What is Skeletal System?

In biology, the skeletal system makes up the framework of human bodies structurally and has two main subdivisions-

1. The axial skeleton
2. The appendicular skeleton

Skeletal system provides us the structure and support needed. Anatomically, skeletal system is composed of several connective tissues namely; bones, tendons, ligaments and joints. Joints are very essential for movement. Without the skeletal system, human body will not have the required rigidity and will most likely just collapse!

Movements in some joints are either unidirectional and at some joints, movements is not restricted to a single direction. Any physical activity like running, jumping, chewing, etc. requires the proper functioning of the joints and the skeletal system.  

Key takeaways-        

• What are the different types under appendicular skeletal system?
• What are its functions?
• Significance
• Detailed structure of the appendicular skeleton

The Appendicular Skeleton

In total, the human adult skeleton consists of 206 bones out of which 80 bones belong to the axial skeleton and the rest 126 bones make up the appendicular skeleton.

The central axis of the body makes up the axial skeleton which provides protection and support to some vital organs, the brain, the spinal cord and provides a surface for the muscles for attachment.

Both the upper and lower limbs, bones of the pelvis and bones of shoulders make up the appendicular skeleton.

The human appendicular skeleton aids in the attachment and functions of the upper and lower limbs of the body.

The Bones Involved

The function of the upper limbs is basically grasping and manipulation of objects whereas the lower limbs function to aid in locomotion of human body in general. It is also inclusive of the pectoral girdle (shoulder bones) which attaches the upper limbs and the pelvic girdle which attaches the lower limbs.

1.    The upper limbs

The upper limbs comprise of 30 bones in total. The upper limbs are divisible into three regions-

• The region from shoulder to elbows called ‘arm’.
• The region from wrist to elbow called ‘forearm’.
• The wrist and hand region.

The upper limb’s longest bone is the humerus. It connects the scapula of the shoulder with the forearm at the elbow.

The forearm bears two bones;

• Radius
• Ulna

Radius forms a connection with the humerus at the elbow and is placed laterally in the forearm. The ulna which is located medially in the forearm connects with the humerus at the elbow, too. Ulna is longer than radius. Both these bones articulate with each other and also with the carpal bones (the eight small bones which make up the wrist are called carpals).

The hand is made up of the 8 carpus bones (wrist), 5 metacarpus bones (palm) and 14 phalanges (bones of human five fingers). In total that would add up to 27 in each hand. Each finger is made of 3 phalanges except the thumb which has only two.

2.    The lower limbs

The lower limb is divisible into thigh, leg and foot. The set of bones that make up the lower limb are-

• Thigh bone called femur
• Kneecap called patella
• Fibula which are bones of the leg
• Tarsals which are the ankle bones
• Metatarsals and phalanges which are the bones in the foot

The lower limb bones are comparatively stronger and thicker as they need to bear the entire body’s weight and are solely responsible for locomotion.

The lower limb equivalent of humerus is called femur and it is the longest, strongest and the heaviest bone in human body. The hip joint is formed by the connection of femur with the pelvis at the proximal end. The knee joint is the resultant of connection between femur, tibia and the patella at the distal end.

The kneecap bone called patella is shaped like a triangle and is placed anteriorly to the knee joint. Patella is embedded in the tendon of quadriceps (femoral extensors). Its function is to reduce friction and improve knee extension.

The shinbone is called tibia which is large and is placed directly below the knee. At the proximal end, tibia connects with the femur and at the distal end, it connects with the fibula and the tarsal bones. Tibia is the second largest bone in human body and its function is to transmit the weight of human body right from the foot to the thigh bones.

The calf bone is called fibula and it articulates with tibia. Although its function does not include bearing weight, it facilitates attachment of muscle for the ankle joint.

The ankle bones are seven in number and are referred to as the tarsals. These bones transmit the body’s weight from tibia and fibula to the feet.

There are five bones in the foot called metatarsals and the 14 bones of the toes are referred to as the phalanges.

3.    The pectoral girdle

The shoulder bones are called the pectoral girdle and this is the point of attachment of axial skeleton with the upper limbs. Pectoral girdle is divisible into scapula and the clavicle.

• The shoulder blades are called scapula which are located posteriorly. They are flat bones, shaped like a triangle placed at the back of the shoulder. Their function is to provide support to the muscles crossing the shoulder joint and the spine runs across the back of this bone, scapula. Scapulae provide a broad area of attachment for muscles with the bone and are bony protrusions.
• The collarbone is called clavicle. They are S-shaped, placed anteriorly and help in positioning the arms on the body. They lie horizontally just above the first rib, across the front part of the thorax.

 4.    The pelvic girdle

Pelvic girdle is the point of attachment of the axial skeleton with the lower limbs through strong ligaments. The pelvic girdle bears the responsibility of carrying the weight of the body and for locomotion. It also securely attaches the femur to the body through sturdy ligaments. Two large hip bones further strengthen the pelvic girdle. This hip joint is a resultant of fusion of three other bones- ilium, ischium and pubis.

At the anterior end, the pelvis forms a connection with the pubic symphyses and at the posterior end; it connects with the bones of the sacrum.

Practice Question

Question: What are the 4 main categories of bones belonging to the appendicular skeleton?

Answer: The appendicular skeleton comprises of the shoulder bones, limb bones, the pectoral girdle, and the pelvic girdle.

1. Pectoral girdle- two bones- scapula and clavicle.
2. Pelvic girdle- two hip bones
3. Upper limb bones- humerus, ulna, radius, carpals, metacarpals, phalanges.
4. Lower limb bones- femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

Context and Applications

This topic is a prerequisite for the completion of certain undergraduate and graduate specialties, especially for;

• M.B.B.S
• Bachelor of Science in Life Sciences
• Master of Science in Life Sciences
• Bachelor of Science in Zoology
• Master of Science in Zoology