The girdles and the limbs · Biology Class 11–12 | StemPy Lab
Course 16.3·Lesson 2 of 3
The girdles and the limbs
The appendicular skeleton holds the bones of the limbs and of the two girdles. Each limb has 30 bones. The pectoral girdle joins the fore limbs to the axis, and the pelvic girdle joins the hind limbs. Together they make 126 bones, and with the 80 of the axial skeleton they make the 206 bones of the whole skeleton.
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From the shoulder to the fingertip
Lift your arm in front of you. It has one long bone above the elbow, two long bones below it, a group of small bones in the wrist, and the many bones of the hand. Your leg follows nearly the same plan. This is the appendicular skeleton.
The bones of a fore limb on the left and of a hind limb on the right. The fore limb, from the top: the humerus, then the radius and the ulna side by side, then 8 carpals of the wrist in two rows, 5 metacarpals of the palm and 14 phalanges in the five fingers. The hind limb, from the top: the femur, the patella in front of the knee, the tibia and the fibula side by side, 7 tarsals of the ankle, 5 metatarsals and 14 phalanges in the five toes. A count under each limb says 30 bones.
The bones of the limbs
Each limb is made of 30 bones.
Fore limb (arm and hand)
Humerus: 1
Radius and ulna: 2
Carpals (wrist bones): 8
Metacarpals (palm bones): 5
Phalanges (finger bones): 14
Total: 30
Hind limb (leg and foot)
Femur (thigh bone, the longest bone of the body): 1
Tibia and fibula: 2
Patella (knee cap): 1
Tarsals (ankle bones): 7
Metatarsals: 5
Phalanges (toe bones): 14
Total: 30
The patella is a cup shaped bone that covers the knee at the front.
The girdles
A girdle joins a pair of limbs to the axial skeleton. Each girdle has two halves.
The two girdles. On the left, the pectoral girdle of one side seen from above and behind: the clavicle, a slim bone with two curves, meets the acromion, the flat end of the spine of the scapula, which is a large triangular flat bone. Below the acromion, the glenoid cavity is a shallow socket that receives the head of the humerus and makes the shoulder joint. On the right, the pelvic girdle from the front: two coxal bones, each made of three fused bones, the ilium above, the ischium below and behind, and the pubis in front. The acetabulum is a cup on the side of each coxal bone that receives the head of the femur. The two halves meet in the middle at the pubic symphysis, which holds fibrous cartilage.
The pectoral girdle
Each half of the pectoral girdle has a clavicle and a scapula.
The scapula is a large, flat, triangular bone at the back of the chest, between the second and the seventh ribs.
The back of the scapula has a raised ridge, the spine. It ends in a flat, expanded process, the acromion. The clavicle meets the acromion.
Below the acromion lies a depression, the glenoid cavity. It receives the head of the humerus, and together they make the shoulder joint.
The clavicle is a long, slim bone with two curves. It is commonly called the collar bone.
The pelvic girdle
The pelvic girdle has two coxal bones. Each coxal bone is made by the fusion of three bones: the ilium, the ischium and the pubis.
At the place where they fuse, a cup called the acetabulum receives the head of the thigh bone.
The two halves of the girdle meet in front at the pubic symphysis, which holds fibrous cartilage.
The whole skeleton
Count the appendicular skeleton. Four limbs have 4×30=120 bones. The two pectoral halves have a clavicle and a scapula each, which makes 4 bones. The two coxal bones make 2 bones. So the appendicular skeleton has 120+4+2=126 bones.
NCERT gives the whole skeleton as 206 bones and the axial skeleton as 80. The difference is 206−80=126. The two counts agree.
You should find that each limb has 30 bones, and that the fore limb carries two girdle bones and the hind limb carries one.
Try it in Python
The program adds the bones of the limbs and the girdles, and checks the total of the skeleton.
Practise with new numbers
Common mistakes
Leaving the patella out of the count of the hind limb. The leg has 30 bones, and the patella is one of them.
Saying that the femur is the longest bone of the arm. It is the thigh bone, and the longest bone of the body.
Mixing the carpals and the tarsals. The carpals (8) are in the wrist. The tarsals (7) are in the ankle.
Saying that the glenoid cavity is in the pelvic girdle. It is in the scapula. The acetabulum is in the coxal bone.
Forgetting that a coxal bone is three fused bones. They are the ilium, the ischium and the pubis.