure on both sides of the eardrum.
– If the air pressure on the outside of the eardrum changes, swallowing or opening the mouth either release a buildup of air in the middle ear or sends air into the middle ear to control air pressure balance.
– People often sense this as a “popping” in their middle ear. The popping is the middle ear adjusting to the changing air pressure. This usually happens at high altitudes.

The sense of smell and taste are closely related.

The sense organs for the nose and mouth are designed to collect chemicals from the environment.

Chemical molecules that enter the nose and mouth are collected by receptors in these areas.

The receptors of smell are located in the mucus membranes of the upper part of the nasal cavities.

The receptors in the nose for smell are very small, hair-like nerve endings. When chemicals contact these nerves, impulses are sent to the OLFACTORY NERVE to the brain.

The brain interprets the nerve impulses sent by the various nerve endings as a particular odor.

The sense off smell is closely linked to the sense of taste.

When chemical molecules dissolve in the salvia of the mouth, they trigger TASTE BUDS.

Taste buds are found on the lining of the tongue.


The 5 taste classes are interpreted an certain area of the tongue.

Chemicals that enter the mouth are also interpreted by receptors in the nose. Much of what we taste is really what we smell.

The sense of touch is obtained through millions of nerve endings located throughout the body in the skin.

These nerve endings respond to the stimuli of PRESSURE, PAIN, and TEMPERATURE.

The entire body is covered with nerve endings; however area like the fingers, the toes and the face have more nerves, that are more sensitive to softer stimuli.

The strength of all impulses sent from the skin to the brain are the same.

The strength of a nerve impulse from the skin depends on 2 things.
1. The number of nerve endings in the skin actually stimulated by the touch.
2. The frequency of the impulses that these nerves send to the brain.

This means that the strongest pain of sense of pressure to the skin causes the highest frequency and the greatest number of impulses to be sent to the brain for interpretation and action.


The Skeletal System (part 1 of 2)

The human skeleton is made up of 2 types of connective tissue: bone and cartilage.

The primary purpose of the skeleton is to carry the weight and maintain the shape of the body, protect and support the internal organs, and provide an area of support where muscles can attach for movement to occur.

The skeleton is strong enough to absorb reasonable amounts of shock without fracturing, and at the same time it is flexible and light enough to allow movement.

Skeletal bones move in response to muscles that work like levers. Bones act as levers to which muscles are attached. When the muscles contract the bones produce movement.

At birth, the human body is made up of 270 bones, due to the fusion of separate bones; the mature skeleton has only 206 bones.

There is considerable difference between the male and female skeletons. The bones of the male are generally larger and heavier than those of the female. The ends of the bones are thicker in relation to the shafts in males. The points of attachment of muscles are larger in males due to the fact that males have larger muscles. The pelvic girdle of females is designed to allow the birth of children.

Bone replaces itself throughout adult life. Worn or injured bone is removed and replaced by new tissue.

Bone replaces itself at different intervals. The distal end (furthest from the center) of the femur is replaced about every 4 months; however the bone in parts of the shaft of the femur may not be completely replaced during a lifetime. This process allows the bone to serve as the body’s storage are for calcium.

Bones are the major site of mineral storage, particularly calcium and phosphorous, these minerals can be distributed to other parts of the body on demand.

– Long bones are longer than they are wide and they are slightly curved for strength.
– A bone that is curved is structurally able to absorb the stress of the body at several points so the stress will be evenly distributed.
– The straight bone would not be able to evenly distribute the weight of the body and the bone would break more easily.
– Long bones are shaped like tubes with rounded ends that are designed to fit into other bones to form JOINTS.
– The ends of long bones are filled with spongy bone; this makes them light but strong.
– Long bones can be found in the thigh, lower leg, toes, arms, forearms and fingers.
– Short bones are cubed-shaped and nearly equal in length and width.
– They are a spongy bone except at the surface, where there is a thin layer of compact bone.
– Short bones can be found in the wrist and ankle.
– Flat bones are usually thin and are composed of several plates of compact bone over a layer of spongy bone.
– They provide considerable protection and provide ample area for muscle attachment.
– cranial bones, the sternum, ribs and the scapula are flat bones.
– Irregular bones have complex shapes and cannot be grouped into any of the other 3 categories.
– The vertebrae and some of the facial bones are irregular.

Most of the human skeleton is living tissue that is growing constantly.

The bones are made up of cells embedded in a matrix.

About 20% of living bone is water. The remaining 80% is a MATRIX that consists of minerals (tri-calcium phosphate, magnesium and other elements) and protein (collagen fibers).

When the minerals are deposited, the bone becomes OSSIFIED (hardened).

The protein fibers and the minerals make up the non-living matrix of the bone.

Bone is not a completely solid substance; bone has some spaces between the hard components.

The spaces provide channels for blood vessels that supply bone cells with nutrients and make bones lighter.

The bone matrix has a network of canals called HAVERSAIN CANALS in it. These canals allow blood vessels and nerve fibers to extend into interior bone.

Bone also contains many living cells and blood vessels that provide the movement of nourishment into the cell and the removal of waste from the cells.

The surface of the bone is covered by a tough membrane called the PERIOSTEUM.

The periosteum has many microscopic blood vessels that provide nourishment to the bone.

Living bone cells are found in small spaces in the mineral matrix of the bone. There are 3 types of living bone cells and each has a special function.

– Osteoblasts repair broken bones and produce new bone material.
– They also secrete the mineral and protein compounds that form the matrix.
– Osteoclasts are the “bone breakers”. They are able to dissolve pieces of bone that are in the way of “efficient” skeletal design.
– The destructive work is often followed by the constructive work of the osteoblast in the rebuilding of the bone.
– Osteocytes function as the “caretaker” of the bone tissue. They provide coverall maintenance of the bone.

Depending on the size and distribution of the spaces in the bone, the regions of the bone can be categorized as COMPACT or SPONGY.
– Compact bone is dense tissue (thicker) with few spaces.
– It is deposited in a layer over the spongy bone tissue.
– Compact bone tissue provides protection and support and helps long bones resist the stress of weight placed on them. It can resist considerable weight and stress.
– Spongy bone is less dense and contains many large spaces. It is filled with soft tissue called RED MARROW, which makes red blood cells and helps the body store fat.
– Red bone marrow consists of immature blood cells, called STEM CELLS, fat cells, and macrophages.
– Red marrow produces red blood cells, some white blood cells and platelets.
– Spongy bone make up most of the short, flat and irregularly shaped bones. It also makes up most of the ends of long bones.

– Cartilage is both firm and flexible, unlike bone that is harder and more brittle.
– Cartilage is usually located where firmness and flexibility are needed, like joints, the nose, the ears, trachea, and larynx and in between the vertebrate.
– Cartilage is made up of circular cells embedded in a rubbery matrix that has supporting fibers.
– During the development of the skeletal system of humans, embryos begin with a cartilaginous skeleton. Gradually most of the cartilage is replaced by bone.

The axial skeleton includes the bones that lie along the longitudinal axis. It provides the protective functions of the skeleton.

– All of the bones of the head make up the skull. There are 2 regions of the skull; the CRANIUM and the FACE.
– The skull is made up of 22 flat and irregular shaped bones.
– There are 8 bones in the cranium; their function is to protect the brain.
– The facial region is made up of 14 bones and it protects the eyes, nose, mouth and ears.
– The SINUSES are air space in the facial bones. They aid in reducing the weight of the skull.
– The 3 bones in the middle ear (the HAMMER, the ANVIL, and the STIRRUP) function in
transmitting sound to the inner ear and are the smallest bones in the body.

– The vertebral column has 26 bones that are called VERTEBREA.
– At birth the vertebral column has 33 bones.
– 5 SACRAL VERTEBREA. The 5 sacral bones fuse into one large triangular bone, called the SACRUM, located at the back of the pelvis.
– 4 CAUDAL/COCCYGEAL (tail) VERTEBREA. The 4 coccygeal bones fuse into a single COCCYX.
– The vertebrae column is called the BACKBONE; it is made flexible by the cartilage and ligaments that join the individual vertebrae.
– This flexibility allows movement of the head and the trunk of the body.
– DISC of CARTILAGE separate the individual vertebrae. The disc prevent friction due to the rubbing of the bones and act as shock absorbers.

– Just below the neck is the THORACIC BASKET (12 pair of ribs attached to the vertebrae column).
– The 1st 10 pair of ribs are attached to the STERNUM (BREASTBONE) by cartilage strips.
– The 11th and 12th pair of ribs are called the FLOATING RIBS, because they are attached to the vertebrae column, but not to the sternum.
– The loose connections of the ribs to the vertebrae and the flexible cartilage connections at the sternum allow the ribs to move when the lungs are inflated.

– Appendicular is the adjective of the word appendage.
– An appendage is an attachment to a main body or structure. Arms and legs are attached to the axial skeleton.
– The moveable limbs attached to the axial skeleton make up the appendicular skeleton.
– The appendicular skeleton forms a system of levers, providing movement and dexterity.
– The arms and hands, the legs and feet, and the bones of the shoulders and the pelvis make up the appendicular skeleton.
– The sites where the arms and legs are attached to the axial skeleton are bones referred to a GIRDLES.
– The PECTORAL(shoulder)GIRDLE, 2 SCAPULA(shoulder blades) and 2 CLAVICAL(collar bones) hold the arm to the axial skeleton.
– The legs are attached to the PELVIC GIRDLE, which is formed by the fusion of 3 bones. The ILIUM, the ISCHIUM, and the PUBIS on each side of the midline of the body.
– The PELVIC GIRDLE receives the weight of the upper body from the vertebral column and transfers it to the leg bones or the surface when seated.
– The arched shape of the foot allow it to withstand tremendous force and weight.

– The fused bones creating the CRANUIM make up the SKULL.
– The lower TEETH are located in the MANDIBLE(jaw).
– The collar bone is the CLAVICAL.
– The wings in the upper back are called SCAPULAS.
– The bone connecting all of the RIBS in the middle of the chest is the STERNUM.
– The ribs are connected in the back to the VERTEBRAL COLUMN(backbone), which is composed of VERTEBREA.
– The vertebrae in the neck are called CERVICAL VERTEBREA; THORACIC VERTEBREA join with the ribs; LUMBAR VERTEBREA descends from the thoracic vertebrae to the pelvis; and together the fused bones in the PELVIS make up the SACRUM.
– The COCCYX(tail) is made up of CAUDAL VERTEBREA.
– The bone in the upper arm is the HUMERUS and the two bones in the lower arm are the RADIUS and ULNA.
– The wrist bones are the CARPALS.
– At the base of the fingers(the palm of the hand) are the METACARPALS.
– The smaller bones extending out to the fingertips are the PHALANGES.
– The largest bones in the body are the FEMURS, the thigh bone connects the upper legs with the pelvis.
– The upper and lower legs meet at the KNEE covering that is the kneecap, or PATELLA.
– Each lower leg has two long bones, the TIBIA and FIBULA.
– The little bones in the ankles are the TARSALS, that connect to the METATARSALS
– The bones extending into the toe tips are called PHALANGES.

The site where 2 bones come together is called a JOINT.

Most joints allow the bones to move at the connection point.

Some joints are immovable. They are usually bones that are fused together, like the SUTURES located between the skull bones.

Moveable joints are held together with LIGAMENTS, a strong, flexible tissue that connects bone to cartilage.

The more mobile the joint is, the weaker it is.

– No movement is allowed by this type of joint.
– Suture joints in the skull are an example.
– Bones meeting at these joints have some ability to move.
– The spaces between the vertebrae in the back are an example.
– Bones meeting at these joints have the possibility of great movement.
– BALL and SOCKET JOINTS (found in the shoulder), HINGE JOINT (a back and forth movement joint, like the knees and elbows), and PIVOT JOINTS (a rotating joint, like the forearm at the elbow) are all examples of freely moving joints.