| Flat bone | |
|---|---|
 Anatomy of a flat bone (Frontal bone) | |
 Flat bones inhuman skeleton. (shown in red) | |
| Details | |
| Identifiers | |
| Latin | os planum |
| TA98 | A02.0.00.013 |
| TA2 | 371 |
| FMA | 7476 |
| Anatomical terms of bone | |
Flat bones arebones whose principal function is either extensive protection or the provision of broad surfaces for muscular attachment. These bones are expanded into broad, flatplates,[1] as in the cranium (skull), the ilium, ischium, and pubis (pelvis),sternum and therib cage.The flat bones are: theoccipital,parietal,frontal,nasal,lacrimal,vomer,sternum,ribs, andscapulae.[1]
These bones are composed of two thin layers ofcompact bone enclosing between them a variable quantity ofcancellous bone,[1] which is the location of redbone marrow. In an adult, mostred blood cells are formed in flat bones. In thecranial bones, the layers of compact tissue are familiarly known as the tables of the skull; the outer one is thick and tough; the inner is thin, dense, and brittle, and hence is termed the vitreous (glass-like) table.[1] The intervening cancellous tissue is called thediploë, and this, in the nasal region of the skull, becomes absorbed so as to leave spaces filled with air–theparanasal sinuses between the two tables.[1]
Ossification is started by the formation of layers of undifferentiatedconnective tissue that hold the area where the flat bone is to come. On a baby, those spots are known asfontanelles. The fontanelles contain connective tissuestem cells, which form intoosteoblasts, which secretecalcium phosphate into a matrix of canals. They form a ring in between the membranes, and begin to expand outwards. As they expand they make a bony matrix.
This hardened matrix forms the body of the bone. Since flat bones are usually thinner than thelong bones, they only havered bone marrow, rather than both red andyellow bone marrow (yellow bone marrow being made up of mostly fat). The bone marrow fills the space in the ring of osteoblasts, and eventually fills the bony matrix.
After the bone is completely ossified, the osteoblasts retract their calcium phosphate secreting tendrils, leaving tiny canals in the bony matrix, known ascanaliculi. These canaliculi provide the nutrients needed for the newly transformed osteoblasts, which are now calledosteocytes. These cells are responsible for the general maintenance of the bone.
A third type of bone cell found in flat bones is called anosteoclast, which destroys the bone using enzymes. There are three reasons that osteoclasts are normally used: the first is for the reparation of bones after a break. They destroy sections of bone that protrude or make reformation difficult. They are also used to obtain necessary calcium that osteoclasts are used is for growing. As the bone grows, its shape changes. The osteoclasts dissolve the part of the bone that must change.[2]
This article incorporates text in thepublic domain frompage 79 of the 20th edition ofGray's Anatomy(1918)
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