Topic: Bones Of Axial And Appendicular Skeleton
The skeletal system or bones in mammals are grouped into two major parts. These are the axial and appendicular skeleton.
1. Axial Skeleton
The axial skeleton is made up of the skull, vertebral column or backbone, the ribs and sternum or breast bone.
i. The Skull: The mammalian skull is made up of several flat bones which are joined together by means of joints called sutures.
The three major parts of the skull are:
i. The cranium or brain box which holds or contains the brain.
ii. The facial skeleton which supports the nose, eyes and the muscles of the cheek.
iii. The jaws which are made up of the upper jaw (maxilla) and lower jaw (mandible) which contain the teeth.
Functions of the Skull
i. It protects the brain.
ii. It gives shape to the head.
iii. It protects vital organs in the head, e.g. Eyes, nose, ears, etc.
iv. It bears the teeth which aid grinding food.
ii. The Vertebral Column: The vertebral column, also known as the backbone or spinal column, is the central supporting structure of the skeleton.
The vetebral column forms the backbone of vertebral animals and protects the spinal cord. It is made up of five groups of bones called vertebrae (singular: vertebra) in mammals. In humans, it consists of 33 vertebrae. The vertebrae are held one to the others with strong ligaments having compressible cartilage pads called intervertebral discs between consecutive vertebrae.
In mammals, the five different vertebrae are:
i. Carvical vertebrae – found in the neck region
ii. Thoracic vertebrae – found in the chest region
iii. Lumber vertebrae – found in the upper abdomen
iv Sacral vertebrae – found in the lower abdomen
v. Caudal vertebrae – found in the tail region.
iii. The Ribs: The ribs are long semi-circular rods connecting the thoracic vertebrae to the breastbone (sternum). It is found in the chest region of mammals. There are twelve pairs of ribs in humans and thirteen pairs in rabbit. The bony cage formed by ribs protects the lungs and heart. It also assists in breathing.
A typical rib consists of a head (which fits between successive thoracic vertebrae), a neck and the main part, the shaft. Each rib articulates with the thoracic vertebrae by two processes: the capitulum which articulates with facets of the centre of two nearby vertebrae and the tuberculum (tubercle) which articulates with a transverse process.
The first seven pairs of the ribs are called the true ribs because they are connected directly with the sternum in front (ventrally) by costal cartilages. The next five pairs of ribs are called false ribs because the eighth, ninth and tenth pair of ribs have a common connection with the sternum, each being attached to the costal cartilage. The eleventh and twelfth pair are called floating ribs because they have no connection whatsoever with the sternum.
2. Appendicular Skeleton
The appendicular skeleton is made up of the limb girdles (pectoral and pelvic girdles) and the limbs (fore limbs and hind limbs).
i. The Pectoral Girdle: The pectoral girdle is found around the shoulder in man. 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 pectorial 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).
ii. The Pelvic Girdle: The pelvic girdle 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 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 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 nerve, 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.
iii. 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.
iv. 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 of the shoulder joint.
The lower ends of the humerus end in a grooved-pulley – like surface called trochlea. At the trochleas, 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 the 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 fingers, they 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.
v. 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 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 nobs called condyles. They articulate with tibia bone. In-between 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-fabula. 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 lie 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.