Hip and Lower Limb Muscles

These muscles, particularly those of the hips and thigh, tend to be large and heavy because they are used to move the entire weight of the body and to resist the force of gravity. Therefore, they are important for movement and balance.

Muscles That Move the Thigh

The muscles that move the thigh have at least one origin on the pelvic girdle and insert on the femur. Notice that the iliopsoas is an anterior muscle that moves the thigh, while the gluteal muscles (”gluts”) are posterior muscles that move the thigh. The adductor muscles are medial muscles (Fig. 1.19).

Iliopsoas (includes psoas major and iliacus) originates at the ilium and the bodies of the lumbar vertebrae, and inserts on the femur anteriorly. This muscle is the prime mover for flexing the thigh and also the trunk, as when we bow. As the major flexor of the thigh, the iliopsoas is important to the process of walking. It also helps prevent the trunk from falling backward when a person is standing erect. The gluteal muscles form the buttocks

Gluteus maximus is the largest muscle in the body and covers a large part of the buttock (gluteus means buttocks in Greek). It originates at the ilium and sacrum, and inserts on the femur. The gluteus maximus is a prime mover of thigh extension, as when a person is walking, climbing stairs, or jumping from a crouched position. Notice that the iliopsoas and the gluteus maximus are antagonistic muscles.

Gluteus medius lies partly behind the gluteus maximus. It runs between the ilium and the femur, and functions to abduct the thigh. The gluteus maximus assists the gluteus medius in this function. Therefore, they are  synergistic muscles.

Adductor group muscles (pectineus, adductor longus, adductor magnus, gracilis) are located on the medial thigh. All of these muscles originate from the pubis and ischium, and insert on the femur; the deep adductor magnus. Adductor muscles adduct the thigh that is, they lower the thigh side ways from a horizontal position. Because they press the thighs inward, these are the muscles that keep a rider on a horse.

Skeletal Muscle Group IV

Muscles of the Arm

The muscles of the arm move the forearm.

Biceps brachii is a muscle of the proximal anterior arm (Fig.1.16) that is familiar because it bulges when the forearm is flexed. It also supinates the hand when a doorknob is turned or the cap of a jar is unscrewed. The name of the muscle refers to its two heads that attach to the scapula, where it originates. The biceps brachii inserts on the radius.

Brachialis originates on the humerus and inserts on the ulna. It is a muscle of the distal anterior humerus and lies deep to the biceps brachii. It is synergistic to the biceps brachii in flexing the forearm.

Triceps brachii is the only muscle of the posterior arm (Fig.1.17). It has three heads that attach to the scapula and humerus, and it inserts on the ulna. The triceps extends the forearm. It is sometimes called the fiboxers muscle because it extends the elbow when a punch is thrown. The triceps is also used in tennis to do a backhand volley.

Muscles of the Forearm

The muscles of the forearm move the hand and fingers. Note that extensors of the wrists and fingers are on the lateral forearm and flexors are on the medial forearm.

Flexor carpi and extensor carpi muscles originate on the bones of the forearm and insert on the bones of the hand. The flexor carpi flex the wrists and hands, and the extensor carpi extend the wrists and hands.

Flexor digitorum and extensor digitorum muscles also originate on the bones of the forearm and insert on the bones of the hand. The flexor digitorum flexes the wrist and fingers, and the extensor digitorum extends the wrist and fingers (i.e., the digits).

Skeletal Muscle Group III

Muscles of the Shoulder

Muscles of the shoulder are shown in Figs. 1.13 and 1.14. The muscles of the shoulder attach the scapula to the thorax and move the scapula; they also attach the humerus to the scapula and move the arm. 

 

Muscles That Move the Scapula

Serratus anterior is located below the axilla (armpit) on the lateral chest. It runs between the upper ribs and the scapula. It depresses the scapula and pulls it forward, as when we push something. It also helps to elevate the arm above the horizontal level.

Muscles That Move the Arm

Deltoid is a large, fleshy, triangular muscle (deltoid in Greek means triangular) that covers the shoulder and causes a bulge in the arm where it meets the shoulder. It runs from both the clavicle and the scapula of the pectoral girdle to the humerus. This muscle abducts the arm to the horizontal position.

Pectoralis major (Fig. 1.13) is a large anterior muscle of the upper chest. It originates from a clavicle, but also from the sternum and ribs. It inserts on the humerus. The pectoralis major flexes the arm (raises it anteriorly) and adducts the arm, pulling it toward the chest.

Latissimus dorsi (Fig. 1.14) is a large, wide, triangular muscle of the back. This muscle originates from the lower spine and sweeps upward to insert on the humerus. The latissimus dorsi extends and adducts the arm (brings it down from a raised position). This muscle is very important for swimming, rowing, and climbing a rope.

Rotator cuff (Fig. 1.15). This group of muscles is so named because their tendons help form a cuff over the proximal humerus. These muscles lie deep to those already mentioned, and they are synergists to them.

Skeletal Muscle Group II

Muscles of the Trunk

The muscles of the thoracic wall are primarily involved in breathing. The muscles of the abdominal wall protect and support the organs within the abdominal cavity.

Muscles of the Thoracic Wall 

External intercostal muscles occur between the ribs, they originate on a superior rib and insert on an inferior rib. These muscles elevate the rib cage during the inspiration phase of breathing.

The diaphragm is a domehaped muscle that, as you know, separates the thoracic cavity from the abdominal cavity. Contraction of the diaphragm also assists inspiration.

Internal intercostal muscles originate on an inferior rib and insert on a superior rib. These muscles depress the rib cage and contract only during a forced expiration. Normal expiration does not require muscular action.

Muscles of the Abdominal Wall

The abdominal wall has no bony reinforcement (Fig. 1.12). The wall is strengthened by four pairs of muscles that run at angles to one another.The external and internal obliques and the transversus abdominis occur laterally, but the fasciae of these muscle pairs meet at the midline of the body, forming a tendinous area called the linea alba. The rectus abdominis is a superficial medial pair of muscles. All of the muscle pairs of the abdominal wall compress the abdominal cavity and support and protect the organs within the abdominal cavity.

External and internal obliques occur on a slant and are at right angles to one another between the lower ribs and the pelvic girdle. The external obliques are superior to the internal obliques. These muscles also aid trunk rotation and lateral flexion.

Transversus abdominis, deep to the obliques,extends horizontally across the abdomen. The obliques and the transversus abdominis are synergistic muscles.

Rectus abdominis has a straplike appearance but takes its name from the fact that it runs straight (rectus means straight) up from the pubic bones to the ribs and sternum. These muscles also help flex and rotate the lumbar portion of the vertebral column.

Skeletal Muscle Group II

Muscles of the Neck

Deep muscles of the neck are responsible for swallowing. Superficial muscles of the neck move the head.

Swallowing

Swallowing is an important activity that begins after we chew our food. First, the tongue (a muscle) and the buccinators squeeze the food back along the roof of the mouth toward the pharynx. An important bone that functions in swallowing is the hyoid. The hyoid is the only bone in the body that does not articulate with another bone.

Muscles that lie superior to the hyoid, called the suprahyoid muscles, and muscles that lie inferior to the hyoid, called the infrahyoid muscles, move the hyoid. The suprahyoid muscles pull the hyoid forward and upward toward the mandible. Because the hyoid is attached to the larynx, this pulls the larynx upward and forward. The epiglottis now lies over the glottis and closes the respiratory passages. Small palatini muscles pull the soft palate backward, closing off the nasal passages. Pharyngeal constrictor muscles push the bolus of food into the pharynx, which widens when the suprahyoid muscles move the hyoid. The hyoid bone and larynx are returned to their original positions by the infrahyoid muscles. Notice that the suprahyoid and infrahyoid muscles are antagonists.

Muscles That Move the Head

Two muscles in the neck are of particular interest: The sternocleidomastoid and the trapezius illustrated in Fig. 1.10 and 1.11. Recall that flexion is a movement that closes the angle at a joint and extension is a movement that increases the angle at a joint. Recall that abduction is a movement away from the midline of the body, while adduction is a movement toward the midline. Also, rotation is the movement of a part around its own axis.

Sternocleidomastoid

muscles ascend obliquely from their origin on the sternum and clavicle to their insertion on the mastoid process of the temporal bone. Which part of the body do you expect them to move? When both sternocleidomastoid muscles contract, flexion of the head occurs. When only one contracts, the head turns to the opposite side. If you turn your head to the right, you can see how the left sternocleidomastoid shortens, pulling the head to the right.

Each trapezius muscle is triangular, but together, they take on a diamond or trapezoid shape. The origin of a trapezius is at the base of the skull. Its insertion is on a clavicle and scapula. You would expect the trapezius muscles to move the scapulae, and they do. They adduct the scapulae when the shoulders are shrugged or pulled back. The trapezius muscles also help extend the head, however. The prime movers for head extension are actually deep to the trapezius.

Skeletal Muscle Group I

Head

Muscles of Facial Expression

The muscles of facial expression are located on the scalp and face. These muscles are unusual in that they insert into and move the skin. Therefore, we expect them to move the skin and not a bone. The use of these muscles communicates to others whether we are surprised, angry, fearful, happy, and so forth. 

Frontalis lies over the frontal bone; it raises the eyebrows and wrinkles the brow. Frequent use results in furrowing of the forehead. 

Orbicularis oculi is a ringlike band of muscle that encircles (forms an orbit about) the eye. It causes the eye to close or blink, and is responsible for ”crow's feet” at the eye corners.

Orbicularis oris encircles the mouth and is used to pucker the lips, as in forming a kiss. Frequent use results in lines about the mouth.

Buccinator muscles are located in the cheek areas. When a buccinator contracts, the cheek is compressed, as when a person whistles or blows out air. Therefore, this muscle is called the ”trumpeter's muscle”. Important to everyday life, the buccinator helps hold food in contact with the teeth during chewing. It is also used in swallowing.

Zygomaticus extends from each zygomatic arch (cheekbone) to the corners of the mouth. It raises the corners of the mouth when a person smiles.

Muscles of Mastication

The musles of mastication are used when we chew food or bite something. Although there are four pairs of muscles for chewing, only two pairs are superficial and shown in Fig. 1.9. As you might expect, both of these muscles insert on the mandible. 

Each masseter has its origin on the zygomatic arch and its insertion on the mandible. The masseter is a muscle of mastication (chewing) because it is a prime mover for elevating the mandible. 

Each temporalis is a fan-shaped muscle that overlies the temporal bone. It is also a prime mover for elevating the mandible. The masseter and temporalis are synergists.

Muscle Connective Tissue Coverings

Muscles are organs, and as such they contain other types of tissues, such as nervous tissue, blood vessels, and connective tissue. Connective tissue is essential to the organization of the fibers within a muscle (Fig. 1.7). First, each fiber is surrounded by a thin layer of areolar connective tissue called the endomysium. Blood capillaries and nerve fibers reach each muscle fiber by way of the endomysium. Second, the muscle fibers are grouped into bundles called fascicles. The fascicles have a sheath of connective tissue called the perimysium.Finally, the.muscle itself is covered by a connective tissue layer called the epimysium. The epimysium becomes a part of the fascia, a layer of fibrous tissue that separates muscles from each other (deep fascia) and from the skin (superficial fascia). Collagen fibers of the epimysium continue as a strong, fibrous tendon that attaches the muscle to a bone. The epimysium merges with the periosteum of the bone.

 

Functions of Skeletal Muscles

Skeletal muscles support the body

Skeletal muscle contraction opposes the force of gravity and allows us to remain upright. Some skeletal muscles are serving this purpose even when you think you are relaxed.

Skeletal muscles make bones and other body parts move

Muscle contraction accounts not only for the movement of limbs but also for eye movements, facial expressions, and breathing. 

Skeletal muscles help maintain a constant body temperature 

Skeletal muscle contraction causes ATP to break down, releasing heat that is distributed about the body. Skeletal muscle contraction assists movement in cardiovascular and lymphatic vessels

The pressure of skeletal muscle contraction keeps blood moving in cardiovascular veins and lymph moving in lymphatic vessels. 

Skeletal muscles help protect internal organs and stabilize joints 

Muscles pad the bones that protect organs, and they have tendons that help hold bones together at joints.

Muscular System

Smooth muscle

is located in the walls of hollow internal organs, and its involuntary contraction moves materials through an organ. Smooth muscle fibers are spindle-shaped cells, each with a single nucleus (uninucleated). The cells are usually arranged in parallel lines, forming sheets. Smooth muscle does not have the striations (bands of light and dark) seen in cardiac and skeletal muscle. Although smooth muscle is slower to contract than skeletal muscle, it can sustain prolonged contractions and does not fatigue easily.

CardiacMuscle

forms the heart wall. Its fibers are uninucleated, striated, tubular, and branched, which allows the fibers to interlock at intercalated disks. Intercalated disks permit contractions to spread quickly throughout the heart. Cardiac fibers relax completely between contractions, which prevents fatigue. Contraction of cardiac muscle fibers is rhythmical, it occurs without outside nervous stimulation or control. Thus, cardiac muscle contraction is involuntary.

Skeletal Muscle

Skeletal muscle fibers are tubular, multinucleated, and striated. They make up the skeletal muscles attached to the skeleton. Skeletal muscle fibers can run the length of a muscle and therefore can be quite long. Skeletal muscle is voluntary because its contraction is always stimulated and controlled by the nervous system.

Accessory Structures of the Skin II

Nails
Grow from special epithelial cells at the base of the nail in the region called the nail root (Fig. 5.4). These cells become keratinized as they grow out over the nail bed. The visible portion of the nail is called the nail body. The cuticle is a fold of skin that hides the nail root. Ordinarily, nails grow only about 1 millimeter per week. The pink color of nails is due to the vascularized dermal tissue beneath the nail. The whitish color of the half-moonshaped base, or lunula, results from the thicker germinal layer in this area.

Glands

The glands in the skin are groups of cells specialized to produce and secrete a substance into ducts.
Sweat Glands

Sweat glands, or sudoriferous glands, are present in all regions of the skin. There can be as many as 90 glands persquare centimeter on the leg, 400 glands per square centimeter on the palms and soles, and an even greater number on the fingertips. A sweat gland is tubular. The tubule is coiled, particularly at its origin within the dermis. These glands become active when a person is under stress. Two types of sweat glands are shown in Fig. 1.4. Apocrine glands open into hair follicles in the anal region, groin, and armpits. These glands begin to secrete at puberty, and a component of their secretion may act as a sex attractant. Eccrine glands open onto the surface of the skin. They become active when a person is hot, helping to lower body temperature as sweat evaporates. The sweat (perspiration) produced by these glands is mostly water, but it also contains salts and some urea, a waste substance. Therefore, sweat is a form of excretion. Ears contain modified sweat glands, called ceruminous glands, which produce cerumen, or earwax

Sebaceous Glands

Most sebaceousglands are associated with a hair follicle. These glands secrete an oily substance called sebum that flows into the follicle and then out onto the skin surface. This secretion lubricates the hair and skin, and helps waterproof them. Particularly on the face and back, the sebaceous glands may fail to discharge sebum, and the secretions collect, forming whiteheads or blackheads. If pus-inducing bacteria are also present, a boil or pimple may result. Acne vulgaris, the most common form of acne, is an inflammation of the sebaceous glands that most often occurs during adolescence. Hormonal changes during puberty cause the sebaceous glands to become more active at this time.

Eccrinel sweat glands are widely distributed over the body especially on the forehead, back, palms, and, soles. These glands are formed before birth and function in evaporative cooling (Fig. 1.4a and 1.4b).
Apocrine sweat glands are much larger than the eccrine glands. They are found in the axillary and pubic regions where they secrete into hair follicles. Apocrine glands are not functional until puberty and their odoriferous secretion is thought to act as a sexual attractant. 

Mammary Glands

The mammary glands are located within the breasts. A female breast contains 15 to 25 lobes, which are divided into lobules (Fig. 1.5). Each lobule contains many alveoli. When milk is secreted, the milk enters a duct that leads to the nipple. Cells within the alveoli produce milk only after childbirth in response to complex hormonal changes occurring at that time.

Accessory Structures of the Skin I

Hair

is found on all body parts except the palms, soles, lips, nipples, and portions of the external reproductive organs. Most of this hair is fine and downy, but the hair on the head includes stronger types as well. After puberty, when sex hormones are made in quantity, there is noticeable hair in the axillary and pelvic regions of both sexes. In the male, a beard develops, and other parts of the body may also become quite hairy. When women produce more male sex hormone than usual, they can develop hirsutism, a condition characterized by excessive body and facial hair. Hormonal injections and electrolysis to kill roots are possible treatments. Hairs project from complex structures called hair follicles. These hair follicles are formed from epidermal cells but are located in the dermis of the skin (Fig. 1.1). Certain hair follicle cells continually divide, producing new cells that form a hair. At first, the cells are nourished by dermal blood vessels, but as the hair grows up and out of the follicle, they are pushed farther away from this source of nutrients, become keratinized, and die. The portion of a hair within the follicle is called the root, and the portion that extends beyond the skin is called the shaft. 

The life span of any particular hair is usually three to four months for an eyelash and three to four years for a scalp hair; then it is shed and regrows. In males, baldness occurs when the hair on the head fails to regrow. Alopecia, meaning hair loss, can have many causes. Male pattern baldness, or androgenic alopecia, is an inherited condition. Alopecia areata is characterized by the sudden onset of patchy hair loss. It is most common among children and young adults, and can affect either sex. Each hair has one or more oil, or sebaceous, glands, whose ducts empty into the follicle. A smooth muscle, the arrector pili, attaches to the follicle in such a way that contraction of the muscle causes the hair to stand on end. If a person has had a scare or is cold, figoose bumpsfl develop due to contraction of these muscles.

Dermis & Hypodermis

The dermis, a deeper and thicker region than the epidermis, is composed of dense irregular connective tissue. The upper layer of the dermis has fingerlike projections called dermal papillae. Dermal papillae project into and anchor the epidermis. In the overlying epidermis, dermal papillae cause ridges, resulting in spiral and concentric patterns commonly known as fingerprints. The function of the epidermal ridges is to increase friction and thus provide a better gripping surface. Because they are unique to each person, fingerprints and foot- prints can be used for identification purposes. The dermis contains collagenous and elastic fibers. The collagenous fibers are flexible but offer great resistance to overstretching, they prevent the skin from being torn. The elastic fibers stretch to allow movement of underlying muscles and joints, but they maintain normal skin tension. The dermis also contains blood vessels that nourish the skin. Blood rushes into these vessels when a person blushes; blood is reduced in them when a person turns cyanotic, or "blue". Sometimes, blood flow to a particular area is restricted in bedridden patients, and consequently they develop decubitusulcers (bedsores). These can be prevented by changing the patients position frequently and by massaging the skin to stimulate blood flow. There are also numerous sensory nerve fibers in the dermis that take nerve impulses to and from the accessory structures of the skin.

Hypodermis, or subcutaneous tissue, lies below the dermis. From the names for this layer, we get the terms subcutaneous injection, performed with a hypodermic needle. The hypodermis is composed of loose connective tissue, including adipose (fat) tissue. Fat is an energy storage form that can be called upon when necessary to supply the body with molecules for cellular respiration. Adipose tissue also helps insulate the body. A well-developed hypodermis gives the body a rounded appearance and provides protective padding against external assaults. Excessive development of adipose tissue in the hypodermis layer results in obesity.

Skin structure

The skin is sometimes called the cutaneous membrane or the integument. Because the skin has several accessory organs, it is also possible to speak of the integumentary system. The skin (Fig.1) has two regions: the epidermis and the dermis. The hypodermis, a subcutaneous tissue, is found between the skin and any underlying structures, such as muscle. Usually, the hypodermis is only loosely attached to underlying muscle tissue, but where no muscles are present, the hypodermis attaches directly to bone. For example, there are flexion creases where the skin attaches directly to the joints of the fingers.

Epidermis 

The epidermis is the outer and thinner region of the skin. It is made up of stratified squamous epithelium divided into several layers; the deepest layer is the stratum basale, and the most superficial layer is the stratum corneum.

Stratum Basale 

The basal cells of the stratum basale lie just superior to the dermis and are constantly dividing and producing new cells that are pushed to the surface of the epidermis in two to four weeks. As the cells move away from the dermis, they get progressively farther away from the blood vessels in the dermis. Because these cells are not being supplied with nutrients and oxygen (the epidermis itself lacks blood vessels), they eventually die and are sloughed off.

Langerhans cells are macrophages found deep in the epidermis. Macrophages are related to monocytes, white blood cells produced in red bone marrow. These cells phagocytize microbes and then travel to lymphatic organs, where they stimulate the immune system to react.

Melanocytes are another type of specialized cell located in the deeper epidermis. Melanocytes produce melanin, the pigment primarily responsible for skin color. Since the number of melanocytesis about the same in all individuals, variation in skin color is due to the amount of melanin produced and its distribution. When skin is exposed to the sun, melanocytes produce more melanin to protect the skin from the damaging effects of the ultraviolet (UV) radiation in sunlight. The melanin is passed to other epidermal cells, and the result is tanning, or in some people, the formation of patches of melanin called freckles. A hereditary trait characterized by the lack of ability to produce melanin is known as albinism. Individuals with this disorder lack pigment not only in the skin, but also in the hair and eyes. Another pigment, called carotene, is present in epidermal cells and in the dermis and gives the skin of certain Asians its yellowish hue. The pinkish color of fair skinned people is due to the pigment hemoglobin in the red blood cells in the capillaries of the dermis.

Stratum Corneum
As cells are pushed toward the surface of the skin, they become flat and hard, forming the tough, uppermost layer of the epidermis, the stratum corneum. Hardening is caused by keratinization, the cellular production of a fibrous, waterproof protein called keratin. Over much of the body, keratinization is minimal, but the palms of the hands and the soles of the feet normally have a particularly thick outer layer of dead, keratinized cells. The waterproof nature of keratin protects the body from water loss and water gain. The stratum corneum allows us to live in a desert or a tropical rain forest without damaging our inner cells. The stratum corneum also serves as a mechanical barrier against microbe invasion. This protective function of skin is assisted by the secretions of sebaceous glands, which weaken or kill bacteria on the skin.