# UNIT 1 Supporting Systems in Animals

##### This unit deals with support system of animals.

STRUCTURE AND LIFE PROCESSES OF ORGANISMS

UNIT ONE

SUPPORTING SYSTEMS IN ANIMALS

The skeleton is the body part that forms the supporting structure of an organism. It can also be seen as the bony frame work of the body which provides support, shape and protection to the soft tissues and delicate organs in animals.

Types of skeleton

There are several different skeletal types: the exoskeleton, which is the stable outer shell of an organism, the endoskeleton, which forms the support structure inside the body, the hydroskeleton, a flexible skeleton supported by fluid pressure, and the cytoskeleton present in the cytoplasm of all cells, including bacteria, and archaea. For our course, we will consider only

Exoskeleton and Endoskeleton

The two major types of skeletons: solid and fluid. Solid skeletons can be internal, called an endoskeleton, or external, called an exoskeleton, and may be further classified as pliant (elastic/movable) or rigid (hard/non-movable).

Endoskeleton

This skeleton is found inside the body and can consist of bone (all vertebrates except sharks) or cartilage (sharks) and some endoskeletons consist of both.

• Living: Endoskeletons consist of living tissue, so it is able to grow steadily within the animal enabling some to reach a large size.
• Structure and support: The endoskeleton provides shape and structural support.
• Structural diversity and adaptation: The bones can vary in size and shape to support the animal’s mass.
• Flexible: The endoskeleton is jointed which allows for flexible movement and support.
• Muscle attachment: The muscles attach directly to the skeletal bones to allow for movement and support.
• Protection: The endoskeleton protects the vital organs such as the heart and lungs which are protected by the ribcage.
• Diversified locomotion: The development of an endoskeleton has allowed for animals to become successfully adapted to locomotion in the environment in which they live. Vertebrates (organisms with a vertebral column and an endoskeleton) have become adapted to move in a number of different modes of locomotion, e.g. running, jumping, swimming, and flying.

• Vulnerable to external environment: The endoskeleton does not offer the animal any protection from the exterior, be it a physical attack or changes in environmental conditions. The animal is therefore very vulnerable.
• Susceptible to disease: The skeleton consists of living tissue so is susceptible to infections and disease

Exoskeleton

An exoskeleton is an external skeleton that supports and protects an animal’s body. The skeleton is non-living and consists of a cuticle strengthened by chitin, a substance secreted by the epidermis (skin). Crustaceans such as crabs have their exoskeleton further strengthened by calcium carbonate. There are muscles attached to the inside of the exoskeleton which provides the resistance needed for muscle action.

The exoskeleton is confined to animals such as insects, spiders, scorpions, crabs etc.,

all of which belong to the Phylum Arthropoda (jointed-legged and jointed-bodied animals). The exoskeleton acts as a hard outer covering, and is made up of a series of plates or tubes. We often call large exoskeletons shells’.

• Muscle attachment: The exoskeleton forms the point of attachment of internal muscles needed for locomotion thereby providing better leverage for muscle action.
• Protection: The exoskeleton protects the soft internal tissues and organs.
• Support: The exoskeleton provides structural support and shape.
• Prevents Dessication: The exoskeleton prevents desiccation (drying out) on land.
• Light-weight: The exoskeleton of insects has a low density and is therefore lightweight, to allow for flight.
• Diversity: The mouth-parts can be modified for biting, sucking, piercing grasping thus providing for a diversified diet for organisms possessing an exoskeleton compared to those that do not.

• Size restriction: The final body size is limited because as the body size increases, the surface area to volume ratio decreases. The larger the animal, the heavier the exoskeleton, making movement more difficult.
• Non-living skeleton does not grow with animal: The overall growth of the animal is restricted due to periodic moulting. Since the exoskeleton restricts growth, moulting is required to accommodate for increases in the size of the animal.
• Vulnerability during moulting: The animal is vulnerable when it is in the moulting process, because the new skeleton is very soft until the new exoskeleton has dried and hardened.
• Sites of structural weakness: Exoskeletons are weaker at the joints.

Skeletal Materials (Cartilage and Bone) in Mammals

Bone

Bones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue.

Bones have a variety of shapes with a complex internal and external structure. They are also lightweight, yet strong and hard. One of the types of tissue that makes up bone tissue is mineralized tissue and this gives it rigidity and a honeycomb-like three-dimensional internal structure.

Other types of tissue found in bones include marrow, endosteum and periosteum, nerves, blood vessels and cartilage. There are 206 bones in the adult human body and 270 in an infant.

The human skeleton showing the major bones of the body.

Cartilage

During embryogenesis the precursor to bone development is cartilage. Much of this substance is then replaced by bone during the second and third trimester, after the flesh such as muscle has formed around it; forming the skeleton. Cartilage is a stiff and inflexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs. It is not as hard and rigid as bone but is stiffer and less flexible than muscle.

Cartilage is composed of specialized cells called chondrocytes that produce a large amount of extracellular matrix composed of Type II collagen (except Fibrocartilage which also contains type I collagen) fibers, abundant ground substance rich in proteoglycan, and elastin fibers. Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in the relative amounts of these three main components.

Unlike other connective tissues, cartilage does not contain blood vessels. The chondrocytes are supplied by diffusion, helped by the pumping action generated by compression of the articular cartilage or flexion of the elastic cartilage. Thus, compared to other connective tissues, cartilage grows and repairs more slowly.

The Morphological Differences between Bone and Cartilage

 Basis for Comparison Bone Cartilage Meaning Bones are the complex structure, made up of connective tissues which are hard and are helpful in providing protection, shape to the body. Cartilage is the simple structure, made up of connective tissue which is soft and are useful in providing flexibility to the joints and also protect from the external and internal shocks. Features They are rigid, non-flexible, and tough. They are flexible and are soft-elastic. Bones grow in both directions (bidirectional). Cartilage grows in single direction (unidirectional). Haversian system and Volkmann’s canals are present. Haversian system and Volkmann’s canals are absent. Bone marrow is present (it is a kind of haematopoietic tissue from which all blood cells are made). Bone marrow is absent. Lacunae possess canaliculi where each lacuna consist of only one cell (osteocyte). Lacunae do not possess canaliculi, and each lacuna has two-three chondrocytes. These are active participants of blood supply. They are not the participants in blood supply, except in perichondrium. Matrix consist of the protein called ossein and can be both organic and inorganic. They occur in lamellae and are vascular. They have the deposit of calcium salts largely of calcium phosphate. The matrix consists of the protein called chondrin, and they are organic. In cartilage, a matrix is said to be as homogenous mass without lamellae. They do not possess calcium salts. Bones cells are also known as Osteocytes. Cartilage cells are also known as Chondrocytes. Bones are hard due to the deposition of phosphates and carbonates of calcium in the matrix. Cartilage is soft, except the calcified cartilage and matrix is made up of proteins and sugars. They are responsible for the formation of the skeletal system, which gives the shape to the body. Cartilage is found in ear, nose, larynx and trachea. Types Two types 1. Compact bone. 2.Spongy bone. Three types 1. Fibrocartilage. 2. Elastic cartilage. 3. Hyaline cartilage.

Classification of the Mammalian Skeletal System

Axial and Appendicular Skeleton

## Human skeleton

Humans have a living endoskeleton (internal skeleton) made of bone, cartilage and connective tissue. At birth, the human skeleton consists of over 270bones. However, in adults this number has reduced to 206 bones due the fusion of smaller bones into larger structures. The adult skeleton is made up of the axial skeleton and the appendicular skeleton

### Axial skeleton

The axial skeleton forms the central axis of the body and consists of the skull, vertebral column and rib cage and sternum.

The Skull

The skull consists of the a) cranium and b) facial bones.

1. a) The cranium consists of eight flat bones joined together by immovable joints called sutures. The cranium surrounds and protects the brain. There is a large opening at the base of the skull called the foramen magnum through which the spinal cord passes. On either side of the foramen magnum is a projection which articulates with the first vertebra (called the atlas) to allow for the nodding movement of the head.

The cranium.

1. b) There are 15 facial bones. These are irregular bones that include the cheek bones, nasal bones, temples, upper jaw bone (maxilla)) and lower jaw bones (mandible). The only movable bone is the lower jaw. The upper and lower jaws bear the sockets for the 32 permanent teeth.

The Human Vertebral Column

The vertebral column typically consists of 24 articulating vertebrae and 9 fused vertebrae in the sacrum and the coccyx. Between the vertebrae are discs of fibrocartilage, which prevent friction between vertebrae, and act as shock absorbers during walking, running and jumping.

Spinal nerves are able to enter and leave the spinal cord through gaps between adjacent vertebrae. Strong ligaments and muscles around the spine stabilize the vertebrae and help to control movement. The vertebrae join up to each other in such a way that there is a continuous spinal canal which runs from the base of the skull to the pelvic girdle. This canal contains the spinal cord. The entire vertebral canal can be divided into five regions.

• Cervical region
• Thoracic region
• Lumbar region
• Sacral region
• Coccyx

Cervical region of the skeleton is highlighted in red.

Human Vertebral Column.

The cervical (neck) region consists of seven vertebrae. The first cervical vertebra, called the atlas, supports the skull and allows for the nodding movement of the head. The second vertebra, called the axis, has a projection on which the atlas pivots to give the side to side movement of the head.

The thoracic region (chest) consists of 12 vertebrae, which each bear a pair of ribs.

The thoracic vertebrae give rise to 12 pairs of ribs.

The lumbar region (lower back) consists of five vertebrae. This region has the largest vertebrae as it carries the weight of the body.

The lumbar region is highlighted by the red circle.

The sacral region consists of five fused vertebrae, forming a bone called the sacrum. The sacrum forms part of the pelvic girdle which provides surfaces for the attachment of muscles and the legs.

The coccyx is made up of four fused bones. These bones form the tail in those mammals that have tails.

In a newborn baby the entire vertebral column curves backwards probably because of the confines of the uterus. Initially a baby cannot support the weight of its head. When after about 3 months it is able to support its head, the cervical forward curve is complete. The lumbar forward curve is complete when the baby is able to stand on its own and ready to learn to walk.

Functions of the vertebral column

• Supports the skull
• Surrounds and protects the spinal cord
• Provides attachment for ribs, girdles, and back muscles
• Separate vertebrae and S-shaped curvature provide flexibility allowing humans to bend backwards, forwards and sideways
• Fibrocartilage discs between the vertebrae act as shock absorbers

The rib cage and sternum

The rib cage is a bony and cartilaginous structure. A typical rib cage consists of 24 ribs (12 pairs), the sternum (an inverted T-shaped structure connecting the rib bones), costal cartilages and the 12 thoracic vertebrae shown in the diagram below. The first seven pairs of ribs connect directly to the sternum and are referred to as true ribs. The remaining five pairs of ribs do not connect directly to the sternum and are referred to as false ribs. The rib cage aids in the protection of the heart and lungs. With the help of the diaphragm and the intercostal muscles, they increase and decrease the volume of the thoracic cavity thereby allowing inhalation and exhalation to take place.

The Human Rib Cage.

### Appendicular skeleton

The appendicular skeleton consists of the pectoral girdle with the arms and the pelvic girdle with the legs. The pectoral girdle and arms and pelvic girdle and legs will be explored in greater detail in the following section.

The Pectoral Girdle and Arms

The pectoral girdle consists of two clavicles (collar bones) and two scapulae (shoulder blades). Each clavicle is attached to the sternum in the front and the scapulae at the sides and they help to support the shoulders. The clavicle is the most frequently broken bone in the body as it often takes the full impact of   falls on outstretched arms or of blows to the shoulder. The pectoral girdle is connected by muscles to the back of the thorax to enable it to have a supporting structure as well as giving the shoulders greater freedom of movement which in turn allows greater mobility of the arms. Any limit to movement is provided by the clavicle.

Each upper arm has a single bone called the humerus which fits into the Glenoid cavity on the scapula to form a ball and socket joint. This cavity is very shallow which allows the arms to move in almost any direction. The forearm consists of two bones namely the ulna in line with the little finger and the radius in line with the thumb. The joint at the elbow is a hinge joint. The wrist consists of eight small carpal bones arranged in two rows of four. The palm of the hand consists of five metacarpal bones. There are 14 digits (short bones) or phalanges in each hand, two in each thumb and three in each of the fingers.

Skeletal framework of the arm and shoulder region.

Functions of pectoral girdle

• Forms a strong support structure for the attachments of the arms.
• Provides large area of bone for the attachment of muscles.
• Forms ball-and-socket joints with the arms which allows the arms to move freely.

Pelvic girdle and the legs

The pelvic girdle consists of hip bones joined at the front by cartilage called the pubic symphysis and they are attached to the sacrum at the back. Each hip bone consists of three fused bones (ilium, ischium and pubis). Portions of all three bones contribute to the formation of the acetabulum, a deep socket into which the head of the femur (thigh bone) joins to form the hip joint.

Skeletal framework of pelvic girdle.

The female pelvic girdle is wider and lighter than the male. This is an adaptation to allow for pregnancy and childbirth.

The femur in the leg is the largest and strongest bone in the body. The upper end forms a ball and socket joint with the hip bone while the lower end articulates with the tibia to form the hinge joint of the knee. The patella or kneecap is a flat triangular bone which is embedded in the tendon of the thigh muscle and attached by a ligament to the tibia.

There are two bones in the lower leg: the tibia (shin bone) which is the larger of the two and supports most of the mass. The upper end articulates with the femur while the lower end articulates with one of the tarsal bones to form the ankle joint. The fibula (calf bone) is thinner than the tibia and serves mainly for the attachment of muscles. It is attached to the femur and is articulated to the top and bottom of the tibia.

Skeletal framework of pelvic girdle and legs.

The structure of the foot is similar to that of the hand. However, the foot supports the weight of the body, so it is stronger and less mobile than the hand. There are seven tarsals or ankle bones, only one of which, the talus, articulates with the tibia. The talus us also know as the ankle bone. The heel bone (calcaneum) is the largest of the tarsal bones and is the bone to which the calf muscle is attached. The heel bone presses firmly on the ground when one stands, walks or runs.

There are 5 metatarsal bones which form the ball and arch of the foot. The 14 phalanges of the toes are the counterparts of those in the fingers, with the big toe having two phalanges and the other 4 having 3 phalanges each. Together these structures consist of the bones form the lower limb shown below

Bones of lower extremity.

### Functions of skeleton

The human skeleton is living and performs many functions in the body. Some important functions are summarised below:

1. Movement: muscles attach to the bones of the skeleton, enabling movement.
2. Protection: the skull protects the brain, the ribcage protects the heart and lungs, and the pelvic bones protect the digestive tract and reproductive organs.
3. Support: provides shape and support to the body.
4. Storage of minerals: bones store minerals such as calcium and phosphate ions.
5. Hearing: bones in the middle ear, called the hammer, anvil and stirrup, amplify sound waves and assist in the hearing process.
6. Red blood cell production: long bones and flat bones contain red bone marrow to produce red blood cells.

Illustration of the relationship of skeleton and muscles during movement

### Dear student, for you to understand the relationship of the skeleton and muscles during movement, we need to discuss the Musculoskeletal tissues

The tissues which provide structure to the body and enable movement are part of the musculoskeletal system. The tissues in this system include the bones, cartilage, joint, tendons, ligaments and muscles. In this section, we will examine each of these types of tissues, so that we can understand how these structures work together to bring about movement.

• Bone: hard mineralized tissue that determines the structure of the body and provides attachments for muscles.
• Cartilage: flexible connective tissue that is usually found in many areas of the body including the joints between bones.
• Joints: region where bones meet, a variety of joint types allow for a range of movements in a number of different planes.
• Ligament: tissue that connects bones to other bones.
• Tendons: fibrous connective tissue that connects muscle to bone and transfers the force generated by the muscles into skeletal movement.
• Muscles: made up of fibres that are capable of contraction and therefore capable of bringing about movement.

### Bones

Bones provide the framework and internal core structure for the attachment of muscles. Bone is a living rigid tissue which forms the support structures for the rest of the body. The process of bone formation is called ossification. The matrix of bone contains a dense arrangement of collagen fibres together with mineral salts of calcium, magnesium and phosphates. The calcium salts give bone its hardness and rigidity while collagen fibres give bones its flexibility and strength.

Microscopic structure of a long bone

Bones are made up of numerous hollow tunnels called Haversian canals. Haversian canals occur within the matrix of bone tissue and run parallel to the length of the bone. Each Haversian canal consists of a nerve to carry impulses, blood vessels to transport gases, food and wastes and a lymph duct to drain tissue fluid. Under the microscope they appear as black circles against a lighter background.

A diagram of a section of compact bone showing Haversian canals.

Each Haversian canal is surrounded by concentric rings of compact bone called lamellae. Each of these layers contains a ring of fluid-filled cavities called lacunae. Each of these lacuna will contain a number of bone cells called osteocytes. The lacunae are linked to each other and to the Haversian canal by a system of very tiny interconnecting canals called canaliculi. Strands of cytoplasm extend through these canals which supply the osteocytes with oxygen and nutrients and remove waste products. The Haversian canals, lacunae, osteocytes and canaliculi together form a unit called an Haversian System and a number of these systems make up compact bones.

Apart from Osteocytes which are embedded in the lacunae of bone there are two other types of bone cells.

Osteoblasts: Bone forming cells. These cells allow the bone to change and remodel its shape as the organism grows and responds to stresses. If a bone is broken or if strengthening is needed, bone cells lay down new tissue and repair damaged tissue

Osteoclasts: Special bone cells for destroying and reabsorbing bone tissue.

Functions of bone

• To serve as a firm support framework for the whole body.
• To protect such delicate structures as the brain and spinal cord.
• To serve as levers, working with attached muscles to produce movement.
• To serve as a storehouse for calcium salts, which may be reabsorbed into the blood if there is not enough calcium in the diet.
• To produce blood cells in the red marrow.

### Cartilage

Cartilage is a tough semi-transparent flexible tissue that consists of a tough matrix or jelly-like substance. The matrix is made up of collagen (a protein) and proteins with special carbohydrate chains called proteoglycans. Cartilage is enclosed by a fibrous capsule called the perichondrium. It consists of living cells called chondrocytes which secrete a rubbery protein matrix called chondrin. Chondrocytes occur in small fluid-filled spaces called lacunae which are scattered throughout the matrix. There are no blood vessels or nerves in the matrix.

Cartilage and bone

Infant and young children do not have bones like those of adults. Their bones are made mostly of cartilage – a firm, elastic, fibrous material. As the individual grows and matures, the cartilage is gradually replaced by bone cells which deposit crystals of calcium carbonate and calcium phosphate. This process called ossification greatly increases the strength of the bone.

 Cartilage Appearance Location Function Hyaline cartilage glass-like, bluish-white, few fibres at ends of bones, forms c-shaped structures in Trachea, joins ribs to sternum, larynx and tip of nose, temporary cartilage in bones reduces friction at joints, allows movement of ribs during breathing, forms permanent structures, allows bones to increase in length Fibrocartilage many white collagen fibres discs between the vertebrae, in the rim of ball and socket joints, between pubic bones acts as shock absorbers, makes the socket deeper while still allowing movement Elastic many yellow fibres in matrix in the pinna of the ear, in the epiglottis maintains the shape of the ear, strengthens the epiglottis

### Joints

A joint is a point at which two bones make contact. It allows movement in many planes.

1. Fibrous joints: Joins bones where no movement is allowed. An example of this includes the bones of your cranium (the skull).
2. Cartilaginous joints: These allow slight, restricted movement. An example is the discs between the vertebrae of the spine.
3. Synovial joints: These allow free movement in one or more directions. Examples include the joints of the pelvic and pectoral girdles. They also facilitate movements like standing, sitting, walking and running.

Another way of categorising joints is movable and immovable joints. Most joints in the skeleton are movable joints. Movable joints are also known as synovial joints. Synovial joints are characterised by the existence of capsules, which contain synovial fluid. The synovial fluid helps to prevent friction during movement.

Example of a synovial joint.

There are a number of different types of synovial joints. The four main types of synovial joints include:

1. Ball and socket joint: Found in structures such as the shoulder. It allows forwards/backwards, up/down and roundabout movement.
2. Hinge joint: Found in structures such as the elbow. It allows the forearm to move up and down and acts like the hinge of a door.
3. Pivot joint: Allows turning of the head in a rotational movement from side to side.
4. Gliding joint: Found in the wrist and foot. It allows bones to slide over one another.

## Movement at joints

Joints occur where two bones meet. Different types of joints allow for different types of movements.

### Tendons and ligaments

Tendon and ligaments are dense bands of dense connective tissue. Ligaments join bone to bone, and tendons join muscles to bone. An example of a ligament is the anterior cruciate ligament (ACL) of the knee, and an example of a tendon is the Achilles tendon, which attaches your calf muscle to your heel. Tendons and ligaments are similar structures, but they have some important differences, which are summarised in Table below.

Comparison of ligaments and tendons

 Ligaments Tendons join bone to bone attach muscles to bones consist of white collagen fibres and a network of yellow elastic fibres consist of non elastic collagen fibres which give tendons a white shiny appearance strong collagen fibres prevent dislocation at joints, and yellow elastic fibres allow flexibility at the joint parallel arrangement of strong collagen fibres in order to efficiently convert muscle contraction into movement of the skeleton

### Antagonistic muscles

Voluntary muscles are normally connected to at least two bones. The point of attachment to the movable bone is called the point of insertion and the point of attachment of a muscle to the immovable bone is called the origin. Most muscles work in pairs and when a muscle works it needs to have an agonist and an antagonist.

An agonist is a muscle that acts to move a limb out of a particular position (contraction). An antagonist is a muscle that acts in opposition to the specific movement generated by the agonist and is responsible for returning the limb back to its original position (relaxation). Antagonistic pairs of muscles are necessary because each muscle can only exert a pulling force. A muscle cannot push itself back to its starting position. Therefore another muscle is required to pull in the opposite direction in order to return the agonist muscle back to its starting position. An example of this can be found in the contraction and relaxation of the biceps and triceps muscles when moving your forearm.

Example: Biceps and triceps

In the case of the biceps the two bones involved are the scapula (origin) and the humerus (insertion). The biceps muscle gets its name from having two tendons attached to the scapula. The tendons join to form a single muscle body, and then splits again into two tendons, one of which inserts at the radius, and the other of which inserts at the ulna. When the biceps muscle contracts, the forearm is lifted or bent, decreasing the angle between the forearm and humerus and flexing your arm. This ability of the biceps to decrease the angle between the joints results in it being called a flexor muscle.

The biceps brachii muscle gets its name from being a two-headed muscle, attaching to the scapula at two points. Although it is commonly referred to as a bicep’, biceps is the correct form even in the singular. Similarly, the triceps brachii muscle joins at three points, and should be referred to as the triceps, whether you are talking about one or both arms.

Illustration of the triceps (extensor) and biceps (flexor) muscles

Straightening of the forearm

When the arm is bent the biceps cannot contract since it is already in a contracted state. Muscles can only cause movement by pulling as they contract, not by pushing when they relax. Therefore, the straightening of the arm is brought about by the contraction of the triceps muscle (an extensor muscle) as it increases the angle between forearm and humerus. The triceps has three points of origin, two on the humerus and one on the scapula, and a single point of insertion on the ulna.

Human locomotion

Locomotion refers to the ability to move. Specifically, it refers to the way in which organisms travel from one place to another. Examples of types of locomotion include running, swimming, jumping or flying. Human locomotion is achieved by the use of our limbs. Below we discuss the major organs and structures that bring about movement in humans.

A marathon event in progress: this locomotion is facilitated by the skeletal framework described in this section.

The structures used during locomotion include:

1. Bones provide the body’s supporting structure. They provide the framework that help maintain the body’s shape and provide a surface for the attachment of muscles.
2. Joints are points of contact between individual bones. They allow bones to move against and past each other to enable movement.
3. Ligaments connect bones the ends of bones together in order to form a joint. Most ligaments limit dislocation, or prevent certain movements that could form breaks. They hold bones in place so that they work in a coordinated manner.
4. Tendons connect muscle to bone. They transfer the force generated by muscle contraction into movement of the skeleton.
5. Muscles work in antagonist pairs to cause bones to move. Muscles are attached to bone via the tendon. Therefore as the muscle contracts, the bone moves.

Summary

The two types of skeleton designs for our discussion are exoskeletons, and endoskeletons. An exoskeleton is a hard external skeleton that protects the outer surface of an organism and enables movement through muscles attached on the inside. An endoskeleton is an internal skeleton composed of hard, mineralized tissue that also enables movement by attachment to muscles. The human skeleton is an endoskeleton that is composed of the axial and appendicular skeleton. The axial skeleton is composed of the bones of the skull, ossicles of the ear, hyoid bone, vertebral column, and ribcage. The skull consists of eight cranial bones and 14 facial bones. Six bones make up the ossicles of the middle ear, while the hyoid bone is located in the neck under the mandible. The vertebral column contains 26 bones, and it surrounds and protects the spinal cord. The thoracic cage consists of the sternum, ribs, thoracic vertebrae, and costal cartilages. The appendicular skeleton is made up of the limbs of the upper and lower limbs. The pectoral girdle is composed of the clavicles and the scapulae. The upper limb contains 30 bones in the arm, the forearm, and the hand. The pelvic girdle attaches the lower limbs to the axial skeleton. The lower limb includes the bones of the thigh, the leg, and the foot.

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