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The Pectoral Girdle
The pectoral girdle is found around the shoulder in mammals. It is made of two halves which are held by muscles. Each half of the girdle is made of three bones. These bones are: i) scapula or shoulder blade (ii) the clavicle or collar bone and (iii) coracoid.
In mammals, the scapula and coracoid are fused 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. It runs towards the glenoid cavity. The scapula spine ends in two projections which are acromion and metacromion. Acromion is shorter than the metacromion which is larger. 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 surface 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).
The Pelvic Girdle
The pelvic girdle is found around the waist in mammals. 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 half is called innominate bone. Each half is made up of three bones which are (i) ilium (ii) ischium and (iii) pubis. They are fused together. 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, an example of ball and socket joint.
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. The diagram shows a generalised fore and hind pentadactyl limbs including the names of the corresponding parts.
The Fore Limb
The fore limb of mammal (Fig. 2.34), e.g. rabbit is made up of an upper arm bone which is a long bone called humerus (Fig 2.34). 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 arm bones are the radius and 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 of 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 the 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 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.
The Hind Limb
The hind limb of mammal (Fig. 2.36) e.g. rabbit is made up of the thigh bone called the femur (Fig 2.37). The femur is the largest and the 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 nob called condyles. They articulate with tibia bon In-between the two condyles is a pulley-lik hoof. The shank is made up of two bones calle tibia and fibula but are fused in rabbit to formHow Can We Make ClassNotesNG Better - CLICK to Tell Us💃
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