These bones are expanded into broad, flat plates ,  as in the cranium skull , the ilium pelvis , sternum and the rib cage. The flat bones are: First metacarpal shown in yellow. The osteoclasts dissolve the part of the bone that must change. The distal articular surface is quadrilateral, wide, and flat; thicker and broader transversely and extends much further palmarly than dorsally. In an adult, most red blood cells are formed in flat bones. These canaliculi provide the nutrients needed for the newly transformed osteoblasts, which are now called osteocytes. On a baby, those spots are known as fontanelles.
The articular surface is delimited by a thick, crest-like ridge extending around its circumference. On the palmar aspect of the articular surface there is a pair of eminences or tubercles which articulate with the radial and ulnar sesamoid bones of the thumb metacarpophalangeal joint; the lateral eminence is larger than the medial. The dorsal surface of the shaft is weakly convex longitudinally, while its palmar, radial, and ulnar surfaces tend to be concave. The configuration of the thumb carpometacarpal joint plays an important role in the mechanism of opposition. The osteoclasts dissolve the part of the bone that must change. In an adult, most red blood cells are formed in flat bones. Characteristics[ edit ] The first metacarpal bone is short and thick with a shaft thicker and broader than those of the other metacarpal bones. These cells are responsible for the general maintenance of the bone. Since flat bones are usually thinner than the long bones , they only have red bone marrow , rather than both red and yellow bone marrow yellow bone marrow being made up of mostly fat. Its narrow shaft connects its widened base and rounded head; the former consisting of a thick cortical bone surrounding the open medullary canal ; the latter two consisting of cancellous bone surrounded by a thin cortical shell. These canaliculi provide the nutrients needed for the newly transformed osteoblasts, which are now called osteocytes. This hardened matrix forms the body of the bone. First metacarpal shown in yellow. They are also used to obtain necessary calcium from the bones. When a person's blood calcium is low, the osteoclasts take calcium from the bone and put it into the blood for necessary things such as nerve and muscle action. As the bone grows, its shape changes. The last reason that osteoclasts are used is for growing. The fontanelles contain connective tissue stem cells , which form into osteoblasts , which secrete calcium phosphate into a matrix of canals. 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. The metacarpal bone of the thumb has two centres of ossification: Base[ edit ] The base is significantly different from the bases of the other metacarpals. The ossification process begins in the shaft during the ninth week of prenatal life, and in the base during the second year of life in girls and the third year in boys. The origin of the first dorsal interosseous muscle is on the ulnar aspect of the base, and can sometimes extend onto the shaft. A third type of bone cell found in flat bones is called an osteoclast , which destroys the bone using enzymes. This contrasts to the other four metacarpal bones where the secondary centre is found in the head. They destroy sections of bone that protrude or make reformation difficult.
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