On April 15, 1811, in Mekong or possibly Bangesau, Siam, two boys were born joined at the waist by a short tubular cartilaginous band through which their circulatory systems functioned. Physicians said that it would be fatal to cut them apart because they shared the same liver tissue. The boys, Chang and Eng, had mirror organs in that what one had on the right side, the other had on the left, what is known scientifically as situs inversus. They could walk, even run fast, and in some ways seemed normal. They had short tempers with other people, but they were solicitous about each other’s comfort.
P. T. Barnum, the great showman, discovered the boys, sons of a Chinese father and Siamese mother, and brought them to America. He named them the Siamese Twins, and the name became generic for twins joined at birth side by side or front to back. Barnum exhibited them until they became adults. They then exhibited on their own until they acquired enough money to become farmers. This they did in North Carolina, whose legislature gave them the surname Bunker. In 1843 they married two sisters and eventually fathered twenty-two children, all physically normal. While returning from a visit to Liverpool, Chang suffered a paralytic stroke. On January 16, 1874, at their home in Mount Airy, North Carolina, Eng awakened to find that his brother had died. Three hours later, Eng was dead.
Some such twins have been surgically separated and are able to live normal lives. The key is the site of the connection.
The name Siamese twins has persisted for physically attached twins. In general usage, the expression means that two persons are so friendly that if you see one, you are bound to see the other.