Bones have a hard and non-pliable ground substance rich in calcium salts and collagen fibres which give bone its strength. It is the main tissue that provides the structural frame to the body. Bones support and protect softer tissues and organs. The bone cells (osteocytes) are present in the spaces called lacunae. These lacunae also have extensions known as canaliculi. Lacunae are present in the matrix of cartilage too but are without canaliculi.