Not Rocket Surgery

Lining the cranial cavity and spinal cavity are the protective membranes known as the meninges.
Inflammation of the meninges is known as meningitis.

Lining the abdomen (belly) is a membrane known as the peritoneum.
Inflammation of the peritoneum is known as peritonitis.

Lining the thorax (chest) is a membrane known as the pleura.
This does not follow the usual naming conventions as inflammation of the pleura is known as pleurisy (although pleuritis is sometime heard).