Internally, the 8c is made out of birch plywood. It is used to create the separate compartments for the drivers and to brace the cabinet, in order to increase the stiffness and strength of the cabinet. This results in an acoustically inert cabinet, which ensures that all you hear is the sound of the loudspeaker drivers.
Solid wood is a living material that continuously changes over time. Under the influence of changes in temperature and humidity, the wood will expand and contract. Hairline cracks are normal with solid wood, and thus to be expected.