The first line of defense against infection are the surface barriers. The surface barriers include intact skin, secretions of skin and mucous membrane. Skin protects external or outside boundaries and mucous membranes protect internal boundaries. Both the skin and mucous membranes release chemical secretions which restrict the growth of microbes on their surfaces if pathogens cannot cross the first line of defense body, they cannot disrupt normal functioning of and cannot cause disease.
Compact bone is the outer layer that is packed together very tightly. This is mainly for support & balance. The spongy bone is strung together loosely and looks like *no surprise here* a sponge. This helps create blood cells & other bodily garbage.