Human Skeleton

It consists of axial skeleton and appendicular skeleton. Axial skeleton includes skull or cranium, facial bones, hyoid bone, vertebral column and ribs. Appendicular skeleton includes pectoral girdle, pelvic girdle, forelimbs and hindlimbs.

Skull

Human skull (side view)
The skull or cranium is a box cavity containing and protecting brain. The bones of the skull are mostly flat and come close together at joints called sutures, they are immovable. The cranium consists of eight bones out of which four are unpaired and two are paired. The bones of the cranium are: (a) Frontal bone (b) Parietal bones (left and right) (c) Occipital bone (d) Temporal bone (left and right) (e) Ethmoid bone (f) Sphenoid bone. 

These are 14 in number out of which 6 are paired and 2 are unpaired, and are attached to the cranium. They support the muscles of the face, mouth and nose. The paired bones of face are:
(a)        Maxilla
(b)        Zygomatic bones
(c)        Nasal bones
(d)        Inferior nasal concha
(e)        Lacrimal bones
(f)        Palatine bones are two irregular bones, which form the back of the hard palate.
            The unpaired bones are
             (i)         Mandible
             (ii)        Vomer (is only movable)

Inferior nasal concha

An inferior view of the skull 


Interior view of Hyoid Bone 
Hyoid Bone
It is a small bone which lies at the tongue. It gives attachment to the tongue and serves as the site for the attachment of muscles associated with swallowing. It does not articulate with any other bone of head. It has a body, two greater and two lesser cornu. It is attached to the temporal bone muscles and ligaments.


Vertebral Column
Vertebral Column
It supports the weight of the body. It provides axial support to the head. Normally the vertebral column has four curvatures that provide more resiliency and strength in an upright posture than a straight column could. The vertebral column in human being consists of 33 vertebrae. The vertebrae are named according to their location in the vertebral column and may be divided into following groups:

a)         Cervical vertebrae        -           (07)
b)         Thoracic vertebrae       -           (12)
c)         Lumbar vertebrae        -           (05)
d)         Sacral vertebrae           -           (05)
e)         Coccygeal vertebrae    -           (04)
Cervical vertebrae are the vertebrae of neck. There are seven such vertebrae in necks of all mammals. The first two vertebrae are modified to allow the movements of the head.
Atlas: The atlas is the first cervical (neck) vertebral
Axis: it’s the second cervical vertebra.
Thoracic vertebrae: These are the rib carrying vertebrae and form a backward curve down the thorax. These are characterized by their large spinous processes. The ribs articulate with the rib facet.
Lumbar vertebrae: These are strong stout bones with no rib articulations. These are the largest bones in the vertebral column.
Sacral vertebrae: These are five fused vertebrae forming the sacrum. The sacrum articulates with the iliac bones of the hip bone to form the back of pelvis.
Coccygeal vertebrae: These are also called coccyx and are four vertebrae fused in adults.

Chest Cage
Chest Cage

It consists of twelve pairs of ribs. The ribs articulate posterior with the thoracic vertebrae. Ten ribs are connected interiorly with sternum either directly or through the costal cartilage. The rib cage provides support for a semi-vacuum chamber called chest cavity. The seven pairs of ribs that attach directly to the sternum are called true ribs. The 8th, 9th and 10th ribs are called false ribs, as these three pairs of ribs are attached to the sternum by means of common costal cartilage. 11th and 12th pair of ribs is known as floating ribs, because they do not attach to the sternum.

Pectoral Girdle

It consists of a pair of clavicles and a pair of scapulas.

Clavicles: These are a pair of collar bones forming the front of the pectoral girdle (shoulders). One end of each curved bone articulates with the sternum. The other end articulates with the scapulas.

Pectoral Girdle
Scapulas: These are two shoulder blades, triangular bones forming the back part of pectoral girdles. They do not articulate with ribs but are embedded in muscles. A depression called glenoid cavity accommodates the ball end of the upper arm bone. The other parts of scapulas are acromion for the attachment of muscles and for the articulation with the clavicle and coracoid process to which muscles of arm and chest are attached.


Forelimb

It consists of the following bones:
Humerus:        Upper arm bone. Radius and Ulna: Forearm Bones.
Carpals:          Eight short bones forming wrist.
Metacarpal:    Five short bones forming the palm.                                                                       
Phalanges:     Three bones forming the fingers. Two bones form the thumb.

Forelimb

Humerus

It is a long bone, the end of which has a spherical shaped head, which fits into the glenoid cavity, forming the shoulder joints. It has greater tubercle and lesser tubercle for the attachment of muscles. The shaft has many rough surfaces which give attachment to muscles; the most marked being the deltoid tuberosity. The lower extremity carries two condyles for articulation, with radius and ulna. Above the condyle on either side are processes called epicondyle, which gives attachment to the muscles. 

Above the condyle there are two depressions the olecranon fossa at the back and coronoid fossa in the front.

Radius: It is a long, outer bone of the fore arm (on the thumb side). Its upper extremity is smaller and carries disc shaped head. Below the head is the neck and on the front of the bone, is a process called radial tuberosity for the attachment of biceps brachii muscles.

Ulna: It is a long bone on the inner side of the forearm, and slightly bigger than radius. There are two processes for muscle attachment. The processes are known as olecranon and coronoid processes. The olecranon is large and gives attachment to the triceps.

Carpals: These consist of two rows of four short bones. The upper row articulates with radius and forms the wrist joint. The lower row articulates with metacarpals of the palm.

Metacarpals: These consist of five bones making up the palm of the hand. The lower ends of the metacarpal bones form the knuckles which articulate with the phalanges or digits.

Humerus
Pelvic Girdle

The pelvic girdle is the complete ring of the bone formed by the lower part of the back bone and the two hip bones.
(a) The hip bones are made up of three units the ileum, ischium and pubis which form coxa.
(b) The two halves of the pelvic girdle are joined at the pubic symphysis. A cavity called acetabulum is also present.

Pelvic Girdle
Hindlimb

It consists of the following bones:
Femur:            Thigh bone.
Patella:           Knee cap
Tibia:              Skin bone.
Fibula:            Outer bone
Tarsal:            Seven bones forming the ankle, heel and instep.
Metatarsal:     Five bones making up the sole of the foot.
Phalanges:     Three bones forming the small toes. Two bones form the big toe.
Femur:         It is a long bone with head, which fits into the acetabulum. Greater and lesser trochanters are for muscle attachment.
Patella:           It is embedded in a long tendon which runs over the knee joint. Its function is to ease                       movement of this tendon across the knee joint.
Tibia:        It is the large bone in the leg. Its upper end articulates with the condyles of the femur, and the lower end articulates with the bones of the ankles.
Fibula:     It is a thinner bone running down at the outer side of the leg. It joins the tibia just below the knee joint and just above the ankle.


Hindlimb
  
Tarsal: It is made of seven bones which are tightly attached to form the ankle joint heel and instep.
Metatarsal: It consists of five bones which articulate with the tarsal and phalanges to form the sole of the foot.
Phalanges: These are the small bones which make up the toes on the same pentadactyl plan as the hand.

Bone

Human skeleton 
Bone is a hard, tough connective tissue. A bone consists of an outer shell of compact bone, with spongy bone in the interior. Compact bones are highly organized and composed of tubular units called osteones. Osteon also called the Haversian system. In the cross section of an osteon, bones cells, called osteocytes, lie in lacunae, which are tiny chambers arranged in concentric circles around a central canal. The matrix of an osteon is laid down in concentric rings called lamella. It is hard and contains calcium phosphate and collagen fibers. Tiny canals called canaliculi run through the matrix, connecting the lacunae with each other and with the central canal. Spongy bones contain numerous minute spikes of bones called trabeculae which give a lattice like appearance. Spongy bone is lighter than the compact bone and is highly vascular. The spaces of spongy bone are often filled with red bone marrow. The osteocytes of spongy bone are irregularly placed within the trabeculae. Several different types of cells are involved in bone growth and repair. Osteoprogenitor cells (stem cells) are present in the inner portion and central canal of the compact bone.


Osteoblasts are bone forming cells. These cells secrete matrix of the bone. Osteocytes are mature bone cells derived from osteoblasts. Once the osteoblasts are surrounded by the matrix they become osteocytes. Osteoclasts thought to be derived from monocytes, perform bone resorption i.e., they break down bone and deposit calcium and phosphate in the blood and are important for growth and repair of bone.

Cartilage


It is not strong as bone. It is present at the end of joints, and supports the flexible portion of nose and external ears. It is more flexible than the bone because the matrix is gel like and contains many collagenous and elastic fibers. The living cells of cartilage are called chondrocytes, which secrete matrix. Cartilage has no blood vessels, and therefore, injured cartilage is slow to heal. There are three types of cartilage, differ according to the type and arrangement of fibers in the matrix. 

Hyaline cartilage

(a) Hyaline Cartilage: It is firm and somewhat flexible. The matrix has abundance of collagenous fibers. It is found at the ends of long bones and in the nose, at larynx and trachea.

(b) Fibro Cartilage: The matrix contains wide rows of thick collagenous fibers. It is found where support is of prime importance such as in the disks located between the vertebrae, cartilage of the knee.

(c) Elastic cartilage: It is more flexible than hyaline cartilage because the matrix contains mostly elastic fibers. It is found in the ear flaps and epiglottis.

Skeletal Tissues

The vertebrate skeleton is composed of either bone or cartilage. Only cartilaginous fish such as dogfish, sharks possess a completely cartilaginous endoskeleton. All other vertebrates have a bony skeleton in their adult form, with cartilage also present in certain region such as joints, or between the vertebrae etc.

Support and Movement in Animals

Animals are active. They are able to obtain food and escape predators. These activities require movement of both individual body parts and the whole organisms. The animals may be sessile e.g. sponges, coelenterates or motile. Mobility by contractile fibers is an animal’s characteristic. In order to create movement, muscle contraction must be directed against some sort of medium. This medium can be internal body fluids (hydrostatic skeleton), a rigid exoskeleton or a rigid endoskeleton.

Hydrostatic skeleton
In animals that lack a hard skeleton, a fluid filled gastrovascular cavity or coelom can act as hydrostatic skeleton. A hydrostatic (GK. Hydrias, water, and stasis a standing) skeleton also offers support and resistance to the contraction of muscles so that the mobility results. It is found in Cnidaria such as Hydra, and flatworms such as Planaria, nematodes and annelids, and other soft bodied invertebrates.

Sea anemone: It has a central digestive cavity filled with seawater and surrounded by the body wall. When the longitudinal muscles contract, the animal becomes shorter and wide and when the circumferential muscles contract, the animals become longer and narrower.

Nematodes: The roundworms have a fluid filled pseudocoelom and move in a whip like manner when their longitudinal muscles contract.

Earthworm: Earthworms are segmented and each segment has its own set of longitudinal and circular muscles. The hydrostatic skeleton consists of this fluid filled segments separated by septa. When the circular muscles contract, the segments become thinner and elongate. When longitudinal muscles contract the segments become thicker and shorten. By alternating circular muscle contraction and longitudinal muscle contraction, the animal moves forward. Movement is aided by paired setae in each segment.

Exoskeleton
An exoskeleton is an exterior skeleton, to which internal muscles can be attached. The exoskeleton is inert and nonliving. Calcium carbonate forms the exoskeleton of some animals such as corals and of some mollusks e.g. marine bivalvia, snail.

Structure of exoskeleton: It is secreted by the ectoderm in animal cells. It is composed of two layers. The epicuticle is the outermost layer, which is made up of waxy lipoprotein. It is impermeable to water and serves as a barrier to microorganisms and insects. The bulk of exoskeleton below the epicuticle is called procuticle. The procuticle is composed of chitin, tough leathery polysaccharide and several kinds of proteins. It is further hardened by deposition of CaCO3.

Mollusks: The shell of mollusks consists of just one or two pieces. In some mollusks shell is composed of crystals of CaCO3 e.g. marine bivalvia, snail. The shell of land snail generally lacks the hard minerals and is lighter. In mollusks such as calms and snails the exoskeleton grows as the animal grows and the growth rings are apparent on the shell. The soft parts of the molluscan body have a hydrostatic skeleton as well.

Arthropods: They have exoskeleton but they have made adaptations:

Chitin: Exoskeleton is made of chitin, which is very light, strong, flexible nitrogenous polysaccharide. It protects the animal from enemies and from drying out.

Segmented exoskeleton: It is divided into several distinct segments. The exoskeletons are movable and attached to each other by joints. The segments are soft and flexible at joints so the animals can move easily.

Gaseous exchange: At joint the exoskeleton is thin, through which gaseous exchange takes place.

Muscles: Exoskeleton has invaginations which form firm ridges and bars for muscle attachment. There is a set of extensor and flexor muscle to facilitate movements.

Moulting: There is a number of moulting or ecdysis to solve the problem of exoskeleton for the growing insects. Moulting occurs in only larval stages. No moulting occurs in adults. During moulting the arthropods are vulnerable to predators. Moulting is controlled by nervous system and hormone ecdysone.

Endoskeleton
The skeleton found inside the body of vertebrates is called endoskeleton. The only other animals with an internal skeleton are certain molluscs (mollusks) such as cuttle fish. In vertebrates the skeleton is made of either cartilage or bone.

Evolution of Endoskeleton

The history of the development of the endoskeleton in various groups of vertebrate is known from the fossil record. The earliest vertebrates were fish. These fish had various adaptations for swimming. The backbone offish is flexible and generally vertebrae are not interlocked for undulating movement.

Functions of Skeletal System

Definition: The rigid body support to which muscles attach and apply force is called a skeleton in both vertebrate and invertebrates.

Functions: Some major functions of the skeletal system are:

a. Support: Bones support soft tissue and serves as attachment site for most muscles.
b. Shapes: It gives the body shape to the animals.
c. Protection: The skeleton protects the internal organs such as brain, spinal cord, heart, lungs, and other viscera.
d. Movement: Skeletal muscles, which are attached to bone help in the movement of the body.
e. Homeostasis: Through negative feedback mechanisms, bones release or take up minerals to maintain homeostasis.
f. Blood Cells: RBCs and WBCs are produced in bone marrow.