Mice have both bones and cartilage. Mice as well as rats can fit into small openings due to their cylindrical shape.
Many people believe that a mouse's body is made of cartilage because mice can fit into small, tight spaces, but this is simply a myth. Mice spend their time tunneling, and their thin, cylindrical shape helps them tunnel and burrow easily. They look at a space and use their whiskers to determine if it is big enough for them to fit through.
Often, a mouse is mistaken as a larger animal than it really is. Mice do not have collar bones, and their largest body part is the head. That means if a mouse can fit its head in a hole, the rest of the body will fit. Juvenile mice are so small that they can fit in a hole the size of a number-two pencil.