Type of Bones

Bones are classified on the basis of place of origin, texture and shape.

On the basis of the place of origin.

• Cartilage or Replacing or Endochondrial Bones. The bones develop through calcification or ossification of cartilage, e.g., long bones like Humerus and Femur.
• Investing or Dermal or Membrane Bones. The bones develop in dermis of skin as thin plates, e.g., frontal, parietal, nasal and vomer of skull.
• Sesamoid Bones. It is a bone formed by the ossification in tendons, e.g., patella or knee cap.
• Visceral Bones or Heterotypic Bones. They are bones which develop in various organs, e.g.,
• (a) Os-cordis. Bones present in heart, e.g., Deer.
• (b) Os-pelpebrae. Bones formed in eyelids, e.g., Crocodile.
• (c) Os-penis. Bone present in the copulatory organ of male, e.g., bats, insectivores, rodents, carnivores (Walrus, Racoons).
• (d) Os-crest. Bone present in crest, e.g., Birds.
• (e) Os-snout. Bone present in snout e.g., Hog.
• (f) Os-clitoris. In clitoris of many carnivores.
• (g) Os-falciparum. In palm of Mole.
• (B) On the Basis of Texture.
• Spongy Bone (Cancellous or Cancellate or Trabecular Bone). It is light weight bone in which matrix is composed of numerous thin, curved, intersecting plates, bars or tubes of variable sizes, called trabeculae. They provide a large surface area which is covered by endosteum. The space between the trabeculae contains red bone marrow. Inside trabeculae the matrix is laid in lamellae but they are irregular, generally parallel to the trabecular surface. Internal supply channels do not occur in trabeculae. Nutrition diffuses directly from endosteum into lacunae through canaliculi. Cancellous bones occurs inside epiphysis and metaphysis of long bones, interior of vertebrae, flat bones of skull and ribs. Spongy bone is usually covered by compact bone. Such a bone having both compact and cancellous part is called diploic bone.
• Compact Bone (Periosteal Bone ; Dense Bone, Ivory Bone). It is that bone which has continuous matrix from outside to inside except for narrow supply channels and a central marrow cavity, if present. The bone contains numerous Haversian systems. The matrix is laid in lamellae. Lamellae are of three types—circumferential, interstitial and concentric. Marrow cavity, if present, is generally filled with fat rich yellow marrow. Compact bone occurs in outer parts of all bones. It is exclusively present in the shaft or diphysial region of long bones, clavicle, scapula, innominates, arm and leg bones.

Table : Differences Between Compact and Spongy Bones

S.No. Compact Bone Spongy Bone

•  It is dense. It is spongy.
•  Matrix is continuous. Matrix occurs in the form of irregular, intersecting plates or trabeculae.
• It is heavier. It is light weight.
• It cannot tolerate excessive stress. It is specialised to tolerate excessive stress.
• Haversian systems are present. Haversian systems are little.
• Lamellae are regular. Lamellae are irregular.

Lamellae are of three types:–

There is no such distinction. circumferential, interstitial and concentric.

• Matrix is richly vascularised. Vascularisation of matrix is little.
• A central marrow cavity may be present. It is absent.
• There is little addition or withdrawal of calcium Most of the addition or withdrawal of calcium occurs here.
• Marrow cavity contains yellow marrow. Space amongst the trabeculae is occupied by red bone marrow.
• Internal surface area is comparatively small. Internal surface area is quite large.
• Marrow contains abundant fat cells. Fat cells are fewer.
• Yellow marrow does not perform Red marrow is active in haemopoesis. haemopoiesis except during emergency.

On the basis of structure or shape.

• Long Bones. These bones are long in size, e.g. humerus, femur, radius, ulna, tibia, fibula.
• Short Bones. These bones are short in size, e.g., carpals, metacarpals and tarsals.
•  Flat Bones. Bones are dorso-ventrally flattened, e.g., scapula, sternum, cranium,
•  Irregular Bones. Irregular shape of bones, e.g., vertebrae, carpals and tarsals.