A Muscle Can Only Pull, Which Is Why Every Joint Needs Two
List what the skeleton does besides holding you up, split its 206 bones into axial and appendicular and check the count, then classify every joint and place the four movable kinds in the body.
Why does bending your arm need two muscles instead of one?
Feel the front of your upper arm while you bend your elbow. The biceps hardens and shortens, and the forearm comes up.
Now straighten the arm again. The biceps softens — and the triceps, at the back of the upper arm, hardens instead.
Why two muscles for one movement in one direction and back?
Because a muscle can do exactly one thing: it can shorten and pull. It cannot lengthen itself and it cannot push. So a muscle that has pulled a bone one way has no way of putting it back, and a second muscle on the opposite side is needed to pull it the other way.
Muscles therefore work in opposing pairs — one to bend the joint, one to straighten it — and neither is redundant.
That is the whole logic of movement in the body, and it explains something about the skeleton too. Bones do not move themselves. They are levers, pulled at by muscles across joints, and the kind of movement a joint allows is decided entirely by the shape of the bone ends that meet there.
So this chapter has three parts that fit together: what the skeleton is for, how its 206 bones are divided, and what kinds of joint hold them together.
This page covers the third part of the ICSE Class 9 Biology chapter on human anatomy and physiology: the functions of the skeleton, the axial and appendicular division, the three classes of joint, and the four kinds of freely movable joint with their locations.
Now straighten the arm again. The biceps softens — and the triceps, at the back of the upper arm, hardens instead.
Why two muscles for one movement in one direction and back?
Because a muscle can do exactly one thing: it can shorten and pull. It cannot lengthen itself and it cannot push. So a muscle that has pulled a bone one way has no way of putting it back, and a second muscle on the opposite side is needed to pull it the other way.
Muscles therefore work in opposing pairs — one to bend the joint, one to straighten it — and neither is redundant.
That is the whole logic of movement in the body, and it explains something about the skeleton too. Bones do not move themselves. They are levers, pulled at by muscles across joints, and the kind of movement a joint allows is decided entirely by the shape of the bone ends that meet there.
So this chapter has three parts that fit together: what the skeleton is for, how its 206 bones are divided, and what kinds of joint hold them together.
This page covers the third part of the ICSE Class 9 Biology chapter on human anatomy and physiology: the functions of the skeleton, the axial and appendicular division, the three classes of joint, and the four kinds of freely movable joint with their locations.
What does the skeleton do besides holding you up?
The skeleton supports and protects, makes movement possible, manufactures blood cells and stores minerals.
Support and shape. The skeleton is the framework on which the body is built. It gives the body its definite shape, holds it upright, and supports the soft organs so that they are not crushed by their own weight.
Protection. Each vital organ is enclosed by bone:
- The skull protects the brain
- The vertebral column protects the spinal cord
- The ribcage and sternum protect the heart and lungs
- The pelvic girdle protects the lower abdominal organs
- The bony sockets protect the eyes
Movement and locomotion. The bones provide surfaces for the attachment of muscles, and each bone acts as a lever at a joint. Without something rigid to pull against, a muscle's contraction would produce no movement at all.
Production of blood cells. The red bone marrow, in the cavities of certain bones, manufactures red blood cells, most white blood cells and platelets.
Storage of minerals. Bone is a store of calcium and phosphorus, which the body can draw on when the blood level falls and replace when it rises. The yellow marrow stores fat.
Breathing movements. The ribs and sternum, moved by muscles, change the volume of the chest — so the skeleton is part of the mechanism of respiration.
Transmission of sound. The three tiny ear ossicles carry vibrations from the eardrum to the inner ear.
Now notice which of those functions have nothing to do with holding the body up, because they are the ones that get forgotten. Making blood cells and storing calcium are jobs of the bone tissue itself rather than of the skeleton's shape, and a question asking for the functions of the skeleton expects both kinds.
The mineral store has a visible consequence. If the diet is short of calcium, the body withdraws it from the bones to keep the blood level steady — because the blood level matters more moment to moment than the bone does. So the bones weaken to protect the blood, and that is why a long calcium or vitamin D shortage produces soft bent bones, as the nutrition chapter described.
Support and shape. The skeleton is the framework on which the body is built. It gives the body its definite shape, holds it upright, and supports the soft organs so that they are not crushed by their own weight.
Protection. Each vital organ is enclosed by bone:
- The skull protects the brain
- The vertebral column protects the spinal cord
- The ribcage and sternum protect the heart and lungs
- The pelvic girdle protects the lower abdominal organs
- The bony sockets protect the eyes
Movement and locomotion. The bones provide surfaces for the attachment of muscles, and each bone acts as a lever at a joint. Without something rigid to pull against, a muscle's contraction would produce no movement at all.
Production of blood cells. The red bone marrow, in the cavities of certain bones, manufactures red blood cells, most white blood cells and platelets.
Storage of minerals. Bone is a store of calcium and phosphorus, which the body can draw on when the blood level falls and replace when it rises. The yellow marrow stores fat.
Breathing movements. The ribs and sternum, moved by muscles, change the volume of the chest — so the skeleton is part of the mechanism of respiration.
Transmission of sound. The three tiny ear ossicles carry vibrations from the eardrum to the inner ear.
Now notice which of those functions have nothing to do with holding the body up, because they are the ones that get forgotten. Making blood cells and storing calcium are jobs of the bone tissue itself rather than of the skeleton's shape, and a question asking for the functions of the skeleton expects both kinds.
The mineral store has a visible consequence. If the diet is short of calcium, the body withdraws it from the bones to keep the blood level steady — because the blood level matters more moment to moment than the bone does. So the bones weaken to protect the blood, and that is why a long calcium or vitamin D shortage produces soft bent bones, as the nutrition chapter described.
How do the 206 bones divide into axial and appendicular?
The axial skeleton is the 80 bones along the main axis of the body; the appendicular skeleton is the 126 bones of the limbs and their girdles.
The axial skeleton — 80 bones.
- Skull — 22 bones: 8 cranial bones forming the box that holds the brain, and 14 facial bones. The only movable bone of the skull is the lower jaw, the mandible
- Vertebral column — 26 bones in an adult. A child has 33 vertebrae: 7 cervical in the neck, 12 thoracic in the chest, 5 lumbar in the lower back, 5 sacral and 4 caudal. The five sacral vertebrae later fuse into one sacrum and the four caudal into one coccyx, which is why the adult count is
- Ribs — 24 bones, that is 12 pairs: 7 pairs true ribs joined directly to the sternum, 3 pairs false ribs joined indirectly through the cartilage above them, and 2 pairs floating ribs joined to nothing in front
- Sternum — 1 bone, the breast bone
- Hyoid — 1 bone, in the throat, supporting the tongue
- Ear ossicles — 6 bones, three in each middle ear
Check the total: .
The appendicular skeleton — 126 bones.
- Pectoral girdle — 4 bones: two clavicles, the collar bones, and two scapulae, the shoulder blades
- Forelimbs — 60 bones, that is 30 in each arm: the humerus in the upper arm; the radius and ulna in the forearm; 8 carpals in the wrist; 5 metacarpals in the palm; and 14 phalanges in the fingers. So per arm
- Pelvic girdle — 2 bones, the hip bones, each formed by the fusion of three parts — the ilium, ischium and pubis
- Hindlimbs — 60 bones, that is 30 in each leg: the femur, the longest bone in the body; the patella or knee cap; the tibia and fibula in the shin; 7 tarsals in the ankle; 5 metatarsals; and 14 phalanges in the toes. So per leg
Check the total: .
And the two together: bones in an adult human skeleton.
Notice that an arm and a leg have the same number of bones — 30 each — and the same plan: one long bone, then two side by side, then a cluster of small ones, then five in a row, then fourteen in the digits. The ankle has 7 tarsals where the wrist has 8 carpals, and the leg has the extra patella, so the two totals come out equal by different routes.
That shared plan is the point of the comparison. A forelimb and a hindlimb are built on one pattern with different proportions, which is why the wrist is flexible and the ankle is strong — the same bones, differently sized for a different job. A question asking you to compare the bones of the arm and the leg wants that correspondence, bone for bone, not two separate lists.
The axial skeleton — 80 bones.
- Skull — 22 bones: 8 cranial bones forming the box that holds the brain, and 14 facial bones. The only movable bone of the skull is the lower jaw, the mandible
- Vertebral column — 26 bones in an adult. A child has 33 vertebrae: 7 cervical in the neck, 12 thoracic in the chest, 5 lumbar in the lower back, 5 sacral and 4 caudal. The five sacral vertebrae later fuse into one sacrum and the four caudal into one coccyx, which is why the adult count is
- Ribs — 24 bones, that is 12 pairs: 7 pairs true ribs joined directly to the sternum, 3 pairs false ribs joined indirectly through the cartilage above them, and 2 pairs floating ribs joined to nothing in front
- Sternum — 1 bone, the breast bone
- Hyoid — 1 bone, in the throat, supporting the tongue
- Ear ossicles — 6 bones, three in each middle ear
Check the total: .
The appendicular skeleton — 126 bones.
- Pectoral girdle — 4 bones: two clavicles, the collar bones, and two scapulae, the shoulder blades
- Forelimbs — 60 bones, that is 30 in each arm: the humerus in the upper arm; the radius and ulna in the forearm; 8 carpals in the wrist; 5 metacarpals in the palm; and 14 phalanges in the fingers. So per arm
- Pelvic girdle — 2 bones, the hip bones, each formed by the fusion of three parts — the ilium, ischium and pubis
- Hindlimbs — 60 bones, that is 30 in each leg: the femur, the longest bone in the body; the patella or knee cap; the tibia and fibula in the shin; 7 tarsals in the ankle; 5 metatarsals; and 14 phalanges in the toes. So per leg
Check the total: .
And the two together: bones in an adult human skeleton.
Notice that an arm and a leg have the same number of bones — 30 each — and the same plan: one long bone, then two side by side, then a cluster of small ones, then five in a row, then fourteen in the digits. The ankle has 7 tarsals where the wrist has 8 carpals, and the leg has the extra patella, so the two totals come out equal by different routes.
That shared plan is the point of the comparison. A forelimb and a hindlimb are built on one pattern with different proportions, which is why the wrist is flexible and the ankle is strong — the same bones, differently sized for a different job. A question asking you to compare the bones of the arm and the leg wants that correspondence, bone for bone, not two separate lists.
How are joints classified by how much they move?
A joint is a place where two or more bones meet, and the three classes differ in how much movement is possible there.
Immovable joints, also called fixed or fibrous joints.
- The bones are firmly united by fibrous tissue, and no movement is possible
- Location: the sutures between the bones of the skull; the joint between a tooth and its socket; the joint between the sacral vertebrae after fusion
- Why fixed here: the skull's job is to protect the brain, and a box that could flex would be a poor shield
Slightly movable joints, also called cartilaginous joints.
- A pad of cartilage lies between the bone ends, and it allows a small amount of movement by being compressed
- Location: between the vertebrae of the backbone, where the discs of cartilage lie; between the ribs and the sternum; the joint between the two hip bones in front
- Why slightly movable here: each vertebral joint gives only a little, but there are many of them — and a little movement repeated twenty-four times over is what lets the whole backbone bend
Freely movable joints, also called synovial joints.
- The bone ends are covered with smooth cartilage; the joint is enclosed in a tough capsule lined with a synovial membrane, which secretes synovial fluid into the cavity
- Location: the shoulder, hip, elbow, knee, wrist, ankle, neck and the joints of the fingers
- Ligaments hold the bones together across the joint
What the synovial fluid does, and it does three things.
- Lubricates the joint, so the cartilage surfaces slide instead of grinding
- Cushions shocks and spreads the load
- Nourishes the cartilage
That third job matters more than it sounds. The tissue chapter established that cartilage has no blood vessels of its own, so the cells inside it cannot be fed by a blood supply. The synovial fluid is how they are fed, which is why a joint that is never moved becomes stiff and its cartilage deteriorates — movement is what circulates the fluid.
So the freely movable joint is the one with a fluid-filled cavity, and the fluid is not merely oil but the cartilage's food supply as well. A question asking why synovial fluid is important should give all three functions, because the nourishment is the one most often left out and the one that explains why exercise protects joints.
Notice how the amount of movement matches the job. Where protection is the priority the joint is fixed; where strength with a little give is wanted it is slightly movable; where movement is the whole point it is freely movable and provided with fluid. There are no freely movable joints in the skull and no fixed ones in a limb, and that alignment is the quickest way to place any joint you are asked about.
Immovable joints, also called fixed or fibrous joints.
- The bones are firmly united by fibrous tissue, and no movement is possible
- Location: the sutures between the bones of the skull; the joint between a tooth and its socket; the joint between the sacral vertebrae after fusion
- Why fixed here: the skull's job is to protect the brain, and a box that could flex would be a poor shield
Slightly movable joints, also called cartilaginous joints.
- A pad of cartilage lies between the bone ends, and it allows a small amount of movement by being compressed
- Location: between the vertebrae of the backbone, where the discs of cartilage lie; between the ribs and the sternum; the joint between the two hip bones in front
- Why slightly movable here: each vertebral joint gives only a little, but there are many of them — and a little movement repeated twenty-four times over is what lets the whole backbone bend
Freely movable joints, also called synovial joints.
- The bone ends are covered with smooth cartilage; the joint is enclosed in a tough capsule lined with a synovial membrane, which secretes synovial fluid into the cavity
- Location: the shoulder, hip, elbow, knee, wrist, ankle, neck and the joints of the fingers
- Ligaments hold the bones together across the joint
What the synovial fluid does, and it does three things.
- Lubricates the joint, so the cartilage surfaces slide instead of grinding
- Cushions shocks and spreads the load
- Nourishes the cartilage
That third job matters more than it sounds. The tissue chapter established that cartilage has no blood vessels of its own, so the cells inside it cannot be fed by a blood supply. The synovial fluid is how they are fed, which is why a joint that is never moved becomes stiff and its cartilage deteriorates — movement is what circulates the fluid.
So the freely movable joint is the one with a fluid-filled cavity, and the fluid is not merely oil but the cartilage's food supply as well. A question asking why synovial fluid is important should give all three functions, because the nourishment is the one most often left out and the one that explains why exercise protects joints.
Notice how the amount of movement matches the job. Where protection is the priority the joint is fixed; where strength with a little give is wanted it is slightly movable; where movement is the whole point it is freely movable and provided with fluid. There are no freely movable joints in the skull and no fixed ones in a limb, and that alignment is the quickest way to place any joint you are asked about.
Which freely movable joint is where, and how does each move?
Four kinds of synovial joint, each allowing a different range of movement decided by the shape of the bone ends.
Hinge joint.
- Shape: a convex surface fits into a concave one, like the hinge of a door
- Movement: in one plane only — bending (flexion) and straightening (extension). No rotation
- Location: the elbow, between the humerus and the ulna; the knee; the ankle; and the joints between the phalanges of the fingers and toes
Ball and socket joint.
- Shape: the rounded head of one bone fits into a cup-shaped socket in another
- Movement: in all directions — back and forth, side to side, and rotation. The freest joint in the body
- Location: the shoulder, where the head of the humerus sits in the scapula; and the hip, where the head of the femur sits in the pelvic girdle
Gliding joint.
- Shape: two flat or slightly curved surfaces face each other
- Movement: the surfaces slide over one another, giving a limited amount of movement in several directions
- Location: between the carpals of the wrist; between the tarsals of the ankle; and between the flat facets of adjacent vertebrae
Pivot joint.
- Shape: a peg-like projection of one bone turns within a ring formed by another
- Movement: rotation only, about one axis
- Location: between the atlas and the axis — the first and second vertebrae of the neck
Here is the neatest illustration in the chapter, and it uses two joints a few centimetres apart.
Nod your head up and down, as if to say yes. That is a hinge-type movement at the joint between the skull and the atlas, which allows bending in one plane.
Now turn your head from side to side, as if to say no. That is the pivot joint, the atlas rotating on the axis — and it does nothing else.
Two different joints in the top of your neck, each doing exactly one thing, and between them your head can nod and turn. A question asking for the location of a pivot joint wants the atlas and axis, and saying "the neck" is not precise enough, because the neck contains both kinds.
Now the muscles that work these joints.
- Tendons attach muscle to bone; ligaments attach bone to bone
- A muscle can only shorten and pull — never push, and never lengthen itself
- So muscles work in antagonistic pairs, one on each side of the joint. The biceps bends the elbow (the flexor); the triceps straightens it (the extensor). When one contracts the other relaxes
That is why every freely movable joint has at least two muscles across it, and it is the answer to the question this page opened with. A single muscle could bend your arm and would then be stuck — which is why the body never has one.
One boundary case worth having. The knee is described as a hinge joint, and it does bend in one plane — but it also allows a small amount of rotation when bent, which is exactly why the knee is so easily injured in sports where the body twists while the foot is planted. A joint built for one plane and asked to work in two is a joint at risk, and that is the practical consequence of the shape.
Hinge joint.
- Shape: a convex surface fits into a concave one, like the hinge of a door
- Movement: in one plane only — bending (flexion) and straightening (extension). No rotation
- Location: the elbow, between the humerus and the ulna; the knee; the ankle; and the joints between the phalanges of the fingers and toes
Ball and socket joint.
- Shape: the rounded head of one bone fits into a cup-shaped socket in another
- Movement: in all directions — back and forth, side to side, and rotation. The freest joint in the body
- Location: the shoulder, where the head of the humerus sits in the scapula; and the hip, where the head of the femur sits in the pelvic girdle
Gliding joint.
- Shape: two flat or slightly curved surfaces face each other
- Movement: the surfaces slide over one another, giving a limited amount of movement in several directions
- Location: between the carpals of the wrist; between the tarsals of the ankle; and between the flat facets of adjacent vertebrae
Pivot joint.
- Shape: a peg-like projection of one bone turns within a ring formed by another
- Movement: rotation only, about one axis
- Location: between the atlas and the axis — the first and second vertebrae of the neck
Here is the neatest illustration in the chapter, and it uses two joints a few centimetres apart.
Nod your head up and down, as if to say yes. That is a hinge-type movement at the joint between the skull and the atlas, which allows bending in one plane.
Now turn your head from side to side, as if to say no. That is the pivot joint, the atlas rotating on the axis — and it does nothing else.
Two different joints in the top of your neck, each doing exactly one thing, and between them your head can nod and turn. A question asking for the location of a pivot joint wants the atlas and axis, and saying "the neck" is not precise enough, because the neck contains both kinds.
Now the muscles that work these joints.
- Tendons attach muscle to bone; ligaments attach bone to bone
- A muscle can only shorten and pull — never push, and never lengthen itself
- So muscles work in antagonistic pairs, one on each side of the joint. The biceps bends the elbow (the flexor); the triceps straightens it (the extensor). When one contracts the other relaxes
That is why every freely movable joint has at least two muscles across it, and it is the answer to the question this page opened with. A single muscle could bend your arm and would then be stuck — which is why the body never has one.
One boundary case worth having. The knee is described as a hinge joint, and it does bend in one plane — but it also allows a small amount of rotation when bent, which is exactly why the knee is so easily injured in sports where the body twists while the foot is planted. A joint built for one plane and asked to work in two is a joint at risk, and that is the practical consequence of the shape.
Exam tip
Exam tip: count the bones, and name the joint with its exact location
Give the skeleton's functions in two groups — those from its shape (support, protection, levers for movement) and those from the bone tissue (blood cell production in the red marrow, calcium and phosphorus storage). The second group is the one that gets forgotten.
Learn the counts and be able to add them: axial 80, appendicular 126, total 206.
Axial breakdown: skull 22 (8 cranial, 14 facial), vertebral column 26 in an adult, ribs 24, sternum 1, hyoid 1, ear ossicles 6. Check: .
The child's 33 vertebrae become 26 because the 5 sacral fuse into the sacrum and the 4 caudal into the coccyx. State the reason, not just the number.
Vertebral regions: 7 cervical, 12 thoracic, 5 lumbar, sacrum, coccyx.
Ribs: 7 pairs true, 3 pairs false, 2 pairs floating.
Appendicular breakdown: pectoral girdle 4, arms 60, pelvic girdle 2, legs 60. Check: .
Each limb has 30 bones. Arm: humerus, radius, ulna, 8 carpals, 5 metacarpals, 14 phalanges. Leg: femur, patella, tibia, fibula, 7 tarsals, 5 metatarsals, 14 phalanges.
Immovable — skull sutures and tooth sockets. Slightly movable — between vertebrae and ribs to sternum. Freely movable — shoulder, hip, elbow, knee, wrist.
Give all three functions of synovial fluid: lubricates, cushions, and nourishes the cartilage, which has no blood supply of its own.
Locate each movable joint precisely: hinge — elbow, knee, fingers; ball and socket — shoulder and hip; gliding — between carpals and tarsals; pivot — between the atlas and the axis.
Tendon joins muscle to bone; ligament joins bone to bone. Do not swap them.
And say that a muscle can only pull, so muscles act in antagonistic pairs — biceps flexes the elbow and triceps extends it.
Learn the counts and be able to add them: axial 80, appendicular 126, total 206.
Axial breakdown: skull 22 (8 cranial, 14 facial), vertebral column 26 in an adult, ribs 24, sternum 1, hyoid 1, ear ossicles 6. Check: .
The child's 33 vertebrae become 26 because the 5 sacral fuse into the sacrum and the 4 caudal into the coccyx. State the reason, not just the number.
Vertebral regions: 7 cervical, 12 thoracic, 5 lumbar, sacrum, coccyx.
Ribs: 7 pairs true, 3 pairs false, 2 pairs floating.
Appendicular breakdown: pectoral girdle 4, arms 60, pelvic girdle 2, legs 60. Check: .
Each limb has 30 bones. Arm: humerus, radius, ulna, 8 carpals, 5 metacarpals, 14 phalanges. Leg: femur, patella, tibia, fibula, 7 tarsals, 5 metatarsals, 14 phalanges.
Immovable — skull sutures and tooth sockets. Slightly movable — between vertebrae and ribs to sternum. Freely movable — shoulder, hip, elbow, knee, wrist.
Give all three functions of synovial fluid: lubricates, cushions, and nourishes the cartilage, which has no blood supply of its own.
Locate each movable joint precisely: hinge — elbow, knee, fingers; ball and socket — shoulder and hip; gliding — between carpals and tarsals; pivot — between the atlas and the axis.
Tendon joins muscle to bone; ligament joins bone to bone. Do not swap them.
And say that a muscle can only pull, so muscles act in antagonistic pairs — biceps flexes the elbow and triceps extends it.
Did you know
Why a giraffe has the same number of neck bones as you do
A giraffe's neck is extraordinarily long. A mouse's neck is barely there. A human neck is somewhere in between.
All three contain seven cervical vertebrae.
Almost every mammal has exactly seven, whatever the length of its neck. The giraffe did not add bones to make its neck long — it made each of the same seven enormously longer. The mouse has seven very short ones.
This is a striking thing about body plans, and it has a useful lesson in it for a student of anatomy. The number of parts in a mammalian skeleton is remarkably conservative, and the proportions are where all the variation is.
The same point runs right through the limb bones on this page. A human arm, a bat's wing, a whale's flipper and a horse's front leg all have the same bones in the same order — one humerus, then a radius and an ulna, then a cluster of small wrist bones, then a set of long bones, then digits. Stretch some, shrink others, fuse a few, and you get a wing, a flipper, a running leg or a hand that can hold a pen.
None of them has an extra bone that the others lack. They have the same list, stretched differently.
Which is why the correspondence noticed earlier — 30 bones in an arm and 30 in a leg, built on the same plan — is not a coincidence to be memorised. It is the plan showing through. Once you have learnt the arm you have very nearly learnt the leg, and the differences are worth attention precisely because there are so few of them: the patella that the arm has no equivalent of, and 7 tarsals where the wrist has 8 carpals.
Learn the pattern once and the exceptions become short and memorable — which is a great deal easier than learning two lists of thirty bones and hoping they stay apart.
All three contain seven cervical vertebrae.
Almost every mammal has exactly seven, whatever the length of its neck. The giraffe did not add bones to make its neck long — it made each of the same seven enormously longer. The mouse has seven very short ones.
This is a striking thing about body plans, and it has a useful lesson in it for a student of anatomy. The number of parts in a mammalian skeleton is remarkably conservative, and the proportions are where all the variation is.
The same point runs right through the limb bones on this page. A human arm, a bat's wing, a whale's flipper and a horse's front leg all have the same bones in the same order — one humerus, then a radius and an ulna, then a cluster of small wrist bones, then a set of long bones, then digits. Stretch some, shrink others, fuse a few, and you get a wing, a flipper, a running leg or a hand that can hold a pen.
None of them has an extra bone that the others lack. They have the same list, stretched differently.
Which is why the correspondence noticed earlier — 30 bones in an arm and 30 in a leg, built on the same plan — is not a coincidence to be memorised. It is the plan showing through. Once you have learnt the arm you have very nearly learnt the leg, and the differences are worth attention precisely because there are so few of them: the patella that the arm has no equivalent of, and 7 tarsals where the wrist has 8 carpals.
Learn the pattern once and the exceptions become short and memorable — which is a great deal easier than learning two lists of thirty bones and hoping they stay apart.
Exam relevance
Why does NEET keep returning to joints and the skeleton?
Because Locomotion and Movement is a Class 11 chapter examined largely as recall of counts, names and joint locations, and all three are on this page.
This is the foundation for Class 11 Biology Locomotion and Movement, examined in NEET. That chapter repeats the whole skeleton with the same 206, 80 and 126 counts, the same vertebral regions and rib groups, and then adds the microscopic machinery this page only gestures at: the sarcomere, the actin and myosin filaments, and the sliding filament theory of how a muscle shortens.
The bone counts are asked directly. Questions give a part of the skeleton and ask for the number of bones, or give a number and ask which part it belongs to. The skull as 22, the vertebral column as 26 in an adult, the ribs as 24 and a limb as 30 are the most reliably examined figures, and the arithmetic on this page is how they are secured.
The fusion that turns 33 vertebrae into 26 is a favourite item. Class 11 explains the sacrum and the coccyx as fused structures, and assertion-reason questions turn on why the adult count differs from the child's. The reason matters as much as the number.
Joint classification is a standing NEET question. You are given a location and asked for the type of joint, or a type and asked for an example. The pivot joint between the atlas and the axis, and the ball and socket at the shoulder and hip, are the two most asked, and the precision of the location is what the question tests — "the neck" will not do for a pivot joint.
Antagonistic muscles become the mechanism of contraction. Class 11 explains that a muscle generates force only by shortening, which is why flexors and extensors are paired — and the sliding filament theory shows why at the molecular level, since actin and myosin can only pull past each other. The one-directional nature of muscle action noted here is the reason the whole theory has the shape it does.
The skeleton's non-structural functions reappear in other chapters. Red bone marrow and blood cell formation are covered in Body Fluids and Circulation; calcium storage and its release under hormonal control appear in Chemical Coordination and Integration, where parathyroid hormone raises blood calcium by withdrawing it from bone. The point made here — that the bones weaken to keep the blood level steady — is exactly the mechanism that chapter names.
Disorders are examined at the end of the chapter. Class 11 covers arthritis, osteoporosis, gout and muscular dystrophy, and myasthenia gravis. The cartilage-has-no-blood-supply point of the tissue chapter is what makes joint wear a permanent condition, and that is the reasoning behind the arthritis questions.
What the questions look like. For board work, expect state the functions of the skeleton, distinguish the axial from the appendicular skeleton and name the bones of each, classify joints with locations, and compare hinge, ball and socket, gliding and pivot joints with an example each. Counts should be given with their addition shown. For NEET, expect bone-count recall, joint-to-location matching, the sarcomere and sliding filament theory, and skeletal disorders.
How board and competitive emphasis differ. A board paper rewards the count with its breakdown and the joint with its precise location. A competitive paper assumes both and asks about actin and myosin, or which hormone mobilises bone calcium.
The single trap that costs the most marks. Swapping tendon and ligament. A tendon joins muscle to bone; a ligament joins bone to bone. The defence is to attach each word to what it has to stretch — a tendon transmits a muscle's pull and must be inelastic, while a ligament holds two bones in place across a joint and must allow the joint to move. Once you know what each is for, the pairing cannot reverse, and a question asking which structure is torn in a sprain becomes obvious.
This is the foundation for Class 11 Biology Locomotion and Movement, examined in NEET. That chapter repeats the whole skeleton with the same 206, 80 and 126 counts, the same vertebral regions and rib groups, and then adds the microscopic machinery this page only gestures at: the sarcomere, the actin and myosin filaments, and the sliding filament theory of how a muscle shortens.
The bone counts are asked directly. Questions give a part of the skeleton and ask for the number of bones, or give a number and ask which part it belongs to. The skull as 22, the vertebral column as 26 in an adult, the ribs as 24 and a limb as 30 are the most reliably examined figures, and the arithmetic on this page is how they are secured.
The fusion that turns 33 vertebrae into 26 is a favourite item. Class 11 explains the sacrum and the coccyx as fused structures, and assertion-reason questions turn on why the adult count differs from the child's. The reason matters as much as the number.
Joint classification is a standing NEET question. You are given a location and asked for the type of joint, or a type and asked for an example. The pivot joint between the atlas and the axis, and the ball and socket at the shoulder and hip, are the two most asked, and the precision of the location is what the question tests — "the neck" will not do for a pivot joint.
Antagonistic muscles become the mechanism of contraction. Class 11 explains that a muscle generates force only by shortening, which is why flexors and extensors are paired — and the sliding filament theory shows why at the molecular level, since actin and myosin can only pull past each other. The one-directional nature of muscle action noted here is the reason the whole theory has the shape it does.
The skeleton's non-structural functions reappear in other chapters. Red bone marrow and blood cell formation are covered in Body Fluids and Circulation; calcium storage and its release under hormonal control appear in Chemical Coordination and Integration, where parathyroid hormone raises blood calcium by withdrawing it from bone. The point made here — that the bones weaken to keep the blood level steady — is exactly the mechanism that chapter names.
Disorders are examined at the end of the chapter. Class 11 covers arthritis, osteoporosis, gout and muscular dystrophy, and myasthenia gravis. The cartilage-has-no-blood-supply point of the tissue chapter is what makes joint wear a permanent condition, and that is the reasoning behind the arthritis questions.
What the questions look like. For board work, expect state the functions of the skeleton, distinguish the axial from the appendicular skeleton and name the bones of each, classify joints with locations, and compare hinge, ball and socket, gliding and pivot joints with an example each. Counts should be given with their addition shown. For NEET, expect bone-count recall, joint-to-location matching, the sarcomere and sliding filament theory, and skeletal disorders.
How board and competitive emphasis differ. A board paper rewards the count with its breakdown and the joint with its precise location. A competitive paper assumes both and asks about actin and myosin, or which hormone mobilises bone calcium.
The single trap that costs the most marks. Swapping tendon and ligament. A tendon joins muscle to bone; a ligament joins bone to bone. The defence is to attach each word to what it has to stretch — a tendon transmits a muscle's pull and must be inelastic, while a ligament holds two bones in place across a joint and must allow the joint to move. Once you know what each is for, the pairing cannot reverse, and a question asking which structure is torn in a sprain becomes obvious.
Key takeaways
The skeleton, its divisions and the joints: quick revision
- Functions of the skeleton: support and shape; protection (skull for the brain, vertebral column for the spinal cord, ribcage for heart and lungs, pelvis for the abdominal organs); movement by providing muscle attachment and acting as levers; production of blood cells in the red bone marrow; storage of calcium and phosphorus, with fat in the yellow marrow; breathing movements by the ribs and sternum; and sound transmission by the ear ossicles.
- If the diet is short of calcium the body withdraws it from bone to keep the blood level steady — so the bones weaken to protect the blood.
- Axial skeleton — 80 bones: skull 22 (8 cranial, 14 facial, with the mandible the only movable one); vertebral column 26 in an adult; ribs 24; sternum 1; hyoid 1; ear ossicles 6. Check .
- Vertebrae: a child has 33 — 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, 4 caudal. The sacral fuse into the sacrum and the caudal into the coccyx, giving in an adult.
- Ribs: 12 pairs — 7 true (joined directly to the sternum), 3 false (joined indirectly), 2 floating (not joined in front).
- Appendicular skeleton — 126 bones: pectoral girdle 4 (2 clavicles, 2 scapulae); forelimbs 60; pelvic girdle 2 (each from ilium, ischium and pubis); hindlimbs 60. Check .
- Each limb has 30 bones. Arm: humerus, radius, ulna, 8 carpals, 5 metacarpals, 14 phalanges. Leg: femur (longest bone), patella, tibia, fibula, 7 tarsals, 5 metatarsals, 14 phalanges.
- **Total: bones.
- Immovable (fibrous) joints — no movement; the sutures of the skull and the tooth in its socket. Fixed because the priority is protection.
- Slightly movable (cartilaginous) joints — a pad of cartilage allows a little movement; between the vertebrae, ribs to sternum, and between the hip bones in front. A little movement repeated many times lets the whole backbone bend.
- Freely movable (synovial) joints — bone ends covered with cartilage, a capsule lined by a synovial membrane secreting synovial fluid, held by ligaments. Shoulder, hip, elbow, knee, wrist, ankle, neck, fingers.
- Synovial fluid does three things: lubricates, cushions, and nourishes the cartilage, which has no blood supply of its own — so a joint never moved becomes stiff.
- Hinge joint — one plane only, bending and straightening: elbow, knee, ankle, finger joints.
- Ball and socket joint — all directions including rotation: shoulder and hip.
- Gliding joint — flat surfaces sliding, limited movement: between the carpals and between the tarsals.
- Pivot joint — rotation only: between the atlas and the axis, which is what lets the head turn; nodding happens at the skull-to-atlas joint.
- Tendon joins muscle to bone; ligament joins bone to bone.
- A muscle can only shorten and pull, so muscles work in antagonistic pairs — the biceps flexes the elbow, the triceps** extends it — and every freely movable joint therefore needs at least two.
Add the axial and appendicular counts from their parts and see whether you reach 206, then name the joint at your elbow, your shoulder, your wrist and the top of your neck.
- If the diet is short of calcium the body withdraws it from bone to keep the blood level steady — so the bones weaken to protect the blood.
- Axial skeleton — 80 bones: skull 22 (8 cranial, 14 facial, with the mandible the only movable one); vertebral column 26 in an adult; ribs 24; sternum 1; hyoid 1; ear ossicles 6. Check .
- Vertebrae: a child has 33 — 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, 4 caudal. The sacral fuse into the sacrum and the caudal into the coccyx, giving in an adult.
- Ribs: 12 pairs — 7 true (joined directly to the sternum), 3 false (joined indirectly), 2 floating (not joined in front).
- Appendicular skeleton — 126 bones: pectoral girdle 4 (2 clavicles, 2 scapulae); forelimbs 60; pelvic girdle 2 (each from ilium, ischium and pubis); hindlimbs 60. Check .
- Each limb has 30 bones. Arm: humerus, radius, ulna, 8 carpals, 5 metacarpals, 14 phalanges. Leg: femur (longest bone), patella, tibia, fibula, 7 tarsals, 5 metatarsals, 14 phalanges.
- **Total: bones.
- Immovable (fibrous) joints — no movement; the sutures of the skull and the tooth in its socket. Fixed because the priority is protection.
- Slightly movable (cartilaginous) joints — a pad of cartilage allows a little movement; between the vertebrae, ribs to sternum, and between the hip bones in front. A little movement repeated many times lets the whole backbone bend.
- Freely movable (synovial) joints — bone ends covered with cartilage, a capsule lined by a synovial membrane secreting synovial fluid, held by ligaments. Shoulder, hip, elbow, knee, wrist, ankle, neck, fingers.
- Synovial fluid does three things: lubricates, cushions, and nourishes the cartilage, which has no blood supply of its own — so a joint never moved becomes stiff.
- Hinge joint — one plane only, bending and straightening: elbow, knee, ankle, finger joints.
- Ball and socket joint — all directions including rotation: shoulder and hip.
- Gliding joint — flat surfaces sliding, limited movement: between the carpals and between the tarsals.
- Pivot joint — rotation only: between the atlas and the axis, which is what lets the head turn; nodding happens at the skull-to-atlas joint.
- Tendon joins muscle to bone; ligament joins bone to bone.
- A muscle can only shorten and pull, so muscles work in antagonistic pairs — the biceps flexes the elbow, the triceps** extends it — and every freely movable joint therefore needs at least two.
Add the axial and appendicular counts from their parts and see whether you reach 206, then name the joint at your elbow, your shoulder, your wrist and the top of your neck.