Teeth vary in size and shape according to how to the animal is using them. Teeth perform different functions such as biting, slicing, chewing, crushing, cracking, gnawing, grooming, digging, defending, and communicating. Teeth that are long with broad, flat surfaces are useful for grinding plant food. Teeth with sharp, pointed cusps can tear apart leaves and insects. The narrow, sharp molars and carnassials of canines are used for cutting meat and crushing bone. Large canines, like those in baboons, can be shown to competitors to display dominance and aggression.
There are three types of bone cells: osteoblasts, osteocytes, and osteoclasts. Osteoblasts form new bone by hardening the protein collagen with minerals. Osteocytes maintain bone by exchanging nutrients and wastes between blood and bone tissues. Osteoclasts destroy bone and release minerals back into the blood.
Bone is made up of both organic and inorganic components. Protein collagen provides the flexibility of bones; the mineral component of bones is mostly hydroxyapatites, formed primarily of calcium and phosphorus. Small amounts of sodium, magnesium, fluorides, and carbonates are also present. One-third of living bone is water.
Bones are actually made up of two different types of bone. Cortical bone, or compact bone, forms the outer thick layer. Cancellous bone, or spongy bone, is formed out of trabeculae, bony struts that provide support. In living bone, cancellous bone contains the red bone marrow.
Bone has three main functions. It supports the body and stores minerals that are needed for metabolic functions. In organisms that have exoskeletons, shells, or bony scales, bone provides protection from predators.
Muscle fibers attach to bone via strong fibrous connective tissues, either directly through muscle fascicles or indirectly through tendons. The connective tissue joins the outer layer of the periosteum, a connective tissue membrane that covers the bone. At the points where tendons attach to bones, raised areas of bone (such as tubercles, trochanters, and crests) may develop to provide extra attachment area.
After birth, increase in the length of bone is caused by growth of hyaline cartilage which is then replaced by bone tissue. Increase in the width of bone is caused by appositional growth, in which bone tissue is added to the surface of the existing bone. This new bone tissue is secreted from the deep layer of the periosteum, a connective tissue membrane that covers the bone.
The process by which bones form, beginning in the eighth week of an embryo's development, is known as osteogenesis or ossification. The majority of the skull bones and the clavicle develop through intramembranous ossification, in which the bones form directly from an embryonic tissue known as mesenchyme. All other bones of the body develop through endochrondral ossification, in which the bones are first formed in hyaline cartilage which then is gradually replaced by bone tissue. Endochrondral ossification of long bones begins in the diaphysis, or shaft, of the bone; the last stage is the ossification of the epiphyses, or the ends. This last stage occurs around the time of birth. Ossification continues throughout life, with the epiphyses finally fusing to the diaphysis in adulthood. Until then, a section of hyaline cartilage known as the epiphyseal plate remains between the diaphysis and the epiphysis.
The Human Skeleton
The smallest bones are the ear ossicles, three bones that are found in the middle ear. They are called the stapes (stirrup), incus (anvil), and malleus (hammer).
The longest bone in the human body is the femur, sometimes called the thighbone.
On average, there are 206 bones in the human skeleton. A baby has over 300 bones, some of which fuse together as the individual grows. Variation means that an adult human may have between 200 and 210 bones in their body.