What are sesamoid bones in the foot?

A runner with pain under the big toe joint said she was told she had a problem with the sesamoid bones in her foot. 

The doctor said maybe it was sesamoiditis, or a sesamoid stress reaction or possibly even a sesamoid stress fracture. 

Her question was, “What exactly are the sesamoid bones in the foot?”

Today on the Doc On The Run Podcast, we're talking about these weird little things called the sesamoid bones in the foot.