's 1863 ''
Evidence as to Man's Place in Nature'', the compared skeletons of apes to humans. Vertebrate skeletons are endoskeletons, and the main skeletal component is bone. Bones are rigid
organs providing structural support for the body, assistance in movement by opposing
muscle contraction, and the forming of a protective wall around internal organs. Bones are primarily made of inorganic minerals, such as
hydroxyapatite, while the remainder is made of an organic matrix and water. The hollow tubular structure of bones provide considerable resistance against compression while staying lightweight. Most cells in bones are
osteoblasts,
osteoclasts, or
osteocytes. Bone tissue is a type of dense
connective tissue, a type of
mineralized tissue that gives rigidity and a
honeycomb-like three-dimensional internal structure. Bones also produce
red and
white blood cells and serve as calcium and phosphate storage at the cellular level. Other types of tissue found in bones include
bone marrow,
endosteum and
periosteum,
nerves,
blood vessels and cartilage. During
embryonic development, bones are developed individually from skeletogenic cells in the ectoderm and mesoderm. Most of these cells develop into separate bone, cartilage, and joint cells, and they are then articulated with one another. Specialized skeletal tissues are unique to vertebrates. Cartilage grows more quickly than bone, causing it to be more prominent earlier in an animal's life before it is overtaken by bone. Cartilage is also used in vertebrates to resist stress at points of articulation in the skeleton. Cartilage in vertebrates is usually encased in
perichondrium tissue.
Ligaments are elastic tissues that connect bones to other bones, and
tendons are elastic tissues that connect muscles to bones.
Amphibians and reptiles The skeletons of turtles have evolved to develop a
shell from the ribcage, forming an exoskeleton. The skeletons of
snakes and
caecilians have significantly more vertebrae than other animals. Snakes often have over 300, compared to the 65 that is typical in lizards.
Birds The skeletons of birds are adapted for
flight. The bones in bird skeletons are hollow and lightweight to reduce the metabolic cost of flight. Several attributes of the shape and structure of the bones are optimized to endure the physical stress associated with flight, including a round and thin
humeral shaft and the fusion of skeletal elements into single
ossifications. Because of this, birds usually have a smaller number of bones than other terrestrial vertebrates. Birds also lack teeth or even a true
jaw, instead having evolved a
beak, which is far more lightweight. The beaks of many baby birds have a projection called an
egg tooth, which facilitates their exit from the amniotic egg.
Fish The skeleton, which forms the support structure inside the fish is either made of cartilage as in the
Chondrichthyes, or bones as in the
Osteichthyes. The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong. The ribs attach to the spine and there are no limbs or limb girdles. They are supported only by the muscles. The main external features of the fish, the
fins, are composed of either bony or soft spines called rays which, with the exception of the caudal fin (tail fin), have no direct connection with the spine. They are supported by the muscles which compose the main part of the trunk. Cartilaginous fish, such as sharks, rays, skates, and chimeras, have skeletons made entirely of cartilage. The lighter weight of cartilage allows these fish to expend less energy when swimming.
Humans The 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,
heart and
spinal cord. The biggest bone in the body is the
femur in the upper leg, and the smallest is the
stapes bone in the
middle ear. In an adult, the skeleton comprises around 13.1% of the total body weight, and half of this weight is water. Fused bones include those of the
pelvis and the
cranium. Not all bones are interconnected directly: There are three bones in each
middle ear called the
ossicles 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. There are 206 bones in the adult human skeleton, although this number depends on whether the pelvic bones (the
hip bones on each side) are counted as one or three bones on each side (ilium, ischium, and pubis), whether the coccyx or tail bone is counted as one or four separate bones, and does not count the variable
wormian bones between skull sutures. Similarly, the sacrum is usually counted as a single bone, rather than five fused vertebrae. There is also a variable number of small sesamoid bones, commonly found in tendons. The patella or kneecap on each side is an example of a larger sesamoid bone. The patellae are counted in the total, as they are constant. The number of bones varies between individuals and with age – newborn babies have over 270 bones some of which fuse together. These bones are organized into a longitudinal axis, the
axial skeleton, to which the
appendicular skeleton is attached. The human skeleton takes 20 years before it is fully developed, and the bones contain
marrow, which produces blood cells. There exist several general differences between the male and female skeletons. The male skeleton, for example, is generally larger and heavier than the female skeleton. In the female skeleton, the bones of the skull are generally less angular. The female skeleton also has wider and shorter breastbone and slimmer wrists. There exist significant differences between the male and female pelvis which are related to the female's pregnancy and childbirth capabilities. The female pelvis is wider and shallower than the male pelvis. Female pelvises also have an enlarged pelvic outlet and a wider and more circular pelvic inlet. The angle between the pubic bones is known to be sharper in males, which results in a more circular, narrower, and near heart-shaped pelvis. ==Invertebrate skeletons==