The human skeleton consists of both fused and individual bones supported and supplemented by ligaments, tendons, muscles and cartilage. It serves as a scaffold which supports organs, anchors muscles, and protects organs such as the brain, lungs and heart.
The longest and heaviest bone in the body is the femur, and the smallest is the stapes bone in the middle ear. In an adult, the skeleton comprises around 20% of the total body weight.
Fused bones include those of the pelvis and the cranium. Not all bones are interconnected directly: There are 6 bones in the middle ear called the ossicles (three on each side) that articulate only with each other. The hyoid bone, which is located in the neck and serves as the point of attachment for the tongue, does not articulate with any other bones in the body, being supported by muscles and ligaments.
Early in gestation, a foetus has a cartilaginous skeleton from which the long bones and most other bones gradually form throughout the remaining gestation period and for years after birth in a process called endochondral ossification. The flat bones of the skull and the clavicles are formed from connective tissue in a process known as intramembranous ossification, and ossification of the mandible occurs in the fibrous membrane covering the outer surfaces of Meckel's cartilages. At birth a newborn baby has approximately 300 bones, whereas on average an adult human has 206 bones (these numbers can vary slightly from individual to individual). The difference comes from a number of small bones that fuse together during growth, such as the sacrum and coccyx of the vertebral column. The sacrum (the bone at the base of the spine) consists of five bones which are separate at birth but fuse together into a solid structure in later years. An infant is born with zones of cartilage, called epiphyseal plates, between segments of bone to allow further growth. Growing is usually completed between ages 13 and 18, at which time the epiphyseal plates of long bones close allowing no further growth.
Much of the human skeleton maintains the ancient segmental pattern present in all vertebrates (mammals, birds, fishes, reptiles and amphibians) with basic units being repeated. This segmental pattern is particularly evident in the vertebral column and in the ribcage.
The skeleton has five main functions:
Provide shape and support
The skeleton provides the framework which supports the body, and maintains its shape. The joints between bones permit movement, some allowing a wider range of movement than others,e.g. the ball and socket joint allows a greater range of movemet than the pivot joint at the neck.
Movement in vertebrates is powered by skeletal muscles, which are attached to the skeleton by tendons. Without the skeleton to give leverage, movement would be greatly restricted. However, biologically speaking, the skeleton does not enable movement.
The skeleton protects many vital organs:
Blood cell production
The skeleton is the site of haematopoiesis – the generation of blood cells, which takes place in red bone marrow.
Bone matrix can store calcium and is involved in calcium metabolism, and bone marrow can store iron in ferritin and is involved in iron metabolism.
The human skeleton can be divided into the axial skeleton and the appendicular skeleton.
- See also: List of bones of the human skeleton
There are more than 200 bones in the human body.
|Bones of head and neck: the face|
|maxilla||Body of maxilla - Maxillary sinus - surfaces of body Anterior (Incisive fossa, Canine fossa, Infraorbital foramen, Anterior nasal spine) - Infratemporal (Alveolar canals, Maxillary tuberosity) - Orbital (Infraorbital groove, Infraorbital canal) - Nasal (Pterygopalatine canal)|
processes Zygomatic process - Frontal process (Agger nasi, Anterior lacrimal crest) - Alveolar process - Palatine process (Incisive foramen, Incisive canals, Foramina of Scarpa, Premaxilla, Anterior nasal spine)
|lacrimal||Posterior lacrimal crest - Lacrimal groove|
|zygomatic||Orbital process - foramina (Zygomaticofacial, Zygomaticotemporal, Zygomaticoörbital)|
|palatine||Pterygopalatine fossa - Pterygoid fossa|
Horizontal plate (Posterior nasal spine) - Perpendicular plate (Pterygopalatine canal, Sphenopalatine foramen, Pyramidal process)
processes (Orbital - Sphenoidal)
|mandible||body (Symphysis menti, Mental protuberance, Mental foramen, Mental spine, Mylohyoid line)|
ramus (Mandibular foramen, Mylohyoid groove, Mandibular canal, Angle, Coronoid process, Condyloid process, Mandibular notch)
|others||nasal bone - inferior nasal conchae (ethmoidal process, maxillary process) - vomer|
|Bones of head and neck: the cranium|
|Occipital||Foramen magnum - Squama occipitalis (Inion - Nuchal lines - Planum occipitale - Planum nuchale - Cruciform eminence - Internal occipital protuberance - Sagittal sulcus - Internal occipital crest)|
Lateral parts (Occipital condyle - Hypoglossal canal - Condyloid fossa - Condylar canal - Jugular process - Jugular tubercle) - Basilar part (Pharyngeal tubercle)
|Parietal||Parietal eminence - Temporal line - Parietal foramen|
|Frontal||Squama frontalis (Frontal suture - Frontal eminence - Superciliary arches - Glabella - Supraorbital foramen - Zygomatic process - Sagittal sulcus - Frontal crest - Foramen cecum) - Pars orbitalis (Ethmoidal notch - Lacrimal fossa - Trochlear fovea - Posterior ethmoidal foramen - Anterior ethmoidal foramen - Frontal sinus - Frontonasal duct)|
|Temporal||Squama temporalis (Articular tubercle - Suprameatal triangle - Mandibular fossa - Petrotympanic fissure) - Mastoid portion (Mastoid foramen - Mastoid process - Mastoid notch - Occipital groove - Sigmoid sulcus - Mastoid antrum) - Petrous portion (Facial canal - Hiatus of the facial canal - Internal auditory meatus - Subarcuate fossa - Carotid canal - Aqueduct of cochlea - Jugular fossa - Inferior tympanic canaliculus - Mastoid canaliculus - Styloid process - Stylomastoid foramen - Petrosquamous suture) - Tympanic part (Suprameatal spine) - Zygomatic process|
|Sphenoid||Body Superior surface (Ethmoidal spine, Chiasmatic groove, Optic foramen, Sella turcica, Fossa hypophyseos, Middle clinoid process, Dorsum sellae, Posterior clinoid processes, Petrosal process, Clivus) - Lateral surface (Carotid groove - Sphenoidal lingula) - Anterior surface (Sphenoidal sinuses) - Great wings (Spine, Foramen rotundum, Foramen ovale, Foramen Vesalii, Foramen spinosum, Infratemporal crest, Sulcus tubae auditivae) - Small wings (Superior orbital fissure, Anterior clinoid process, Optic foramen) - Pterygoid processes (Pterygoid fossa, Scaphoid fossa, Lateral pterygoid plate, Medial pterygoid plate, Pterygoid canal, Pterygoid hamulus) - Sphenoidal conchae|
|Ethmoid||Cribriform plate (Crista galli) - Perpendicular plate - Labyrinth (Ethmoid sinus) - Lateral surface Lamina papyracea - Uncinate process - Medial surface Middle nasal concha - Superior meatus - Superior nasal concha - Middle meatus|
|Bones of upper limbs|
|Pectoral girdle, clavicle||conoid tubercle - trapezoid line - costal tuberosity - subclavian groove|
|Scapula||spine of scapula - suprascapular notch - acromion - glenoid cavity - glenoidal labrum - coracoid process - fossae (subscapular, supraspinatous, infraspinatous) - borders (superior, lateral/axillary, medial/vertebral) - angles (superior, inferior, lateral) - tubercles (infraglenoid, supraglenoid)|
|Humerus||upper extremity: necks (anatomical, surgical) - tubercles (greater, lesser) - intertubercular sulcus
body: radial sulcus - deltoid tuberosity
lower extremity: capitulum - trochlea - epicondyles (lateral, medial) - supracondylar ridges (lateral, medial) - fossae (radial, coronoid, olecranon)
|Forearm||ulna: upper extremity (olecranon, coronoid process, tuberosity, radial notch, semilunar notch) - body of ulna - lower extremity (styloid process)|
radius: upper extremity (tuberosity) - body - lower extremity (ulnar notch, styloid process)
|Hand||carpus: scaphoid - lunate - triquetral - pisiform - trapezium - trapezoid - capitate - hamate (hamulus) - metacarpus: 1st metacarpal - 2nd - 3rd - 4th - 5th - phalanges of the hand|
|Bones of torso|
|Sternum||Suprasternal notch, Manubrium, Sternal angle, Body of sternum, Xiphisternal joint, Xiphoid process|
|Rib||specific ribs (1, 2, 10, 11, 12, false - 8-12, floating - 11-12) - parts (Angle, Tubercle, Costal groove, Neck, Head)|
|General vertebral structures||body of vertebra, vertebral arch (pedicle, lamina, vertebral notch), foramina (vertebral, intervertebral), processes (transverse, articular / zygapophysis, spinous)|
|Cervical vertebrae||C1 (anterior arch, posterior arch, lateral mass), C2 (dens), C7, anterior tubercle, posterior tubercle, foramen transversarium|
|Thoracic vertebrae||costal facets (superior, inferior, transverse)|
|Lumbar vertebrae||accessory process, mammillary process|
|Sacrum/coccyx||pelvic surface (anterior sacral foramina), dorsal surface (posterior sacral foramina, median sacral crest, medial sacral crest, lateral sacral crest), lateral surface (sacral tuberosity), base, sacral hiatus - presacral space - sacral promontory - sacral canal - ala of sacrum - sacrovertebral angle|
|Bones of lower limbs|
|Femur||head of femur (fovea capitis femoris) · neck of femur · greater trochanter · trochanteric fossa · lesser trochanter · tubercle of the femur · intertrochanteric line · intertrochanteric crest · linea quadrata
body (linea aspera, third trochanter, pectineal line, adductor tubercle)
lower extremity (lateral condyle, medial condyle, lateral epicondyle, medial epicondyle, patellar surface)
||medial condyle · lateral condyle · intercondyloid eminence · tuberosity of the tibia · posterior intercondyloid fossa · anterior intercondyloid fossa
|Fibula||head of fibula · body of fibula · lateral malleolus|
|Tarsus||calcaneus (sustentaculum tali, trochlear process) · talus · navicular · cuboid · cuneiform (medial, intermediate, lateral)|
|Metatarsus||1st metatarsal · 2nd · 3rd · 4th · 5th|
|Other||patella · phalanges of the foot|
|Bones of pelvis/pelvic cavity|
|General||sacrum, coccyx, hip bone|
|Ilium||Body (Arcuate line)|
Wing: gluteal lines (Posterior, Anterior, Inferior) - Fossa - iliac spines (Anterior superior, Anterior inferior, Posterior superior, Posterior inferior) - Crest - Tuberosity
|Ischium||Body (Ischial spine, Lesser sciatic notch) - Superior ramus (Tuberosity of the ischium) - Inferior ramus|
|Pubis||Body - Superior ramus (Pubic tubercle, Pubic crest, Obturator crest) - Inferior ramus (Pectineal line)|
|Compound||Acetabulum (Acetabular notch) - Iliopubic eminence/Iliopectineal line - Linea terminalis - Ischiopubic ramus/Pubic arch
Obturator foramen - Greater sciatic foramen/Greater sciatic notch - Lesser sciatic foramen
Lesser pelvis (Pelvic inlet, Pelvic brim, Pelvic outlet) - Greater pelvis
There are many differences between the male and female human skeletons. Most prominent is the difference in the pelvis, owing to characteristics required for the processes of parturition (childbirth). The shape of a female pelvis is flatter, more rounded and proportionally larger to allow the head of the fetus to pass. Men tend to have slightly thicker and longer limbs and digit bones (phalanges), while women tend to have narrower rib cages, smaller teeth, less angular mandibles, less pronounced cranial features such as the brow ridges and external occipital protuberance (the small bump at the back of the skull), and the carrying angle of the forearm is more pronounced in females.
- See also: List of skeletal disorders
There are many disordes of the skeleton. One of the more prominent is osteoporosis.
Osteoporosis is a disease of bone - leading to an increased risk of fracture. In osteoporosis, the bone mineral density (BMD) is reduced, bone microarchitecture is disrupted, and the amount and variety of non-collagenous proteins in bone is altered. Osteoporosis is defined by the World Health Organization (WHO) in women as a bone mineral density 2.5 standard deviations below peak bone mass (20-year-old sex-matched healthy person average) as measured by DXA; the term "established osteoporosis" includes the presence of a fragility fracture. Osteoporosis is most common in women after the menopause, when it is called postmenopausal osteoporosis, but may develop in men and premenopausal women in the presence of particular hormonal disorders and other chronic diseases or as a result of smoking and medications, specifically glucocorticoids, when the disease is called steroid- or glucocorticoid-induced osteoporosis (SIOP or GIOP).
Osteoporosis can be prevented with lifestyle advice and medication, and preventing falls in people with known or suspected osteoporosis is an established way to prevent fractures. Osteoporosis can be treated with bisphosphonates and various other medical treatments.
|Endocrine system: hormones/endocrine glands (Peptide hormones, Steroid hormones)|
|Hypothalamic-pituitary|| Hypothalamus: TRH, CRH , GnRH, GHRH, somatostatin, dopamine - Posterior pituitary: vasopressin, oxytocin - Anterior pituitary: α (FSH, LH, TSH), GH, prolactin, POMC (ACTH, MSH, endorphins, lipotropin)|
|Adrenal axis|| Adrenal medulla: epinephrine, norepinephrine - Adrenal cortex: aldosterone, cortisol, DHEA|
|Thyroid axis|| Thyroid: thyroid hormone (T3 and T4) - calcitonin - Parathyroid: PTH|
|Gonadal axis||Testis: testosterone, AMH, inhibin - Ovary: estradiol, progesterone, inhibin/activin, relaxin (pregnancy)|
|Other end. glands|| Pancreas: glucagon, insulin, somatostatin - Pineal gland: melatonin|
|Non-end. glands|| Placenta: hCG, HPL, estrogen, progesterone - Kidney: renin, EPO, calcitriol, prostaglandin - Heart atrium: ANP - Stomach: gastrin, ghrelin - Duodenum: CCK, GIP, secretin, motilin, VIP - Ileum: enteroglucagon - Adipose tissue: leptin, adiponectin, resistin - Thymus: Thymosin - Thymopoietin - Skeleton: Osteocalcin - Liver/other: Insulin-like growth factor (IGF-1, IGF-2)|
|Target-derived||NGF, BDNF, NT-3|