What is the purpose of a cylinder head?

it's holding the valves, the injection and some includes a cooling line in them. They are responsible for the needed compression in the cylindres and as they include the valves, that are responsible, that the right portion of petrol gets into the combustion chamber at the right time and so on. If there was no pressure and no compression, the explosion would not built the pressure to get the engine run. A broken cylinder head and a burnt gasket avoids the engine from running forward