|In biology the skeleton is a biological system, providing support in live organisms. In a wider sense, non-biological structures, such as bridges or buildings can have skeletons too.
Usually skeletons are divided into three types: external (exoskeleton), internal (endoskeleton) and liquid (hydrostatic skeleton) though the last type does not always belong to skeletons due to the absence of hard support structures.
The exoskeleton is a characteristic for the majority of invertebrates in which it is present in the form of a shell (such as mollusks) or cuticles (the chitinous shell of arthropods). The prominent feature of these formations is that they do not contain cellular elements.
The endoskeleton of vertebrates is formed by cartilage and bone tissues. It is accepted to divide it into axial (vertebral column, skull, thorax) and additional (bones of extremities and girdles).
An example of a hydro skeleton is the formation of mesenteron, filled with liquid in octactinal nematophores which supports the polyp form.
The human skeleton is a set of bones, the passive part of the musculo-skeletal system. The human skeleton serves as support to soft tissues, the point of attachment to muscles (lever system) and the receptacle and protection of internal organs.
The human skeleton consists of two hundred odd individual bones and almost all of them are connected into a single piece by means of joints, ligaments and other joints.
During life the skeleton constantly undergoes changes. During antenatal growth, the cartilaginous skeleton of the fetus is gradually replaced with bone. This process continues as well for several years even after the birth. The newborn baby’s skeleton is made up of almost 270 bones, which are much more than in an adult. Such differences occur since the children's skeleton consists of a considerable quantity of small bones, which grow together into large bones only at a certain age. For example, the bones of skull, pelvis and backbone. The sacral vertebra, for example, grows together into a single bone (the sacral bone) only by the age of 18 to 25 years.
Six special bones ( three on each side), located in the middle ear do not directly belong to skeleton; the auditory ossicle (ear bones) are connected only with each other and assist in the functioning of the hearing organ by transferring the vibrations from the tympanic membrane (eardrum) onto the internal ear.
The hyoid bone is a unique bone which is not directly connected to other bones, topographically it is located on the neck but traditionally, it belongs to the front section of bones of the skull. It is suspended by muscles to bones of the skull and connected with the throat.
The longest bone of the skeleton is the femoral bone (thigh) and the smallest is the stirrup in the middle ear.
Besides mechanical functions for supporting the body frame, ensuring movement and protection of the internal organs, the skeleton is also a blood formation place: the formation of new blood cells takes place in the bone marrow. (One of the most widespread diseases, which affect the bone marrow - leukemia, often leads to death in spite of treatment). Apart from this, the skeleton, being a storehouse of the major portion of the organism’s calcium and phosphorus, plays an important role in the exchange of mineral substances.
The human skeleton is made according to a general principle for all the vertebrates. The skeleton bones are subdivided into two groups: axial skeleton and additional skeleton. The bones lying in the middle and forming the framework of the body belong to the axial skeleton; the bones of the head and neck, backbone, ribs and breast are bones of the axial skeleton. The additional skeleton is made up of collar bones, scapula (blade bones), the bones of the thoracic limbs (upper extremity), and bones of the pelvis and inferior limbs (lower extremity).
All the bones of the skeleton are divided into subgroups:
* Skull — the bony framework of the head, it's a receptacle of the brain and also organs for vision, hearing and the olfactory organs. The skull has two sections: cerebral and facial;
* Thorax — has the form of a truncated compressed cone, is the bone framework of the breast and receptacle for the internal organs. It is made up of 12 dorsal vertebrae, 12 ribs and a breastbone;
* Vertebra or spinal column — is the main axis of the body, supports all the skeleton; the spinal cord passes through inside the vertebral channel.
* Thoracic girdle - provides joining of the upper limbs to the axial skeleton. It is made up of a pair of scapula and collar bones;
* Upper limbs - are as much as possible adapted for executing labor activity. The limb is made up of three sections: shoulder, forearm and the hand;
* Pelvic girdle - ensures the joining of the lower limbs to the axial skeleton and also is a receptacle and support for the digestive organ, urinary and sexual systems;
* Lower limbs — are adapted for moving the body in space.
Male and female skeletons, as a whole, are organized as one type without cardinal differences. The difference is only in minor changes in the form or sizes of the individual bones and accordingly, structures consisting of them. Here are some of the most obvious distinctions. Bones of the limbs and fingers in males, on an average, are longer and thicker. In females, a wider pelvis and also a narrow thorax, less angular jaws and poorly apparent superciliary arches and occipital condyles are found. Still many tiny differences exist between male and female skeletons.
By mistake, popular belief existed in the olden days that the male skeleton has one rib less than the female skeleton. The Bible legend about Eve's creation from Adam's rib has no reflection actually and occurred due to a mistake in the translation of the Hebrew word “Aim” (Hebrew: צלע), meaning both “rib” and “side”. Skeletons of males and females have 24 ribs or 12 pairs.
Many diseases of the bone system are known. Many of them are accompanied by mobility restriction and some can lead to the complete immobilization of a person. Malignant and benign tumors of bones, requiring frequent radical surgical treatment pose serious threat to life and health; usually the infected limb is amputated. Besides bones, even the joints are quite often infected. Joint diseases are often accompanied with a considerable infringement of mobility and severe pain. In the case of osteoporosis, the fragility of bones increases and bones become breakable; this disease of the skeleton appears more often in older persons and in women after menopause. |