Spongy bone is found towards the head of a long bone, under the compact bone. If you consider that real sponge is made of sponge and air spaces, then the sponge is made of bone or Trabeculae -{1} and the air is made of red bone marrow -{3}; the marrow produces red blood cells. The trabeculae is interspersed with cancellous cavities which contain Osteoblasts -{2}. The osteoblasts produce Osteocytes which are embedded in the trabeculae. If you look at the trabeculae just at the end of the indication line marked as {1}, you can see a single osteocyte as a very tiny circle. These produce the inorganic calcium phosphate that the trabeculae are made of. In this slide, the trabeculae are surrounded by a layer of protective connective tisue called the Periosteum -{4}.
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