138Anatomy and physiology of domestic animals associated with bone formation, it can occur in other tissues. There are two general classes of bone formation. Intramembranous ossiﬁ cation occurs when bone develops from a ﬁ brous membrane. The ﬂ at bones of the skull and face, the mandible, and the clavicle if present, are formed by this method.
A typical long bone shows the gross anatomical characteristics of bone. The wider section at each end of the bone is called the epiphysis (plural = epiphyses), which is filled with spongy bone. Red marrow fills the spaces in the spongy bone.
The dynamic nature of bone means that new tissue is constantly formed, and old, injured, or unnecessary bone is dissolved for repair or for calcium release. The cell responsible for bone resorption, or breakdown, is the osteoclast.