Bones In The Human Skeleton

The entire framework of bones and their cartilages, along with ligaments and tendons, constitutes the skeletal system.

1.0Divisions of the Skeletal System

• The adult human skeleton consists of 206 named bones, most of which are paired, with one member of each pair on the right and left sides of the body.
• Bones of the adult skeleton are grouped into two principal divisions: the axial skeleton and the appendicular skeleton. presents the 80 bones of the axial skeleton and the 126 bones of the appendicular skeleton. Both divisions join to form the complete skeleton .

Skeleton has 2 Major divisions 

• Axial skeleton
• Appendicular skeleton

Axial Skeleton consists of

(1) Skull 

(2) Vertebral column 

(3) Ribs and Sternum (bones of thorax)

(4) Hyoid bone.

Appendicular Skeleton consists of 

(1) Bones of upper limb and shoulder.

(2) Bones of lower limb and pelvis.

2.0Axial Skeleton

1. Skull

Consists of 28 bones and is divided into 2 groups.

Facial bones: comprise bones of face.

Cranial bones : comprise that portion of skull which lodges the brain and special sense organs.

Facial Bones (14)

Paired

• Nasal 2
• Maxillae 2
• Zygomatic 2
• Palatine 2
• Lacrimal 2
• Inferior Nasal Conchae 2

Unpaired 

• Mandible
• Vomer

2. Vertebral Column

• Forms the central axis of the body with the skull resting upon it. It consists of 33 irregular bones stacked one upon the other. Vertebral column gives support to the trunk, provides articulation with ribs and pelvic bones, allows a good deal of movement, and lodges and protects the spinal cord. Muscles and ligaments bind the vertebrae together.
• Movement is possible because of the presence of fibrocartilaginous intervertebral discs between adjacent vertebrae.
• Vertebral column shows several bends or curvatures, cervical, thoracic and lumbar curvatures.
• Vertebral column consists of 33 vertebrae. They are named according the region in which they lie-
• Cervical-7,
• Thoracic-12,
• Lumbar-5,
• Sacral-5, fused into one bone, the sacrum.
• Coccyx-4, fused into one bone, the coccyx.
• Vertebrae of different regions have common features as well as special features (specific of their own region)

3. Thoracic Bones consist of Ribs and Sternum

Ribs

• Normally there are 12 pairs of ribs which form a large part of the thoracic wall. They all articulate behind with the vertebral column. 1st to 7th ribs are called True Ribs, as they are connected with the sternum in front by means of costal cartilages.
• 8th to 12th ribs are called False ribs. The cartilages of these ribs are not connected to the sternum directly.
• Vertebro-chondral ribs 8th. The 9th, and 10th ribs belong to this type. They are attached to cartilages of the rib just above.
• Floating ribs 11th and 12th ribs are free at their anterior. Ends.
• Typical ribs are those which present common features.
• Atypical ribs are those which present special features e.g.. 1st, 2nd, 11th and 12th ribs.
• Ribs lie one above the other and the spaces between them are called 'Intercostal spaces'.

Sternum (Breast bone)

• A long flat bone lying in the median portion of anterior thoracic wall. It consists of 3 parts-

(i) Manubrium sterni (ii) Body and (iii) Xiphoid process.

• The body is made up of four fused sternabrae.
• The junction of manubrium with the body of the sternum forms the sternal angle, which is a very important landmark. The 2nd rib Joins the sternum at this point.
• Manubrium sterni articulates with medial ends of the clavicles. The margins of sternum articulate the costal cartilages of the first seven pairs of ribs.

3.0Appendicular Skeleton

1. Bones of Shoulder Girdle and Upper Limb

• 'Shoulder girdle' consists of 2 bones-(1) Clavicle and (2) Scapula which attach the arm to the trunk. Large number of muscles are attached to these bones, which help to bind the upper limb to the trunk and compensate for the presence of only one joint between upper limb and trunk (Sterno-clavicular joint).
• Clavicle: It is also known as Collar bone. It is a doubly curved bone which lies horizontally in front of the root of the neck and is subcutaneous throughout its extent. It articulates medially with the sternum and laterally with the acromion process of scapula.
• It transmits weight of the upper limb to the trunk, prevents drooping of the point of the shoulder and enables the limb to swing clear of the trunk. It serves as a mobile strut.
• Scapula: It is also known as shoulder blade. It is a large triangular flat bone which lies at the postero-lateral part of the chest wall opposite 2nd to 7th ribs. It possesses 2 surfaces, 3 borders, 3 angles and 3 processes. The glenoid cavity, located at the lateral angle, articulates with the head of the humerus to form 'Shoulder joint'. The acromion process articulates with the lateral end of the clavicle.

• Humerus: It is the only bone of (upper) arm. At the elbow it articulates with bones of the forearm, radius and ulna. Its rounded upper end called 'Head' articulates with Glenoid fossa of scapula.
• Radius: It is the lateral bone of the forearm and is shorter than the ulna. Its upper end consists of a disc-like head which articulates with the capitulum of humerus and on its medial side it articulates with the radial notch of ulna. The lower end is larger and has a small conical bony projection at its lateral part. called the styloid process. The inferior surface of the lower end articulates with two of the wrist bones (carpal bones), scaphoid and lunate.
• Ulna: It is the medial bone of the forearm and is longer than the radius. Its upper end is strong and has a "hook-like" projection called the Trochlear notch. It articulates with the trochlea of humerus. A radial notch on the upper part of the lateral surface of the coronoid process articulates with the side of the head of radius. The distal end is smaller and consists of a small, smooth, rounded head and a short rounded process called the Styloid process. The head articulates on its lateral part with the ulnar notch of radius. Inferior surface of the head is separated from carpal bones by the articular disc of Inferior radio-ulnar joint.

Bones of Wrist and Hand

• Carpal Bones: Wrist is anatomically known as Carpus and the eight small bones making it are known as Carpals. They are arranged in 2 irregular rows of 4 bones each. They are from lateral to medial:
• In Proximal row: Scaphold, Lunate, Triquetral, Pisiform.
• In Distal row: Trapezium, Trapezoid, Capitate, Hamate.
• Carpal bones are bound together quite closely by ligaments to allow limited movements. Bones of the proximal row articulate distally with proximal surfaces of bones of distal row except Pisiform which lies upon Triquetral.
• Bones of distal row articulate with metacarpal bones.
• Metacarpals & Phalanges (Anatomical term for hand is manus). Metacarpals are the bones of the hand, and phalanges are the bones of digits. Metacarpals do not have individual names and are numbered from 1 to 5, beginning with the metacarpal of the thumb and counting medially to metacarpal of the little finger.. Each metacarpal possesses a Base which articulates with carpals, and a Head which articulates with the proximal phalanx of corresponding digit. The heads of metacarpals form the 'Knucles'. Shaft, which is relatively long and curved, lies between head and base.
• There are 14 phalanges in each hand.
• Each digit has 3 phalanges-proximal, middle and distal phalanx, except the thumb, which has only two, proximal and distal.

Bones of Pelvic Girdle and Lower Limb

• Pelvic girdle is formed by:
• (1) Two Hip bones, (ii) Sacrum (iii) Coccyx. 
• Pelvic cavity lodges and protects the reproductive organs, nerves and blood vessels of the lower limb, urinary bladder and terminal part of the large intestine. 
• The combined weight of trunk, head and upper limbs rests on the pelvis at its articulation with 5th lumbar vertebra.

• Hip Bones (Os Coxae): It consists of 3 parts-ilium, ischium and pubis which are 3 separate bones in early life. The line of union of 3 parts is in the acetabular cavity, which is a deep socket for articulation with the head of the femur to form a hip joint.
• Ilium is the large, irregularly expanded upper part of the hip bone.
• Ischium forms the postero-inferior part of the bone: It has the large ischial tuberosity, which supports the body weight in sitting posture. One sits on ischial tuberosity.
• Pubis forms the anterior part of the bone. Two pubic bones articulate in front forming a secondary cartilaginous joint called "Pubic symphysis". The rami of pubis and ischium form a greater part of the obturator foramen.
• Articulated Pelvis: Each hip bone articulates behind with the sacrum, forming Sacro-iliac joint and in front with its fellow opposite side at the Pubic symphysis.
• Pelvis is divided into 2 parts by the Pelvic brim:

(1) True pelvis or Pelvis minor: lies below pelvic brim.

(2) False pelvis or Pelvis major: lies above pelvic brim.

• Pelvic brim is formed by (1) Sacral promontory. (2) Arcuate line of the sacrum and ilium (3) Illiopectineal line, (4) Pubic crest and (5) upper end of Pubic symphysis.
• Femur (Thigh bone): It is the longest, heaviest and strongest bone in the body. It is covered all around by large thigh muscles. The rounded head of the femur articulates with the acetabulum to form a Hip joint. Below it articulates with the tibia and patella.

• Patella (Knee cap): It is a flattened, triangular, sesamoid bone, lying within the tendon of quadriceps femoris muscle in front of the knee joint. Its posterior surface articulates with femoral condyles.
• Tibia (Shin bone): It is the largest and medial bone of the leg. Its anterior border is subcutaneous, sharp and prominent in its upper 2/3 and is called 'Shin'. The lower end is smaller than the upper end and has a strong process on its medial side, called 'Medial malleolus. The tibia takes part in formation of several joints, knee, ankle and superior and inferior tibio-fibular joints. It helps in transmitting body weight.
• Fibula: It is a long, slender lateral bone of leg. The upper end is expanded and is called the 'Head'. The lower end is expanded antero-posteriorly and flat from side to side and is called "Lateral Malleolus.” It does not articulate with either patella or femur. It articulates with tibia at its upper end and with tibia and talus at its lower end.

Bones of Ankle and Foot

• Tarsal bones: Correspond to carpals of the wrist. They form the skeleton of the posterior part of the foot. They are 7 in number and are arranged in 2 rows.
• Proximal row: (a) Talus and (b) Calcaneus.
• Distal row : (from medial to lateral side): 

(a) Medial cuneiform 

(b) Intermediate cuneiform 

(c) Lateral cuneiform and 

(d) Cuboid.

• On the medial side Navicular lies between Talus and the 3 Cuneiforms, occupying an intermediate position.

Talus

• It possesses a head, neck and body. It articulates with the lower end of tibia above and calcaneus below, thus receiving the body weight and transferring it to calcaneus. 
• It also articulates with the lower end of the fibula and with the navicular. 
• It helps in forming 3 joints (1) Ankle (2) Talo-calcaneal and (3) Talo-calcaneo-navicular. 
• It does not give attachment to any muscle.

Calcaneus

• It is the largest and strongest of all tarsal bones and its posterior part forms the heel. 
• It transmits body weight to the ground. 
• It helps in forming 3 joints (1) Talo-calcaneal (2) Calcaneo- cuboidal and (3) Talo-calcaneo-navicular.

Navicular

• It lies in front of the talus on the medial aspect of the foot. 
• It is boat-shaped and articulates behind with talus and in front with all 3 cunciform bones.

Cuboid

• It lies laterally in the foot between the calcaneus behind and the 4th metatarsal in front. 
• It articulates medially.th navicular and lateral cuneiform, behind with calcaneus and in front with the 4th and 5th metatarsals.

Cuneiforms

• These are 3 wedge-shaped bones lying behind the first four metatarsals and articulate with them.
• 'Medial Cuneiform' articulates with navicular, intermediate cunciform and 1st metatarsal bone.
• 'Intermediate cuneiform' articulates with navicular, medial and lateral cuneiforms and base of 2nd metatarsal bone.
• Lateral Cuneiform articulates with navicular, cuboid, Inter. mediate cuneiform and 2nd, 3rd and 4th metatarsals.

Metatarsals

• Like metacarpals of the hand, metatarsals are not named but numbered from 1 to 5. 
• Medial most metatarsal is numbered 1. 
• They lie between the tarsals and phalanges. The small rounded head lies distally and articulates with a proximal phalanx of its own toe. 
• The expanded base articulates proximally with the distal row of tarsal bones.

Phalanges

• As in hand, there are 14 phalanges. Each digit has 3 phalanges except the big toe which has only two.

4.0Functions of the Skeletal System

• Support. The skeleton serves as the structural framework for the body by supporting soft tissues and providing attachment points for the tendons of most skeletal muscles. 
• Protection. The skeleton protects the most important internal organs from injury. For example, cranial bones protect the brain, and the rib cage protects the heart and lungs. 
• Assistance in movement. Most skeletal muscles attach to bones; when they contract, they pull on bones to produce movement. 
• Mineral homeostasis (storage and release). Bone tissue makes up about 18% of the weight of the human body. It stores several minerals, especially calcium and phosphorus, which contribute to the strength of bone. Bone tissue stores about 99% of the body’s calcium. On demand, bone releases minerals into the blood to maintain critical mineral balances (homeostasis) and to distribute the minerals to other parts of the body. 
• Blood cell production. Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called hematopoiesis. Red bone marrow consists of developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibers. It is present in developing bones of the fetus and in some adult bones, such as the hip (pelvic) bones, ribs, sternum (breastbone), vertebrae (backbones), skull, and ends of the bones of the humerus (arm bone) and femur (thigh bone). In a newborn, all bone marrow is red and is involved in hematopoiesis. With increasing age, much of the bone marrow changes from red to yellow.
• Triglyceride storage. Yellow bone marrow consists mainly of adipose cells, which store triglycerides. The stored triglycerides are a potential chemical energy reserve.