Textbook of Human Osteology Inderbir Singh
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1Individual Bones of the body2

Introduction to the Skeleton1

The human skeleton may be divided into (a) the axial skeleton (consisting of the bones of the head, neck, and trunk; and (b) the appendicular skeleton consisting of the bones of the limbs.
 
 
A preliminary look at the skull
The skeleton of the head is called the skull. It is seen from the lateral side in Fig. 1.1 and from above in Fig. 1.2. The skull contains a large cranial cavity in which the brain is lodged. Just below the forehead the skull shows two large depressions, the right and left orbits, in which the eyes are lodged. In the region of the nose and mouth there are apertures that lead to the interior of the skull.
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Fig. 1.1: Skull seen from the right side.
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Fig. 1.2: Skull seen from above.
The skull is made up of a large number of bones that are firmly joined together. Some of these are as follows. In the region of the forehead there is the frontal bone. At the back of the head (also called the occiput) there is the occipital bone. The top of the skull, and parts of its side walls, are formed mainly by the right and left parietal bones. The region of the head just above the ears is referred to as the temple, and the bone here is the temporal bone (right or left). The bone that forms the upper jaw, and bears the upper teeth, is the maxilla. The prominence of the cheek is formed by 4the zygomatic bone. In the floor of the cranial cavity there is an unpaired bone called the sphenoid bone. The bone of the lower jaw is called the mandible. It is separate from the rest of the skull. In addition to these large bones, there are several smaller ones that will be identified when we take up the study of the skull in detail.
 
The vertebral column
Below the skull the central axis of the body is formed by the backbone or vertebral column (Fig. 1.3). The vertebral column is made up of a large number of bones of irregular shape called vertebrae. There are seven cervical vertebrae in the neck. Below these there are twelve thoracic vertebrae that take part in forming the skeleton of the thorax. Still lower down there are five lumbar vertebrae that lie in the posterior wall of the abdomen. The lowest part of the vertebral column is made up of the sacrum, which consists of five sacral vertebrae that are fused together; and of a small bone called the coccyx. The coccyx is made up of four rudimentary vertebrae fused together. There are thus thirty three vertebrae in all. Taking the sacrum and coccyx as single bones the vertebral column has twenty six bones.
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Fig. 1.3: Skull and vertebral column (right lateral view).
 
Skeleton of the thorax
The skeleton of the thorax forms a bony cage that protects the heart, the lungs, and some other organs (Fig. 1.4). Behind, it is made up of twelve thoracic vertebrae. In front, it is formed by a bone called the sternum. The sternum consists of an upper part, the manubrium; a middle part, the body; and a lower part, the xiphoid process. The side walls of the thorax are formed by twelve ribs on either side.
Each rib is a long curved bone that is attached posteriorly to the vertebral column. It curves round the sides of the thorax. Its anterior end is attached to a bar of cartilage (the costal cartilage) through which it gains attachment to the sternum. This arrangement is seen typically in the upper seven ribs (true ribs). The 8th, 9th and 10th costal cartilages do not reach the sternum, but end by getting attached to the next higher cartilage (false ribs). The anterior ends of the 11th and 12th ribs are free: they are, therefore, called floating ribs.5
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Fig. 1.4A: Skeleton of the thorax as seen from the front. The bones of the shoulder girdle are also shown.
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Fig. 1.4B: Section across thorax.
 
Skeleton of the Upper Limb
The skeleton of each upper limb (Fig. 1.5) consists of the bones of the pectoral girdle (or shoulder girdle) that lie in close relation to the upper part of the thorax (Fig. 1.4A), and those of the free limb.
The pectoral girdle consists of the collar-bone or clavicle, and the scapula. The clavicle is a rod like bone placed in front of the upper part of the thorax. Medially, it is attached to the manubrium of the sternum, and laterally to the scapula. The scapula is a triangular plate of bone placed behind the upper part of the thorax.
The bone of the arm is called the humerus. There are two bones in the forearm: the bone that lies laterally (i.e., towards the thumb) is called the radius; and the bone that lies medially (i.e., towards the little finger) is called the ulna. The humerus, radius and ulna are long bones each having a cylindrical middle part called the shaft, and expanded upper and lower ends.
In the wrist there are eight small, roughly cuboidal, carpal bones. The skeleton of the palm is made up of five rod like metacarpal bones, while the skeleton of the fingers (or digits) is made up of the phalanges. There are three phalanges – proximal, middle and distal – in each digit except the thumb that has only two phalanges (proximal and distal).6
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Fig. 1.5: Skeleton of the right upper limb. The manubrium sterni is included for orientation.
The upper end of the humerus is joined to the scapula at the shoulder joint, and its lower end is joined to the upper ends of the radius and ulna to form the elbow joint. The wrist joint is formed where the lower ends of the radius and ulna meet the carpal bones. The upper and lower ends of the radius and ulna are united to one another at the superior and inferior radioulnar joints. There are numerous small joints in the hand: the intercarpal between the carpal bones themselves; the carpometacarpal between the carpal and metacarpal bones; the metacarpo-phalangeal between each metacarpal bone and the proximal phalanx; and the inter-phalangeal joints between the phalanges themselves.
 
Skeleton of the Lower Limb
The skeleton of the lower limb consists of the bones of the pelvic girdle, and those of the free limb (Fig. 1.6). The pelvic girdle is made up of one hip bone on each side. Each hip bone is made up of three parts that are fused together. The upper expanded part of the bone is called the ilium. A small part in front (shaded in the figure) is called the pubis. The lower part of the bone is called the ischium. Anteriorly, the two pubic bones meet in the midline to form a joint called the pubic symphysis. Posteriorly, the sacrum is wedged in between the two hip bones. The hip bones and sacrum (along with the coccyx) form the bony pelvis.7
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Fig. 1.6: Skeleton of the pelvis and right lower limb.
The bones of the free part of the limb are arranged in a pattern similar to that in the upper limb. The bone of the thigh is called the femur. There are two bones in the leg. The medial of the two (lying towards the great toe) is called the tibia, while the outer bone is called the fibula. The femur, tibia and fibula are long bones having cylindrical shafts with expanded upper and lower ends. In the region of the ankle, and the posterior part of the foot, there are seven roughly cuboidal tarsal bones. The largest of these is the calcaneus, which forms the heel. Next in size we have the talus. In the anterior part of the foot there are five metatarsal bones. Each digit (or toe) has three phalanges, proximal, middle and distal: however, the great toe has only two phalanges proximal and distal.
The upper end of the femur fits into a deep socket in the hip bone (called the acetabulum) to form the hip joint. The lower end of the femur meets the tibia to form the knee joint. A small bone, the patella, is placed in front of the knee. The tibia and fibula are joined to each other at their upper and lower ends to form the superior and inferior tibiofibular joints. The lower ends of the tibia and fibula join the talus to form the ankle joint. Within the foot there are intertarsal, tarsometatarsal, metatarso-phalangeal and interphalangeal joints on a pattern similar to those in the hand.