What keeps the trachea from collapsing in fetal pig?

The trachea is kept from collapsing in a fetal pig by cartilage rings. These rings are made of a flexible material that allows the trachea to expand and contract as the pig breathes, but they are also strong enough to prevent the trachea from collapsing. The cartilage rings are also lined with a layer of mucus, which helps to protect the trachea from damage.