Anatomy of head injuries
The brain is a jelly-like organ that is bathed in fluid (cerebro-spinal fluid or CSF) and enclosed in a layered membranous bag. This all then sits in a rigid bony skull.
When there is a blow to the head, the brain wobbles a bit like a jelly and, if a concussion occurs, can be stunned (no structural injury to the brain) or much less commonly, and far more seriously, the brain can be bruised or tear on the sides or within the brain causing bleeding. The membranes outside the brain are known as the meninges and carry blood vessels on the surface of the brain between it and the skull.
Anatomy of the head