What are open cavities in the skull called? What is the name of the sacs surrounding the body's movable joints?
Sinus or antrum is a cavity within a bone. Para-nasal sinuses are a connected system of hollow cavities in the bones of the skull and face (frontal, ethmoid, sphenoid and maxillary bone).
Diagrams below show the Sinuses :
The sacs surrounding the body's movable joints are called Bursae (bursa in singular). They prevent friction during joint movements.
Following diagram is showing a bursa around a joint :
Several bursae around shoulder :