Soft on the inside
Some parts of the body are soft on the inside and can't take a direct hit, or will rupture
Soft on the inside means not having tough fibrous strands to hold things in place. The brain is soft on the inside, but the skull protects it. The spleen is normally protected by the rib cage. In some diseases, the spleen enlarges and sticks down below the rib cage. Hit hard enough, it ruptures.