Support Systems in Animals
Support systems in animals
Overview
Animals occupy a wide range of habitats and many can move when conditions become unfavourable. Different environments have led to different types of support systems. The main kinds of support systems are:
- hydrostatic skeleton
- exoskeleton
- endoskeleton
Among animals, vertebrates have an especially advanced support system - the endoskeleton - which provides support, protection and enables flexible movement. This chapter compares support systems across animals and then describes the human support system and locomotion in detail.
1 Skeletons - types and basic features
All living organisms need some form of structural support to keep shape and, where applicable, to move. The form of support varies with organisation and habitat.
Unicellular and soft-bodied animals
Unicellular organisms are shaped and supported by their cell membrane or cell wall. Soft-bodied multicellular invertebrates that live in water (for example many sea anemones and jellyfish) or in moist environments (for example earthworms and some land snails) commonly use a hydrostatic skeleton: a body compartment filled with fluid under pressure that helps maintain shape and enables movement.
Exoskeleton
Arthropods (insects, crustaceans and arachnids) have an exoskeleton - a hard external covering made mainly of chitin (sometimes reinforced with calcium salts). The exoskeleton supports and protects internal tissues and provides attachment surfaces for muscles.
Endoskeleton
Many multicellular animals such as fish, amphibians, reptiles, birds and mammals possess an endoskeleton - an internal rigid framework made of cartilage or bone. Endoskeletons protect soft tissues and organs, give shape to the body, allow for greater growth and provide lever systems for movement.
Cartilaginous endoskeletons (for example sharks) are mainly composed of cartilage. Bony endoskeletons (for example mammals) are composed of bone, joints, ligaments, tendons and muscles.
Developmental progression for terrestrial life
During the colonisation of land, animals developed stronger support systems to cope with gravity and locomotion on solid surfaces. A typical progression of support systems seen in evolution is:
unicellular → multicellular with hydrostatic support → multicellular with exoskeleton → multicellular with endoskeleton
2 Comparative advantages and disadvantages of support systems
| Feature | Hydrostatic skeleton | Exoskeleton | Endoskeleton |
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| Advantages |
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| Disadvantages |
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3 The human skeleton
The human skeleton is the rigid internal framework of the body. In an adult it normally consists of 206 bones, which are grouped by shape into long bones, short bones, flat bones and irregular bones.
3.1 Axial skeleton
The axial skeleton forms the central axis of the body and includes:
- the skull
- the vertebral column
- the ribs and sternum (thoracic cage)
3.1.1 The skull
The human skull comprises 22 bones forming the cranium (which encloses the brain) and the facial skeleton. The skull houses the sense organs for smell, sight and hearing. The facial skeleton includes the maxilla (upper jaw) and mandible (lower jaw) that carry the teeth.
At the base of the cranium are two occipital condyles that articulate with the atlas (the first cervical vertebra). Between these condyles is the foramen magnum, an opening through which the spinal cord passes from the brain into the vertebral column. This articulation enables nodding movements of the head.
3.1.2 Types of teeth and dental formula
Mammals generally have two sets of teeth in their lifetime: a temporary (deciduous or milk) set and a permanent set. Humans usually have 20 milk teeth and 32 permanent teeth. Teeth are arranged symmetrically on each side of the upper and lower jaws. There are four types of teeth with distinct functions:
- Incisors - for cutting and biting food.
- Canines - for tearing and holding food.
- Premolars - for crushing and grinding.
- Molars - for grinding food into fine particles.
The dental formula for a half of the adult human jaw is: 2 : 1 : 2 : 3 (incisors : canines : premolars : molars). This means each side of the upper or lower jaw has 2 incisors, 1 canine, 2 premolars and 3 molars.
3.1.3 The vertebral column
The vertebral column (backbone or spinal column) is a flexible column of irregular bones (vertebrae) extending from the skull to the pelvis. It protects the spinal cord which runs through the vertebral canal, serves as the point of attachment for ribs and limb girdles, and absorbs shock to allow flexible movement.
- Cervical vertebrae: 7
- Thoracic vertebrae: 12
- Lumbar vertebrae: 5
- Sacral vertebrae (fused): 5
- Coccygeal vertebrae (fused): 4
The vertebral column also provides openings for passage of spinal nerves and blood vessels.
3.1.4 The thorax (rib cage)
The thoracic cage is formed by 12 pairs of ribs, the sternum and the thoracic portion of the vertebral column. The rib cage protects the heart and lungs and participates in the mechanism of breathing. Ribs are attached anteriorly to the sternum either directly (true ribs) or indirectly via costal cartilage (false ribs); the lowest ribs are called floating ribs because they do not attach anteriorly to the sternum.
3.2 Appendicular skeleton
The appendicular skeleton connects the limbs to the axial skeleton and includes the limb bones and the girdles that attach them:
- the pectoral (shoulder) girdle and upper limbs
- the pelvic (hip) girdle and lower limbs
The pectoral girdle (scapula and clavicle) connects the arms to the trunk and provides a shallow socket (glenoid cavity) for the head of the humerus. The pelvic girdle (two hip bones fused with the sacrum) provides a deep socket (acetabulum) for the head of the femur.
| Pectoral girdle | Pelvic girdle |
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4 Functions of the human skeleton
The major functions of the human skeleton are:
- Movement and locomotion - bones act as levers; joints and muscles produce movement.
- Protection - bones protect vital organs (for example the skull protects the brain; the rib cage protects the heart and lungs).
- Support and shape - provides a framework that supports soft tissues and maintains body shape.
- Storage of minerals - bones store mineral salts such as calcium and phosphorus and release them as needed.
- Hearing - the three small bones (ossicles) of the middle ear transmit and amplify sound vibrations.
- Haemopoiesis - formation of blood cells occurs in the red bone marrow (produces red and white blood cells and platelets).
5 Structure of a long bone
A typical long bone (such as the femur or humerus) shows the following parts and features:
- Epiphysis - the expanded ends of the bone; often covered with articular (hyaline) cartilage to reduce friction at joints.
- Diaphysis - the shaft of the bone, largely made of compact bone surrounding the marrow cavity.
- Compact (cortical) bone - dense, strong tissue that forms the outer layer of the shaft and provides resistance to bending and torsion.
- Spongy (cancellous) bone - porous network found mainly in the epiphyses; contains red bone marrow.
- Medullary (marrow) cavity - the central cavity of the shaft filled with yellow marrow (rich in fat) in adults; red marrow for blood formation is found in spongy bone and certain long bone regions.
- Periosteum - a fibrous membrane covering the outer surface of bone; contains blood vessels and nerves and is important for growth and repair.
- Epiphyseal line (or plate in growing bones) - the region of growth between the epiphysis and diaphysis during development.
- Nutrient foramen and vessels - provide blood supply to the bone.
6 Relationship between structure and function: tissues of the skeleton
Bones are organs composed of several tissues that together perform the functions of the skeletal system.
- Bone tissue - a living connective tissue with a mineralised extracellular matrix. It is strong and rigid, provides structural support, protects organs, stores minerals and contains marrow for blood cell formation.
- Blood - a connective tissue that transports oxygen, nutrients, hormones and waste products; blood vessels penetrate bone to supply cells and remove waste.
- Striated (skeletal) muscle - attaches to bone by tendons and generates movement by contraction.
- Hyaline cartilage - tough but flexible; covers joint surfaces to reduce friction and acts as a shock absorber.
- Dense fibrous connective tissue - includes tendons (connect muscle to bone) and ligaments (connect bone to bone); these tissues are strong and resist tension.
7 Joints
A joint is a place where two or more bones meet. Joints hold the skeleton together and provide varying degrees of movement. The amount of movement a joint allows depends on its structure and function.
7.1 Classification by movement
Joints may be classified into three functional groups:
- Immovable (fixed) joints - allow little or no movement; for example, cranial sutures.
- Partly movable joints - allow limited movement; for example, the joints between adjacent vertebrae.
- Freely movable (synovial) joints - allow a wide range of movements; most limb joints are synovial.
Fibrous joints (for example the periodontal ligament) hold teeth in their sockets.
7.2 Structure of a synovial joint
Synovial joints are designed to allow movement while preventing bone surfaces from rubbing directly against one another. Main features include:
- Articular cartilage - a layer of hyaline cartilage covering the bone ends to reduce friction and absorb shock.
- Synovial cavity - a small space between the articulating bones filled with synovial fluid.
- Synovial fluid - a viscous fluid that lubricates the joint and nourishes the articular cartilage.
- Synovial membrane - the inner lining of the joint capsule that secretes synovial fluid.
- Fibrous capsule (capsular ligament) - surrounds the joint and provides stability.
- Ligaments - strong bands of fibrous tissue that connect bone to bone and add stability.
- Tendons - attach muscles to bones and may pass over the joint.
- Bursae (where present) - fluid-filled sacs that reduce friction between moving structures.
Major kinds of synovial joints:
- Ball-and-socket joints - allow rotation and movement in many planes; examples: shoulder and hip.
- Hinge joints - permit movement in one plane (flexion and extension); examples: elbow and knee.
- Pivot joints - allow rotation around a central axis; example: the atlanto-axial joint between the atlas and axis.
- Gliding (plane) joints - allow sliding movements; examples: small bones of the wrist and ankle.
8 Roles of bones, joints, ligaments, tendons and cartilage in locomotion
8.1 Role of bones
Bones provide a rigid framework that gives the body shape and strength. They act as levers that transmit forces generated by muscles to produce movement. Long bones must resist deformation when carrying loads.
8.2 Role of joints
Joints connect bones, permitting controlled movements and contributing both stability and mobility to the skeleton. The type of joint determines the range and direction of permitted movement.
8.3 Role of ligaments
Ligaments are strong connective-tissue bands that connect bone to bone. They stabilise joints, limit excessive movement and help maintain joint alignment.
8.4 Role of tendons
Tendons attach muscles to bones and transmit the force produced by muscle contraction to the skeleton. In activities requiring repetitive or elastic loading (for example steady running), tendons store and release elastic energy, reducing the metabolic work required by muscles and improving mechanical efficiency.
8.5 Antagonistic muscle pairs
Muscles often work in antagonistic pairs where one muscle contracts while the other relaxes to produce movement. A common example is the biceps and triceps in the upper arm: contraction of the biceps flexes the forearm at the elbow, while contraction of the triceps extends it.
8.6 Role of cartilage
Cartilage reduces friction at joint surfaces and acts as a shock absorber during movement. It also forms structures such as the intervertebral discs and the cartilage of the rib cage that provide flexibility and cushioning.
9 Structure and function of voluntary (skeletal) muscles
Voluntary skeletal muscles produce consciously controlled movements such as walking, lifting and talking. Key structural features and functional notes:
- Muscle tissue is organised into bundles (fascicles) that are surrounded by connective tissue sheaths; tendons attach the muscle as a whole to bone.
- Each muscle fibre (cell) contains a cytoplasm called sarcoplasm, multiple nuclei, many mitochondria (to supply ATP for contraction) and abundant contractile elements.
- Muscle fibres contain myofibrils, which show alternating light and dark bands; contraction of myofibrils shortens the fibre and therefore the whole muscle.
- Skeletal muscle contraction is under voluntary (somatic) nervous control and can generate rapid, forceful contractions.
10 Diseases and disorders that affect the skeleton
The musculo-skeletal system can be affected by genetic factors, ageing, habitual mechanical stresses, nutritional deficiencies and disease. Common conditions include:
10.1 Rickets
Rickets is a disease of growing children in which bones become weak and soft and may bend under body weight. It is primarily caused by inadequate mineralisation of bone due to deficiencies of vitamin D, calcium and phosphorus in the diet or insufficient exposure to sunlight (which is necessary for vitamin D synthesis).
Signs include bowed legs or deformities of the long bones. Prevention and treatment include adequate dietary calcium and vitamin D and exposure to sunlight where safe.
10.2 Osteoporosis
Osteoporosis is a condition in which bone mass and strength are lost, increasing the risk of fractures. It results when bone resorption outpaces bone formation. Risk factors include ageing, hormonal changes (for example after menopause), inadequate dietary calcium or vitamin D, physical inactivity and certain medical conditions or medications.
Management emphasises adequate nutrition (calcium and vitamin D), regular weight-bearing exercise and medical treatment where appropriate to reduce bone loss and fracture risk.
10.3 Arthritis
Arthritis causes pain, stiffness and swelling of joints. Two common forms are:
- Osteoarthritis - a degenerative condition associated with wear and tear of articular cartilage, often related to ageing or joint injury. It causes joint pain and reduced mobility.
- Rheumatoid arthritis - an autoimmune disorder in which the body's immune system attacks joint tissues, causing inflammation, pain, joint deformity and sometimes involvement of other organs.
Treatment depends on the type of arthritis and may include physiotherapy, anti-inflammatory medication, lifestyle modification and in some cases surgical interventions.
Summary
Animals use different support systems adapted to their environments and modes of life. Vertebrates possess an internal endoskeleton made of bone or cartilage that provides support, protection, mineral storage and a scaffold for powerful and precise movements. The human skeleton is divided into axial and appendicular parts, and its bones, joints, muscles, ligaments and tendons act together to produce movement and to protect vital organs. Good nutrition, regular exercise and appropriate medical care help maintain skeletal health and prevent or manage common disorders such as rickets, osteoporosis and arthritis.
