These are examples of bits used today to keep horses in line. Similar devices were used on slaves who were considered insubordinate. The bit rendered the slave unable to speak or even swallow, causing a great deal of discomfort and difficulty breathing.
“The importance of being voiceless is perhaps one of the most dehumanizing aspects of slavery. The wildness in the eyes, even when the bit is removed shows how part of their humanity is stripped away…They [the bits] stopped you from talking, little by little that should destroy you.” -Toni Morrison, BBC Interview