Skeleton - Movement and Locomotion | ICSE Class 9 Biology Notes

This note covers movement and locomotion, the functions of the human skeleton, bone and cartilage, the axial and appendicular skeletons, the main bones of the body, and the types, locations and movements of joints.

How do movement and locomotion differ?

Movement is a change in the position of the body or a body part. Locomotion is a voluntary movement that changes an animal's place or location. Voluntary means under conscious control. The distinction depends on whether the whole animal changes place.

Human beings move their limbs, jaws, eyelids and tongue. Walking, running, climbing and swimming are forms of locomotion. Moving the jaw while chewing is movement, but it does not by itself carry the person from one place to another.

Definition: All locomotion is movement, but all movement is not locomotion. A moving body part does not necessarily mean that the whole body has changed its location.

Can the same structures perform both?

The limbs, meaning the arms and legs, help in both changes of posture and locomotion. Posture is the position in which the body is held. A limb's role therefore depends on the movement being performed, rather than on a separate set of locomotory bones.

Animals generally move from place to place to find food, shelter, mates, suitable breeding grounds or favourable climatic conditions, or to escape enemies. The word generally matters: this describes usual purposes rather than a compulsory reason for every movement.

In humans, locomotion involves coordinated activity of muscles, the skeleton and the nervous system. Coordination means that these parts work together. Bones provide the framework, muscles provide force, and the nervous system controls the activity.

What are the main functions of the human skeleton?

The skeleton is the body's supporting framework of bones and cartilage. Bone is a hard, specialised supporting tissue. Cartilage is a connective tissue with a slightly pliable supporting material. Connective tissue supports or connects other body tissues. The human adult skeleton contains 206 bones and also includes cartilage.

How does the framework support and protect?

Support gives the body a firm framework. The backbone supports the head and forms the main framework of the trunk, which is the central body region to which the head and limbs are attached. The skeleton also helps the body remain upright.

Protection depends on the arrangement of particular bones. The skull encloses the brain, the backbone protects the spinal cord, and the rib cage protects the heart and lungs. The spinal cord is the nervous structure running within the backbone.

What other roles do bones have?

Bone marrow is soft tissue inside bones. Red bone marrow forms red blood cells in adults. Red blood cells are blood cells that carry oxygen. This function is different from the mechanical roles of supporting, protecting and helping movement.

Bones also store minerals, including calcium and phosphorus. Their compounds help give bone its rigidity. Bone is therefore more than a hard outline around organs: its tissue contributes to support, movement, blood cell formation and mineral storage.

When explaining a function, connect it to a structure. For example, “the vertebral column protects the spinal cord” is more precise than saying that every bone protects every internal organ.

How do bones, cartilage, tendons and ligaments work together?

Bone is a specialised connective tissue with a very hard matrix, the material surrounding its cells. Calcium salts contribute to this hardness. Cartilage has a slightly pliable matrix, so it provides flexibility and cushioning where a rigid structure alone would be unsuitable.

A tendon connects muscle to bone. A ligament connects bone to bone and helps stabilise a joint. A joint is a point of contact between bones, or between bone and cartilage. These structures have related but different jobs.

How does muscular force move a bone?

Skeletal muscles are muscles associated with the skeleton; their activity is under voluntary control. Contraction is the shortening action by which a muscle produces a pull. Bones do not perform this muscular action themselves, and a joint cannot move bones on its own.

1. The nervous system sends instructions to the skeletal muscle involved in the movement.
2. The muscle contracts and produces a pulling force.
3. The tendon transmits this force from the muscle to the bone to which it is attached.
4. The bone moves at a joint, with the type of joint determining the movement permitted.

Ligaments help keep the bones connected while limiting excessive movement. Cartilage cushions contacting regions. Effective movement therefore needs both a force-producing part and supporting connections; a hard bone alone cannot explain how a limb moves.

Note: Tendons transmit a muscle's pull to bone. Ligaments connect bones and help stabilise joints. Reversing these two connections changes the biological meaning of the explanation.

How does a nerve signal activate a muscle fibre?

A muscle fibre is a muscle cell. Its cell membrane is called the sarcolemma. The neuromuscular junction is the meeting point between a motor neuron, which carries the signal to the muscle, and this membrane.

1. A signal from the central nervous system travels along a motor neuron to the neuromuscular junction.
2. The arriving signal causes the release of acetylcholine, a chemical messenger called a neurotransmitter.
3. Acetylcholine generates an electrical signal, called an action potential, in the sarcolemma.
4. The electrical signal spreads through the muscle fibre and causes calcium ions to be released into its fluid interior, the sarcoplasm.

How do sliding filaments produce contraction and relaxation?

Muscle fibres contain thin actin filaments and thick myosin filaments. A sarcomere is a contracting unit bounded by two Z lines, to which the thin filaments attach. Sliding of thin filaments over thick filaments shortens this unit.

1. The released calcium ions bind to troponin, a protein associated with the thin filaments. This exposes sites on actin where myosin can attach.
2. Using energy from the breakdown of ATP, myosin heads attach to the exposed sites on actin, forming connections called cross bridges.
3. The myosin heads pull the attached actin filaments towards the centre of the sarcomere. The Z lines move inwards, shortening the sarcomere and producing contraction.
4. A fresh ATP molecule binds to myosin and breaks the cross bridge. Further ATP breakdown allows the cycle of attachment, pulling and detachment to repeat, causing further sliding.
5. When calcium ions are pumped back into their storage compartments inside the muscle fibre, the binding sites on actin become masked again. The Z lines return to their original positions and the muscle relaxes.

How do the axial and appendicular skeletons differ?

The skeleton has two principal divisions. The axial skeleton lies along the body's main axis, meaning its central line. The appendicular skeleton consists of the limb bones and their girdles. A girdle is a group of bones helping attach limbs to the axial skeleton.

The axial skeleton contains 80 bones. Its main parts are the skull, vertebral column, ribs and sternum, the breastbone at the front of the chest. The appendicular skeleton contains the bones of the upper and lower limbs, together with the pectoral (shoulder) and pelvic (hip) girdles.

Which features distinguish the divisions?

The pectoral girdle, or shoulder girdle, attaches the upper limb region. The pelvic girdle, or hip girdle, attaches the lower limb region. Both belong to the appendicular skeleton even though they lie close to the trunk.

Location near the chest does not automatically make a bone axial. For example, the shoulder blade belongs to a limb girdle. Classification depends on its membership of the girdle, rather than simply on its position beside the ribs.

The two divisions work together. Walking uses limb bones and joints, while the central skeleton provides the supporting framework. Likewise, protection and movement are not exclusive labels that separate all axial bones from all appendicular bones.

To organise a labelled skeleton, first identify the central skull, backbone and chest framework. Then identify the upper and lower limb bones and their girdles. This gives a clear arrangement without treating girdles as a third principal division.

How is the skull organised to protect the brain?

The skull consists of cranial and facial bones. Cranial bones form the cranium, the hard case around the brain. Facial bones form the front part of the skull. There are 8 cranial bones and 14 facial bones, giving 22 bones in these two sets.

This count refers specifically to the cranial and facial sets. A hyoid, a single U-shaped bone, lies at the base of the mouth cavity. Each middle ear also contains three tiny bones, collectively called ear ossicles.

Which structures can be identified in a skull view?

The maxilla is the upper jaw bone and the mandible is the lower jaw bone. The skull diagram distinguishes these from the bones enclosing the brain. Naming the jaws correctly also helps separate feeding movements from the protective role of the cranium.

In this view, frontal refers to the forehead region, parietal to the upper side of the cranium, temporal to its side near the ear, and occipital to its back. These names identify regions of the brain's bony covering.

The side view shows the rounded cranial region and the facial region. Labels identify frontal, parietal, temporal and occipital bones, along with the maxilla and mandible. The U-shaped hyoid is shown below the mandible.

See Fig. 17.6 in your NCERT textbook

Why are cranial joints immovable?

The flat cranial bones join through sutures, seams of dense fibrous connective tissue, meaning tissue containing closely packed fibres. These joints do not allow movement. Their firm union forms a protective covering around the brain rather than a series of freely moving plates.

The useful distinction is between the skull as a collection of bones and the cranium as its brain-protecting part. A question about cranial protection should refer to the cranium and its fixed sutures, rather than treating every structure near the head as identical.

How does the vertebral column support the body and allow flexibility?

The vertebral column, or backbone, extends from the base of the skull along the back of the trunk. It supports the head, protects the spinal cord, and provides attachment for ribs and back muscles. Its individual bony units are called vertebrae; one is a vertebra.

Each vertebra has a central passage called the neural canal, through which the spinal cord passes. The column combines support with flexibility. It is not one uninterrupted rigid rod, so movement can occur between its neighbouring movable parts.

What are its regions?

These give 26 units in the adult column. The fused sacral unit is the sacrum; the fused terminal unit is the coccyx. “Fused” means that originally separate elements have joined. It does not describe several freely moving bones within that adult unit.

The first cervical vertebra is the atlas, which meets the skull. The second is the axis. The pivot joint between the atlas and axis is important for turning the head, and must be distinguished from the joints where cartilage connects neighbouring vertebral bodies.

The right side view shows the curved column. Labels mark cervical, thoracic and lumbar vertebrae, an intervertebral disc, the sacrum and the coccyx.

See Fig. 17.7 in your NCERT textbook

How is limited movement useful?

An intervertebral disc is a cartilage cushion between neighbouring vertebral bodies. Cartilage cushions these regions and permits limited movement. The resulting flexibility helps the back bend and twist while the column continues to support the body and protect the spinal cord.

The term slightly movable describes the limited movement at an individual cartilaginous connection. It does not mean that the entire backbone is fixed, nor that each of its joints permits the broad movement of a shoulder joint.

How do the ribs and sternum form the rib cage?

The rib cage is formed by the thoracic vertebrae, ribs and sternum. There are 12 pairs of ribs. The sternum is a flat bone along the front midline of the chest, while the thoracic vertebrae lie at the back.

The arrangement forms a protective cage around the heart and lungs. Cartilage contributes flexibility, allowing the cage to move during breathing. The protective function therefore does not require the chest to be completely rigid.

How do rib attachments differ?

The first seven pairs are true ribs. They attach to the thoracic vertebrae at the back and connect to the sternum through cartilage at the front. This direct connection distinguishes them from the following three pairs.

The eighth, ninth and tenth pairs are false ribs with an indirect front connection: their cartilage joins that of the seventh rib. They do not each attach directly to the sternum. The eleventh and twelfth pairs are floating ribs, with no front attachment.

The front view labels the sternum, ribs and vertebral column. Brackets distinguish true ribs numbered 1 to 7, false ribs numbered 8 to 10, and floating ribs numbered 11 and 12.

See Fig. 17.8 in your NCERT textbook

Floating does not mean unattached everywhere. These ribs remain attached at the back. The term identifies the absence of a front connection, so a correct description must specify which end is free.

The breathing movement of the chest is also a useful example of movement without locomotion. The rib cage can expand and contract while the person remains in the same place. Its flexibility and protective role operate together.

Which bones make up the upper and lower limbs?

Each upper limb has 30 bones, and each lower limb has 30 bones. Their arrangement includes long bones near the trunk and smaller groups in the hands and feet. The limbs belong to the appendicular skeleton, together with their attachment girdles.

What are the bones of the upper limb?

The humerus is the upper arm bone. The radius and ulna are the two forearm bones. The wrist contains eight carpals; the palm contains five metacarpals; and the fingers contain fourteen phalanges, the bones of the digits.

What are the bones of the lower limb?

The femur, or thigh bone, is the longest bone. The tibia and fibula are the lower leg bones. The patella, or kneecap, covers the front of the knee. Seven tarsals form the ankle group and five metatarsals form the next foot group.

The toes contain fourteen phalanges. Thus, phalanges is used for both finger and toe bones, while carpals and tarsals identify different groups. “Hand” and “whole upper limb” must also be distinguished when describing bone locations.

The table compares corresponding regions; it is not a complete lower limb inventory because the patella is described separately. Keep the kneecap in the full lower limb list when accounting for its 30 bones.

These names also locate joints precisely. A shoulder joint involves the humerus, a hip joint involves the femur, and a gliding joint between carpals lies among the small wrist bones. Naming the region first makes the longer bone names easier to organise.

How do the girdles connect the limbs to the central skeleton?

The pectoral girdle helps connect the upper limbs with the axial skeleton, while the pelvic girdle does the same for the lower limbs. Each girdle has two halves. Their bones provide the limb attachments and the sockets for the shoulder or hip joints.

What forms the shoulder girdle?

Each half of the pectoral girdle contains a clavicle, or collarbone, and a scapula, or shoulder blade. The scapula is a large, triangular, flat bone at the back of the chest. The clavicle is a long, slender bone with two curvatures.

A hollow on the scapula called the glenoid cavity receives the head of the humerus, forming the shoulder joint. Here, the head means the rounded end of the bone that meets the socket, not the person's head.

The front view shows the clavicle and scapula above the humerus. Below them, labels identify the radius, ulna, carpals, metacarpals and phalanges.

See Fig. 17.9 in your NCERT textbook

What forms the hip girdle?

The pelvic girdle consists of two coxal bones, meaning hip bones. Each forms by fusion of three bones: the ilium, ischium and pubis. These are the three named components of a hip bone, rather than three separate adult girdles.

At their meeting point is the acetabulum, the socket receiving the head of the femur. This forms the hip joint. The two halves meet at the front through the pubic symphysis, a connection containing fibrous cartilage.

The front view labels the coxal bone and its ilium, ischium and pubis regions. The limb labels include femur, patella, tibia, fibula, tarsals, metatarsals and phalanges.

See Fig. 17.10 in your NCERT textbook

Remember the two socket-bone associations: the glenoid cavity meets the humerus, and the acetabulum meets the femur. The girdle supplies the socket, while the limb supplies the rounded bone end that fits into it.

How are joints classified by the movement they allow?

Joints differ in how their bones are connected and how much movement they permit. The three functional groups are immovable, slightly movable and freely movable joints. These descriptions concern the permitted movement, rather than the size of the bones involved.

What are immovable joints?

Immovable joints do not permit movement between the joined bones. The sutures between cranial bones are examples. Dense fibrous tissue joins the bone edges, so these are described structurally as fibrous joints. Their firm union protects the brain.

What are slightly movable joints?

Slightly movable joints permit limited movement. Cartilage joins the bones at these connections. The connections between neighbouring vertebral bodies are examples of cartilaginous joints, meaning joints in which cartilage joins the bones. Their limited movements contribute to flexibility of the column.

What are freely movable joints?

Freely movable joints permit considerable movement. They are synovial joints, with a fluid-filled space called the synovial cavity between the surfaces of the meeting bones. Synovial fluid is the fluid in this cavity; it lubricates the joint and reduces friction.

The word freely is comparative. It does not mean that every synovial joint can move equally far or in every direction. A hinge joint permits a different pattern from a ball and socket joint, although both belong to this group.

When identifying a joint, give its location and then its type. “Between cranial bones: immovable” is precise. “In the head” is too broad, because different connections near the head and neck permit different movements.

Similarly, the atlas-axis connection should be identified separately from cartilaginous connections between vertebral bodies. A region can contain more than one kind of joint, so the exact pair of bones matters.

Where are hinge, ball and socket, gliding and pivot joints found?

The four freely movable joint types considered here differ in the shapes and movements of their meeting surfaces. Knowing both the location and the movement helps distinguish them. Naming a body region without identifying the relevant connection can give an incomplete description.

How do hinge and ball and socket joints compare?

A hinge joint permits bending and straightening in one plane, like a door hinge. A plane here means the flat direction within which that movement occurs. The elbow and knee are examples. Bending the elbow brings the forearm towards the upper arm.

A ball and socket joint has the rounded end of one bone fitting into a hollow in another. The shoulder allows forward, backward, sideways and circular movements. The head of the humerus fits into the glenoid cavity of the scapula.

The hip is another ball and socket joint: the rounded head of the femur fits into the acetabulum. Its socket belongs to the pelvic girdle. In both examples, identify the rounded limb-bone end and the girdle socket as distinct parts.

How do gliding and pivot joints compare?

A gliding joint allows adjoining bone surfaces to slide over one another. Examples occur between the carpals, the small bones of the wrist. This describes movement between individual wrist bones, rather than classifying every connection in the wrist region as identical.

A pivot joint allows rotation, meaning turning around an axis. Here, an axis means a line about which turning occurs. The pivot joint between the atlas and axis permits the head to turn from side to side.

The second vertebra named axis and the general word axis for a line of rotation are related uses but different meanings. In a location answer, “atlas and axis” names the bones; in a movement description, “rotation about an axis” explains the action.

Glossary

• Locomotion — Voluntary movement that changes the place or location of the whole animal.
• Skeleton — The supporting framework of bones and cartilage within the human body.
• Axial skeleton — The central division containing the skull, vertebral column, ribs and sternum.
• Appendicular skeleton — The division containing the limb bones and their pectoral and pelvic girdles.
• Cartilage — Connective tissue with a slightly pliable matrix, providing flexibility and cushioning.
• Tendon — A connective structure joining muscle to bone and transmitting muscular force.
• Ligament — A connective structure joining bone to bone and helping stabilise a joint.
• Cranium — The hard protective case formed by cranial bones around the brain.
• Vertebra — One of the bony units arranged in the vertebral column.
• Girdle — A group of bones helping attach limbs to the axial skeleton.
• Suture — An immovable fibrous joint forming a seam between cranial bones.
• Synovial joint — A freely movable joint with a fluid-filled cavity between the meeting bone surfaces.
• Hinge joint — A joint permitting bending and straightening in one plane, as at the elbow.
• Ball and socket joint — A joint where a rounded bone end fits into a socket, permitting movements in several directions.
• Pivot joint — A joint allowing rotation, exemplified by the connection between atlas and axis.

Common errors and misconceptions

• Misconception: Any moving body part shows locomotion. Correct: Locomotion changes the animal's place. Chewing involves jaw movement without necessarily changing the person's location.
• Misconception: The skeleton consists of bones alone. Correct: The human skeletal framework includes bones and cartilage, with cartilage contributing flexibility and cushioning.
• Misconception: Tendons and ligaments have the same connections. Correct: Tendons connect muscles to bones; ligaments connect bones to bones and help stabilise joints.
• Misconception: The girdles form a third principal skeletal division. Correct: The pectoral and pelvic girdles belong to the appendicular skeleton along with the limbs.
• Misconception: Floating ribs have no attachments. Correct: They attach to the vertebral column at the back but have no front attachment.
• Misconception: All joints in the vertebral column are identical. Correct: Cartilaginous connections allow limited movement between vertebral bodies, while the atlas-axis connection is a pivot joint.
• Misconception: Every freely movable joint moves in every direction. Correct: Hinge, ball and socket, gliding and pivot joints permit different patterns of movement.
• Misconception: Bones provide the pulling force for movement. Correct: Muscles produce the force, tendons transmit it, and bones move at joints under that pull.

Exam-style questions with model answers

Q1. A person chews while remaining seated, then walks to another place. Classify each action as movement without locomotion or as locomotion, giving a reason. [2 marks]

1. Chewing is movement without locomotion because the jaw moves while the person's location remains unchanged.
2. Walking to another place is locomotion because the voluntary movement changes the location of the whole person.

Q2. Give three protective functions of the human skeleton. For each, name the skeletal structure and the organ or nervous structure protected. [3 marks]

1. The cranium forms a hard bony covering around the brain. Its cranial bones join firmly through immovable sutures, creating a protective case.
2. The vertebral column protects the spinal cord, which passes through the central passages of its vertebrae along the back of the body.
3. The rib cage protects the heart and lungs. The ribs, sternum and thoracic vertebrae together form the surrounding chest framework.

Q3. Distinguish the axial and appendicular skeletons using four points: position, main components, membership of girdles, and one illustrative function of each division. [4 marks]

1. The axial skeleton lies along the central axis of the body; the appendicular skeleton comprises the limbs and their attachment girdles.
2. The axial division includes the skull, vertebral column, ribs and sternum; the appendicular division includes upper and lower limb bones with their girdles.
3. The pectoral and pelvic girdles belong to the appendicular division, while providing connections between the limbs and the axial framework.
4. The axial skeleton protects central structures such as the brain and spinal cord; appendicular limb bones participate in locomotion and other limb movements.

Q4. Describe five joint examples: cranial sutures, cartilaginous connections between neighbouring vertebral bodies, the elbow, the shoulder, and the atlas-axis connection. For each, state its type and permitted movement. [5 marks]

1. Cranial sutures are immovable fibrous joints. Dense fibrous tissue connects the cranial bones firmly, so movement does not occur between them at these joints.
2. The cartilaginous connections between neighbouring vertebral bodies are slightly movable joints. They permit limited movement and help the vertebral column retain flexibility.
3. The elbow is a freely movable hinge joint. It permits the forearm to bend and straighten in one plane rather than move freely in every direction.
4. The shoulder is a freely movable ball and socket joint. The humeral head fits the glenoid cavity, permitting forward, backward, sideways and circular movements.
5. The atlas-axis connection is a freely movable pivot joint. It permits rotation associated with turning the head from side to side.

Q5. Explain how skeletal movement occurs, giving one point for each of these five structures: nervous system, skeletal muscle, tendon, joint and ligament. [5 marks]

1. The nervous system controls and coordinates the activity. It sends instructions to the skeletal muscles involved, linking their action with the movement being performed.
2. The skeletal muscle contracts and produces a pulling force. This muscular action supplies the force; the bone does not produce a muscular contraction itself.
3. The tendon connects the muscle to a bone and transmits the pulling force to that bone, allowing muscular activity to affect the skeleton.
4. The joint is the connection at which movement occurs. Its type determines the pattern permitted, such as bending and straightening at a hinge joint.
5. The ligament connects bone to bone and helps stabilise the joint. It limits excessive movement while maintaining the connection between the bones involved.

Q6. An adult vertebral-column inventory lists 7 cervical units, 12 thoracic units, 5 lumbar units, 1 fused sacral unit and 1 fused coccygeal unit. Calculate the total, explain how the fused units are counted, and state two functions of the column. [4 marks]

1. The total is 7 + 12 + 5 + 1 + 1 = 26 units in the adult vertebral column.
2. The sacral and coccygeal regions each count as one fused adult unit, following the inventory; their originally separate elements are not counted individually here.
3. The column protects the spinal cord within the central passages of the vertebrae, providing a bony covering along its course.
4. The column supports the head and provides the main framework of the trunk, helping support the central part of the body.

Q7. Compare the front attachments of true ribs, the eighth to tenth rib pairs, and floating ribs. Name the rib pairs in each group. [3 marks]

1. True ribs are the first seven pairs. Each pair connects at the front to the sternum through cartilage, giving these ribs a direct sternal connection.
2. The eighth, ninth and tenth pairs join the seventh rib's cartilage. They therefore connect indirectly rather than attaching individually and directly to the sternum.
3. Floating ribs are the eleventh and twelfth pairs. They have no front attachment, although they remain connected to the vertebral column at the back.

Q8. Name the type of joint between carpals and the type between atlas and axis. State the movement permitted by each. [2 marks]

1. Between carpals are gliding joints, which allow adjoining bone surfaces to slide over one another.
2. Between atlas and axis is a pivot joint, permitting rotation that turns the head from side to side.

Key takeaways

• Locomotion changes the location of the animal; movements such as chewing can occur without locomotion.
• The skeleton supports the body, protects organs and helps movement; bone marrow and bone minerals have additional functions.
• The axial skeleton includes the skull, vertebral column, ribs and sternum along the body's central axis.
• The appendicular skeleton includes the upper and lower limb bones together with the pectoral and pelvic girdles.
• Tendons connect muscles to bones, while ligaments connect bones to bones and help stabilise joints.
• True, false and floating ribs differ in their front attachments; floating ribs still attach at the back.
• Immovable, slightly movable and freely movable joints differ in the amount of movement their connections permit.
• Hinge, ball and socket, gliding and pivot joints have different locations and permit different patterns of movement.

Test yourself

Why is chewing not necessarily locomotion?

The jaw moves during chewing, but this need not change the person's location. Locomotion involves a change of place.

Which two principal divisions make up the human skeleton?

The axial skeleton forms the central framework; the appendicular skeleton contains the limb bones and their girdles.

How do tendons differ from ligaments?

Tendons connect muscles to bones and transmit force. Ligaments connect bones to bones and help stabilise joints.

Why does a count of 26 adult vertebral units include fused regions?

The sacral region counts as one fused unit, and the coccygeal region also counts as one fused unit.

Which ribs are floating, and what does that name mean?

The eleventh and twelfth pairs are floating ribs. They have no front attachment but remain attached at the back.

Which bones and socket meet at the shoulder joint?

The rounded head of the humerus meets the glenoid cavity of the scapula, forming a ball and socket joint.

Where is the pivot joint that allows side-to-side turning of the head?

It is between the atlas and axis, the first and second cervical vertebrae respectively.

Why does “freely movable” not mean “movement in every direction”?

Synovial joints permit considerable movement, but their structures differ. A hinge joint bends and straightens in one plane, unlike a ball and socket joint.