APPENDICULAR SKELETON

The appendicular skeleton is made up of the limb girdles (pectoral and pelvic girdles) and the limbs (fore limbs and hind limbs).

The pectoral girdles

The pectoral girdle consists of a triangular plate of bone called scapula or shoulder blade, ribs and sternum.

a) Scapula

Scapula is a flat bone. It has one head-like end with two processes, the coracoid and acromion with a deep notch called the glenoid fossa or cavity. The rounded head of humerus fits into the glenoid cavity in a ball and socket joint. Another bone of the girdle is the collar bone or clavicle which glides over shoulder joint to keep the bones in correct position.

The scapula is not fixed to the spine, it is only bound to the posterior of the thorax by muscles. This allows considerable range of movement at the shoulder.

b) Ribs

There are thirteen pairs of ribs in a rat. The first seven pairs of ribs are attached to the sternum and they are called true ribs. The eighth, ninth, tenth pairs of ribs have their coastal cartilages attached to the seventh pair of ribs. The eighth, ninth and tenth are therefore called false ribs. The last three pairs are attached only to the thoracic vertebrae. They are therefore called floating ribs. Each rib consists of a slightly curved shaft. The dorsal end of the shaft is the Capitulum and head. A little behind the capitulum is a tuberculum The capitulum articulates with the demi-facet of the centrum of the thoracic vertebra while the tuberculum articulates with the transverse process of the thoracic vertebra.

c) Sternum

The sternum is a jointed and an elongated structure. It is made up of a broad manubrium at the upper part, four sternebrae and a xiphisternum into which a xiphoid cartilage is attached.

Functions

1. It protects the heart and lungs as it forms a protective basket with the ribs.
2. It articulates with the ribs to bring about breathing movement of the chest cavity.

Fore-limb

The fore-limb is made up of a humerus in the upper arm, an ulna and radius in the lower arm, carpals in the wrist and phalanges in the digits.

a) Humerus

The humerus has a shaft and two extremities. The proximal end consists of a rounded head without a neck. The greater tuberosity lies at the opposite side of the head while the lesser tuberosity is attached below the head. The flattened area from the head midway the shaft is the deltoid ridge. A bicipital groove lies in between the head and the greater tuberosity.

The distal end of humerus forms the trochlea. In between the trochlea is the supratrochlear foramen. Above the supratrochlear foramen at the posterior side lies a shallow groove called the olecranon fossa.

Functions

1. It forms attachment surface for biceps and triceps muscles.
2. The bicipital groove anchors the biceps muscles.
3. It forms the ball and socket joint at the shoulder.
4. It forms the hinge joint at the elbow.
5. It allows free movement of the arm.

b) Radius and Ulna

The two bones in the lower arm are the radius and ulna. The radius is shorter and it lies on top of the ulna. They both form the sigmoid notch into which the trochlea of humerus fits in the elbow joint. The ulna projects behind to form the olecranon fossa of humerus to keep the joint in position. The distal ends of radius and ulna form the styloid process at the wrist joint.

Functions

1. They form the point of attachment for the nit insertion of the biceps and triceps muscles.
2. They provide large surface areas for the attachment of the extensor and flexor muscles of the lower arm. The muscles attached to the radius and ulna contract and relax to move the hand and digits.

c) Carpals

The carpals are the wrist bones, which are eight in number, and irregular in shape. They are irregularly arranged in the wrist and held together by tough ligaments.

Functions

They form a hinge joint which allows for the movement of the hand along one plane.

Metacarpals

The metacarpals are the bones of the hand. They are long and slender like sticks of chalk or cigarettes. The metacarpal of the thumb is much shorter than the four others. There are five metacarpals in one hand.

Functions

1. Metacarpals form the shape of the hand.
2. They provide surface area for the attachment of the muscles of the palm.

d) Phalanges

The bones of the digits are called phalanges. There are two in the thumb and three in each of the four others.

Functions

1. They support the digits.
2. They are used for griping and holding on to objects.
3. They are used for walking in tetrapod animals.
4. They bear claws which are used for climbing and digging in some animals such as rat.

Hind-limb

The hind-limb is made up of the femur of thigh bone, the tibia and fibula in the leg, the tarsals in the ankle, the metatarsals of the foot and the phalanges of the digits.

a) Femur

The femur is the longest bone in the body. It has a cylindrical shaft and two extremities. The proximal extremity bears a head with a separating neck, and on the opposite side is the greater trochanter. Two lesser trochanter are located below the head and the greater trochanter. The distal end bears the condyle and the patella groove.

Functions

1. Femur forms a ball and socket joint with the ilium allowing the free movement of the hind limb.
2. Forms attachment surface for the thigh muscles.
3. The distal end forms a hinge joint with the patella, tibia and fibula.

b) The tibia and fibula

The skeleton of the leg consists of the tibia and fibula. That tibia is bigger than the fibula which fuses with it at the distal end to form the tibio-fibula. The proximal end is broad and has facets for articulation with the condyles of the femur and a notch for the patella to form the knee. The distal end has facets for articulation with the ankle bones.

Functions

1. Tibia and fibula provide large surface area for the attachment of muscles of the leg.
2. Muscles that move the foot have their origin attached to the tibia and fibula.
3. They form the ankle joint which allows the movements of the foot along one plane.

c) Tarsals

The tarsals are the ankle bones. They are eight in number and irregularly arranged in two rows. The bones are of unequal sizes and irregular in shape. The three behind articulate with the tibio-fibula in the ankle. The five in the second row articulate with the metatarsals to the anterior of the foot.

Functions

1. The tarsals provide attachment surface for the strong muscles of the legs.
2. They form the ankle for the movement of the foot along one plane.
3. Muscles for moving the foot and toes are attached to the tarsals.

d) Metatarsals

There are five bones in the foot, called metatarsals. The first metatarsal is shorter than the rest four.

Functions

1. The metatarsals support the foot which is used for walking.
2. The metatarsal bear the phalanges of the toes and support then.
3. Metatarsals provide surface area for the attachment of the foot and toe muscles.

e) Phalanges

The bones of the digits or toes are called phalanges. The first toe called big toe has two phalanges. The remaining four toes have three phalanges each. The last phalanx in each digit is pointed to fit into the strong and sharp claw.

Functions

1. The phalanges support the toes which are used for walking, gripping and digging.
2. They provide surface area for the attachment of muscles of the foot and toes.

The Pelvic girdle

The pelvic girdle consists of the ilium (hip bone), ischium and pubis all of which are fused together on each side of the body as the innominate bone. These two halves are rigidly joined at the last lumbar vertebra to the base of the spine at the back while in the front they are less rigidly joined at the pubis by the fibro-cartilage of pubis symphysis. The distal ends of these bones form a cavity called acetabulum into which the head of femur fits to form the ball and socket hip joint.