Flat bones are

bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...

s whose principal function is either extensive protection or the provision of broad surfaces for muscular attachment. These bones are expanded into broad, flat plates,''

Gray's Anatomy ''Gray's Anatomy'' is a reference book of human anatomy written by Henry Gray, illustrated by Henry Vandyke Carter and first published in London in 1858. It has had multiple revised editions, and the current edition, the 42nd (October 2020 ...

'' (1918). (See infobox) as in the cranium (

skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

), the ilium, ischium, and pubis (

pelvis The pelvis (: pelves or pelvises) is the lower part of an Anatomy, anatomical Trunk (anatomy), trunk, between the human abdomen, abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also c ...

sternum The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...

and the

rib cage The rib cage or thoracic cage is an endoskeletal enclosure in the thorax of most vertebrates that comprises the ribs, vertebral column and sternum, which protect the vital organs of the thoracic cavity, such as the heart, lungs and great ve ...

. The flat bones are: the occipital, parietal, frontal,

nasal Nasal is an adjective referring to the nose, part of human or animal anatomy. It may also be shorthand for the following uses in combination: * With reference to the human nose: ** Nasal administration, a method of pharmaceutical drug delivery * ...

, lacrimal,

vomer The vomer (; ) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones. The vomer forms ...

ribs The rib cage or thoracic cage is an endoskeletal enclosure in the thorax of most vertebrates that comprises the ribs, vertebral column and sternum, which protect the vital organs of the thoracic cavity, such as the heart, lungs and great vessels ...

, and scapulae. These bones are composed of two thin layers of

compact bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, and ...

enclosing between them a variable quantity of

cancellous bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...

, which is the location of red

bone marrow Bone marrow is a semi-solid biological tissue, tissue found within the Spongy bone, spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It i ...

. In an adult, most

red blood cells Red blood cells (RBCs), referred to as erythrocytes (, with -''cyte'' translated as 'cell' in modern usage) in academia and medical publishing, also known as red cells, erythroid cells, and rarely haematids, are the most common type of blood cel ...

are formed in flat bones. In the cranial 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. The intervening cancellous tissue is called the

diploë Diploë ( or ) is the spongy cancellous bone separating the inner and outer layers of the cortical bone of the skull. It is a subclass of trabecular bone. In the cranial bones, the layers of compact cortical tissue are familiarly known as the ...

, and this, in the nasal region of the skull, becomes absorbed so as to leave spaces filled with air–the

paranasal sinuses Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity. The maxillary sinuses are located under the eyes; the frontal sinuses are above the eyes; the ethmoidal sinuses are between the eyes and the sphe ...

between the two tables.

Ossification in flat bones

Ossification Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

is started by the formation of layers of undifferentiated

connective tissue Connective tissue is one of the four primary types of animal tissue, a group of cells that are similar in structure, along with epithelial tissue, muscle tissue, and nervous tissue. It develops mostly from the mesenchyme, derived from the mesod ...

that hold the area where the flat bone is to come. On a baby, those spots are known as

fontanelle A fontanelle (or fontanel) (colloquially, soft spot) is an anatomical feature of the infant human skull comprising soft membranous gaps ( sutures) between the cranial bones that make up the calvaria of a fetus or an infant. Fontanelles allow ...

s. The fontanelles contain connective tissue

stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can change into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of cell ...

s, which form into

osteoblast Osteoblasts (from the Greek combining forms for " bone", ὀστέο-, ''osteo-'' and βλαστάνω, ''blastanō'' "germinate") are cells with a single nucleus that synthesize bone. However, in the process of bone formation, osteoblasts fu ...

s, which secrete calcium 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 the

long bone The long bones are those that are longer than they are wide. They are one of five types of bones: long, short, flat, irregular and sesamoid. Long bones, especially the femur and tibia, are subjected to most of the load during daily activities ...

s, they only have

red bone marrow Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It is composed of hematopoietic c ...

, rather than both red and yellow 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 as canaliculi. These canaliculi provide the nutrients needed for the newly transformed osteoblasts, which are now called

osteocyte An osteocyte, an oblate-shaped type of bone cell with dendritic processes, is the most commonly found cell in mature bone. It can live as long as the organism itself. The adult human body has about 42 billion of them. Osteocytes do not divide an ...

s. These cells are responsible for the general maintenance of the bone. A third type of bone cell found in flat bones is called an

osteoclast An osteoclast () is a type of bone cell that breaks down bone tissue. This function is critical in the maintenance, repair, and bone remodeling, remodeling of bones of the vertebrate, vertebral skeleton. The osteoclast disassembles and digests th ...

, 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.

Additional images

File:Flat bones - animation.gif, Flat bones in human skeleton. (shown in red) File:Flat bones in skull - lateral view - with legend.png, Flat bones in

human skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominen ...

. (shown in red) File:Blausen 0229 ClassificationofBones.png, Classification of bones by shape.

References

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