The Appendicular skeleton is a skeletal system, which consists of the girdles and the bones of the limbs. The girdles consist of:
- pectoral girdles and
- pelvic girdles
- fore limbs and
- hind limbs
THE GIRDLES
1. The Pectoral Girdle: In man, this is found around the shoulder. It is made of two halves which are held by muscles. Each half of the girdle is made of three bones:
- the scapula
- the clavice or collar bone
- the coracoid
In mammals, the scapula and coracoid are used to form what is called scapula-coracoid. The scapula is a flat triangular bone. At the apex is a hollow or cavity called glenoid cavity into which the head of the humerus fits to form the shoulder joints. Above the glenoid cavity is a small hook-shaped bone called coracoid bone. On the outer surface of the scapula, there is ridge or spine called the scapular spine.
The scapula spine ends in two projections which are acromion and metacromion. Acromion is shorter than the metacromion. The clavicle is a small rod of bone attached to a ligament joining the sternum to the acromion of the scapular.
The pectoral girdle especially the surfaces of the scapula, the two processes and the spine are important for the attachment of muscles and ligaments. Pectoral girdles also provide firm support for the fore limbs (arms).
2. The Pelvic Girdle: This is found around the waist in man. It consists of two halves which are joined to each other ventrally and to the sacrum dorsally. The line of fusion of the two halves is called pubis symphysis. Each halve is called innominate bone. Each half is made of three bones which are fused together and they are:
- ilium
- ischium
- pubis
At the top is the ilium which is the largest and the longest of the three bones. At the lower end are the fused bones - ischium and pubis. The ischium and pubis enclose an opening or a hole called obturator foramen. It is through this hole that nerves, blood vessels and muscles pass.
On the other surface of each half of the girdle where the three bones meet, there is a deep hollow or depression called acetabulum where the head of the femur of the hind limb fits to form the hip joint, and is an example of ball and socket joint.
THE LIMBS
The limbs are made up of the fore limbs (bones of the hands) and the hind limbs (bones of the legs). The limbs of most vertebrates are built on the same basic plan, i.e pentadactyl limb plan (5-digit plan). It is made up of a long bone, followed by a pair of two long bones which lie side by side.
The pair of these long bones is followed by a set of nine small bones which are arranged in three rows. The nine small bones are followed by five digits. Each digit is made up of small bones called phalanges.
1. The Fore Limb: The fore limb of mammal, e.g rabbit, is made up of an upper arm bone which is a long bone called humerus. It has a rounded head which fits and articulates with the glenoid cavity of the scapula of the pectoral girdle of the shoulder joint.
The lower ends of the humerus end in a grooved-pulley - like surface called trochlea. At the trochlea, the humerus articulates with radius and ulna to form the elbow joint.
The humerus is followed by the bones of the fore-arm. The fore-am bones are the radius and ulna. The radius is a long bone, It lies in front of the ulna. The radius is a long bone. It lies in front of the ulna and is slightly curved. The ulna is longer than the radius. It has a cavity called sigmoid cavity.
The trochlea of the humerus fits into this cavity. The ulna also projects backwards to form a projection called olecranon process. The radius/ulna bone is followed by bones od the wrists which are made of nine small bones arranged in three rows. These bones are called carpals. Proximally (in front), the carpals articulate with radius-ulna and dixtally with bones of the digits.
The wrist bones are followed by bones of the digits. The digit bones are five and they are called metacarpals. In man, the metacarpals are called fingers, they are also referred to as phalanges. In man, each digit has three phalanges with the exception of the thumb, which has two phalanges. In rabbit, the phalanges end in claws.
2. The Hind Limb: The hind limb of mammals, e.g rabbit, is made up of the thigh bone called the femur. The femur is the largestand strongest bone in the body. It is rounded at the proximal end to form a head which fits into the acetabulum of the pelvic girdle to form a hip joint.
Very close to the head of the femur are three projections called trochanters. They are important for the attachment of muscles. At the distal end of the femur are two rounded nobs called condyles. They articulate with tibia bone. In-between the the two condyles is a pulley-like hoof.
The shank is made up of two bones called tibia and fibula but are fused in rabbit to form what is called tibio-fibula.The tibia is longer and larger than the fibula. At its proximal end, there are two grooves into which the condyles of the femur fits into. The fibula is a small bone which lies outside the tibia. It is free at its proximal end but joined to the tibia at the distal end.
The knee joint is found at the junction of the femur and the tibia. In front of this joint is a small round bone called the patella or knee cap. The ankle is made up of six bones called tarsals. The inner tarsal projects backwards to form the heel bone. The foot of the rabbit has four digits called metatarsals. Each digit is made up of three phalanges. The last phalanges bear the claws. Most mammals, including man, has five metatarsals.
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