or: The Thing Inside
Caution: may meander into the squeamish zone in both words and pictures... be warned.
I don't exactly know why I am fascinated by cojoined twins. Sometimes I guiltily cruise YouTube, watching videos about successful and unsuccessful separations. Very often the way the twins are joined is terribly difficult: at the top of the head, sharing organs, or with one twin unable to function because of their relationship to the other child. One of the most famous examples of a successful pair is Abigail and Brittany Hensel, a pair of "highly symmetric dicephalic parapagus" (ie, joined at the bottom, with two heads). It is difficult to tell but it looks as though they only have two arms, as well. They are both conscious, functioning girls with very separate personalities and interests, and are growing up apparently without problems, a very unusual case.
In fact, when you look at how their body is arranged, it's nearly miraculous: they have two heads and two stomachs but their small intesting has a "Y" intersection which, with a few minor problems, works fairly well. They have separate spines but one ribcage, two hearts and four lungs, but a shared diaphram. They can only feel their own half of the body, but they can do things such as type, ride a bike, and drive a car without mishap or misunderstanding. It's rather amazing.
More often, cojoined twins do not do well. Their mortality rate is about 75%. There are just too many complications, and all the things which seem to have gone right for the Hensel girls could just as easily fail through lack of alignment or poor formation of organs, which leads to their failure. Twins that are cojoined at the head often cannot function together, but separation can lead to the death of one or both of them, as the brains are involved.
What causes twins to be cojoined? There seem to be two separate theories on that. The older theory, the one I'm more familiar with and which leads to cases like the Hensel girls, is that during development the fetus separates as it does in the case of identical twins, who are two children made by one fertilized egg splitting completely in half. Except that in the case of cojoined twins, the split is incomplete. So you get children who share pelvises, skulls, etc.
Another theory is that in some cases, "a fertilized egg completely separates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuse the twins together." [wiki] Which implies that in the kinds of cases for whom this theory would apply, the joint is less extreme: some fleshy part of the (mostly separate) twins is joined. In the case of Chang and Eng Bunker, a pair of Thai cojoined twins who traveled with P.T. Barnum's circus for many years and from whom the term "siamese twins" comes, they were joined by "a band of flesh, cartilage, and their fused livers at the torso." This type of cojoining is very easy nowadays to separate.
Then there is the scenario where one twin is described as "parasitic," because it does not have a heart and/or a head of its own, and therefore lives off the circulatory system and resources of its twin. Parasitic twins can be attached in many different places, and are often much smaller than their fully-formed twin. A well-known example of this was the recent case of Lakshmi Tatma, found in a remote area of India at age two, who was joined to her twin pelvis-to-pelvis (with the twin between her legs). Thirty-six doctors took turns separating her spine and moving her bladder, then moving her pelvis together so her legs could point downward instead of off to the side. She had one non-functioning kidney, so the doctors replaced it with one of her twin's; she was then put in a cast designed to keep her legs together. A year later, it seems she was doing fine, beginning to learn to walk, and so on.
Parasitic twins generally do not improve the life of the main twin, not just because it is uncomfortable or embarrassing, but because it is a drain on their health and resources. One form of parasitic twin, called fetus in fetu is another, more rare and often devastating case of poor twinning, where the fetus of one twin, it seems, actually forms around the other, in effect born with another body inside it. One man, in India, lived for thirty-six years with his twin inside him, growing as he grew. I can only imagine how much weight it was to carry around, and how much he must have eaten to keep them both healthy.
Another boy in India went to the doctor because of his abdominal swelling, which was seen to move. A nearly-fully-formed twin was found inside him, with hair, toes, arms, legs, and even a vague face. This kind of completeness is rare in parasitic twins, especially among fetus in fetu, which tend not to be "alive" in the sense we think of, but rather a sort of growth in human form.
There seems to be a grey area between a fetus in fetu and what is called a teratoma: a kind of encapsulated tumor which contains "tissue or organ components resembling normal derivatives of all three germ layers. There are rare occasions when not all three germ layers are identifiable. The tissues of a teratoma, although normal in themselves, may be quite different from surrounding tissues, and may be highly disparate; teratomas have been reported to contain hair, teeth, bone and very rarely more complex organs such as(...) hands, feet, or other limbs." [wiki] ...or tissues from various organs. Teratomas are more common, and more benign, in women and girls, because they often form on the ovaries, where undifferentiated cells can be found; in men, they are more often malignant, for no reason that I've been able to find. Teratomas are considered to be congenital, in other words, they are present at birth, but not found until later on.
You can get teratomas in your brain or other organs, on the outside of your body (such as on the skull sutures or on the back of the pelvis), around your tongue and in your throat. I find them incredibly creepy, because they can contain pieces and parts of a person, encapsulated in a little pod in the body, as if someone had been trying to form, and failed miserably.
Curiously, I don't always find the parasitic twin phenomenon creepy; as the wonderful Human Marvels website describes, in the case of Ernie Defort, who was born with his "brother" Lem attached to his sternum, some people, if left to themselves, will become quite emotionally attached (hur, hur) to their extra parts.
It strikes me that this is not so different from the imperfections we all experience: if left to ourselves, we can become used to them and even like them, as being an integral part of ourselves. It is only when we listen to the media, or other external voices, that we begin to doubt who we should be.
Some interesting books on freaks/human marvels (which is what many cojoined twins and people with parasitic twins did to make a living in the old days):
Monsters: Human Freaks in America's Gilded Age: The Photographs of Chas Eisenmann, a book of really quite wonderful photos taken in the late 1800s by Mr. Eisenmann in his studio in New York.
American Sideshow, a lovingly-detailed and warmly described history of sideshow performers in America from the 1830s to the present. With recommendations on Amazon by some of the participants.
Colorado Doctor Finds Foot in Baby's Brain [link]
More info on parasitic twins, thanks to The Human Marvels