I went with my fourth grade son on a field trip to the local museum of natural science. I noticed the skeleton on my display had a very small shoulder socket. The round part of the upper arm barely fit. Is that normal? What holds it together?

The shoulder has the most motion of any joint in the body. To have that much mobility, the joint gives up a lot of stability. That means the head of the humerus (upper arm bone) pivots off the shoulder socket more than snapping in tightly.

In order to do this without dislocating the joint capsule, ligaments, and muscles hold it firmly in place. There's a special group of muscles that surround the shoulder called the rotator cuff. They hold the humeral head in place while allowing arm motion in all directions.

The skeleton you saw was likely very normal. But to see the bones without the soft tissues can be misleading.