Question

What are open cavities in the skull called? What is the name of the sacs surrounding the body's movable joints?

Answer 1

A cavity inside a bone is known as an antrum. The paranasal sinuses are a network of hollow cavities in the skull and face bones (frontal, ethmoid, sphenoid and maxillary bone).

Complete answer:
Sinuses, Bursa sacs surrounding the body's movable joints
A sinus is a sac or depression in any organ or tissue, as well as an unusual hole or entry caused by tissue obliteration. Similarly, the term "sinus" generally refers to the paranasal sinuses, which are air cavities in the cranial bones, particularly those close to and interfacing with the nose. The majority of people have four combined holes in their cranial bone or skull. A sinus is a sac or pit in any organ or tissue, or a strange cavity or section caused by tissue obliteration. Similarly, the term "sinus" usually refers to the paranasal sinuses, which are air holes in the cranial bones, particularly those near and interfacing with the nose. The majority of people have four matched depressions in their cranial bone or skull.
Ethmoid sinus are depressions between the eyes.
Sinus pits on the front of the nose that can be found over the eyes (more in the temple district).
Maxillary sinus can be found on either side of the nostrils (cheekbone territories).
Sphenoid sinuses are located behind the eyes and in the deeper openings of the skull.
Bursae are found between bones and bones, bones and muscles, or muscles and skin, and serve to prevent grinding at pressure points throughout the body. The bursae are either clearly visible (on top of the patella) or mostly hidden among bone and muscle in the image.

Note:
The capacity of the sinus depressions within the cranial bone (skull) is unclear. Convictions about their potential include:
Voice reverberation is taken into account by sinus cavities.
They aid in the channelling and dampening of any air breathed in through the nasal entries, as well as the expulsion of undesirable particles from the sinus depressions.
Sinus depressions provide relief to the skull.
Sinus cavities provide space for development.
When injured, sinus depressions may serve as stun assimilation.