The skeletal system is composed of a network of hard and soft tissues including bone, cartilage, ligaments, and tendons. Together, these tissues form an overall framework, thus giving the body its characteristic shape and providing structural support. Additionally, the skeletal system protects internal organs and facilitates movement by providing attachment sites for muscles. Other functions of the skeletal system include a storage facility for minerals such as calcium, formation of blood cells, and an energy reserve of lipids.
The bones of the skeletal system can be classified according to their shape and location. The types of bones categorized by shape--long, short, flat, and irregular--also provide evidence of their function. Long bones consist of an elongated shaft called the diaphysis. Each end of the diaphysis is an expanded portion of the shaft and is called an epiphysis. Examples of long bones include the femur in the thigh and the humerus in the arm. These bones function as levers when muscles contract, thus providing support to enable movement. Short bones often have equal dimensions, like those of a cube. Compared to long bones, short bones have a limited range of motion but are able to withstand force. Examples of short bones include the carpals in the wrist and the tarsals in the ankle. Flat bones are thin, flat bones that protect internal organs and provide sites for muscle attachment. The ribs, cranial bones, and scapula are all examples of flat bones. Irregular bones are not shaped like any of the three aforementioned bones and therefore form their own category. The vertebrae and facial bones are categorized as irregular bones.
Location rather than shape classifies other types of bones, such as sesamoid and sutural bones. Sesamoid bones bear pressure as the result of being buried in tendons. The kneecap, or patella, is the best-known example of a sesamoid bone. Sutural bones are tiny bones located between the joints, or sutures, of the cranial bones.
The adult skeleton consists of 206 bones. A baby is born with 270 bones, many of which fuse together during adolescence and adulthood. The bones of males and females differ in that male bones tend to be larger and heavier than female bones.
The skeletal system can be divided into the axial skeleton and the appendicular skeleton. The axial skeleton is composed of the bones that surround the midline or axis of the body, forming the head and trunk. These bones include the skull bones, auditory ossicles, hyoid bone, vertebral column, sternum, and ribs.
The skull can be subdivided into eight cranial bones and 14 facial bones. The cranial bones include the frontal bone, two parietal bones, two temporal bones, occipital bone, sphenoid bone, and ethmoid bone. The facial bones include two lacrimal bones, two nasal bones, two inferior nasal conchae, vomer, two zygomatic bones, two maxillae, two palatine bones, and mandible. Within the middle ear are three auditory ossicles: the maleus, incus, and stapes. These tiny bones transmit vibrations from the eardrum to the inner ear. The hyoid bone is located in the superior part of the neck and attaches the muscles of the tongue.
The vertebral column typically consists of 26 vertebrae that protect the spinal cord and provide attachment sites for ribs and back muscles. The seven most superior vertebrae are the cervical vertebrae. The first vertebra is called the atlas and enables the head to move forward and backward. The second vertebra, the axis, is unique in that it is the only vertebra that has a process called the dens or odontoid process. The axis enables the head to rotate from side to side. The vertebrae immediately inferior to the cervical vertebrae are the 12 thoracic vertebrae. These vertebrae are larger than the cervical vertebrae and, except for the eleventh and twelfth thoracic vertebrae, have facets that articulate with the ribs. The point where two bones meet forms a joint and the bones are said to articulate with one another. Just below the thoracic vertebrae are the five lumbar vertebrae. The lumbar vertebrae are the largest of the vertebrae because they support a tremendous amount of the body's weight. The five sacral vertebrae are actually fused together in adults to form the sacrum. Inferior to the lumbar vertebrae, the sacrum articulates with the pelvic girdle to form the pelvis. The four remaining bones of the vertebral column constitute the coccyx. These individual bones also become fused together in adults.
The sternum, also known as the breastbone, consists of three parts. The manubrium and the body are the superior and middle parts of the sternum that articulate with the ribs. Additionally, the manubrium articulates with the clavicles. The xiphoid process is the inferior part of the sternum that provides attachment for abdominal muscles.
There are 12 pairs of ribs that make up the rib cage. The first seven pairs are true ribs because they are attached directly to the sternum by cartilage. The next three pairs of ribs are false ribs because they are indirectly attached to the sternum by the cartilage of the seventh pair. The two remaining ribs are known as floating ribs because they do not connect to the sternum at all.
The appendicular skeleton is comprised of two pectoral girdles, two pelvic girdles, and the bones of the upper and lower extremities. Each pectoral girdle, or shoulder girdle, includes the clavicle and scapula responsible for attaching the upper extremities to the axial skeleton. The clavicle, or collarbone, is the anterior component of the shoulder that articulates with the scapula and manubrium of the sternum. The scapula, or shoulder blade, is positioned posterior to the clavicle and articulates with the humerus. The humerus constitutes the upper arm and articulates with the two bones of the forearm. The radius is the lateral bone and the ulna is the medial bone of the forearm. The distal end of the radius articulates with the carpals, the first row of bones in the hand. The proximal row of carpals located from lateral to medial includes the scaphoid, lunate, triquetrum, and pisiform. The distal row of carpals that articulates with the metacarpals are the trapezium, trapezoid, capitate, and hamate. The metacarpals are numbered one through five beginning on the lateral palm of the hand extending medially. The 14 bones of the fingers, named phalanges, articulate with the metacarpals. Each finger has a proximal, middle, and distal phalanx except for the thumb, which only has two phalanges.
Each pelvic girdle, or hipbone, in an adult is made of three fused bones. Also known as the coxal bones, the hipbones consist of the ilium, ischium, and pubis. The ilium articulates posteriorly with the sacrum. The ischium connects the ilium and pubis. The two pubis bones meet anteriorly to form the pubis symphysis. Together, the hipbones, sacrum, and coccyx constitute the pelvis. One major difference between the male and female skeleton is the bones of the pelvis. In the female, the pelvic bones form a wide, round opening called the pelvic inlet to accommodate for childbirth. The pelvic inlet of males is heart shaped and much narrower than in women. Additionally, the sacrum is wider and shorter in women than in men.
The bones of the lower extremities include the femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges. The femur is the leg bone that articulates with the pelvic girdle. The distal end of the femur articulates with the foreleg to form the knee. Anterior to the knee lies the patella, or kneecap. Each foreleg consists of two bones: tibia and fibula. The tibia is the larger of the two bones and forms the shin. The fibula is the lateral bone in the foreleg. At the distal end of the forelegs are the proximal bones of the foot called the tarsals. The tarsals include the calcaneus, talus, navicular, cuboid, and three cuneiforms. The metatarsals form the sole of the foot and are labeled one through five beginning on the medial side of the foot. Each toe consists of three phalanges, the proximal, middle, and distal phalanx. The exception is the big toe that contains only two phalanges.
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