These monsters have been known to live a considerable time, their capacity for life being probably owing to the separation of the hearts and the absence of malformation in the more important organs of the body. The umbilical cord is single, and never has a double set of vessels ; an apparent proof (confirmed by si milar examples in other classes) that the one body is not formed of the materials of two ; a conclusion which is supported by the coinci dent singleness of the anus and urinary blad der, and the union of the intestinal canals.
4. Posterior duplicity.
The fourth chief form is posterior duplicity, in which two bodies are united by their backs, or a part of theni. The union may be at the pelvis (which is most common), and occurred in the well-known Hungarian sisters, who lived to their twenty-second year ; or at the back of the vertebral column, or at the back part of the heads.
5. Superior duplicity.
The fifth is the superior duplicity, in which the two children are connected by their skulls, the bones of which are united so as to form a single skull. In these also the place of union varies greatly. The frontal bone of one coa lesces with the parietal or the occipital of the other, or the foreheads are attached to one another, or the side of one head to the front of the other. But all these are very rare, and of each kind only one or two examples can be found on record.
All these are true double monsters. Of tripled-bodied monsters but one instance is known in the human subject (Atli dell' Acade mia de Cattania, t. viii. p. 203., 1834).
To conclude the description of duplicity, those which occur in individual parts of the body, the rest being single, viz. in the head, chest, abdomen, and limbs, ought to be re corded ; such as, for example, two mouths, su pernumerary teeth and horns, two cesophagi or duodena, double hearts, or supernumerary cavities in one heart otherwise well formed, a double penis and urethra, a double clitoris, supernumerary breasts, kidneys, vertebra?, ribs, fingers, toes, or whole limbs. But for all these I refer to my monograph.
At present I prefer to give all those con siderations upon the origin of double mon sters, of which this highly interesting subject is capable. From all the facts I have published, I point out the following generalisations:— The double monsters form collectively one class of organic beings, which, however dif ferent in their several degrees of malforma tion, may be arranged in one continued series. As the lowest degree of duplicity, may be mentioned that of a single part of the body; for example, a double or supernumerary fin ger; as the highest, a complete double mon ster with two heads, four upper and four lower limbs, and two trunks, such as the Siamese twins. And between these two ex tremes there are different forms of duplicity, which gradually run one into the other.
There is no positive or constant relation between the external and the internal organs as to their degrees or modes of duplicity. In the completest duplicity of the exterior, for example, the heart is often single, or even shows signs of having been arrested in its developement ; and, on the other hand, in the more nearly single forms, the heart is usually either partially or completely double. The
histories of the cases of anterior and lateral duplicity, which I have given in my mono graph, furnish abundant proofs of this. Nor is there any closer relation between the con dition of any other internal organ and that of the exterior, than there is between it and the heart ; for in nearly complete external dupli city any of the internal organs may be single; when there are two trunks indeed, the urinary and genital organs are commonly double ; but as for the stomach, the liver, and the lungs, the correspondence between them and exter nal duplicity follows no other rule than that where there are two necks there are two trachea: and two oesophagi, and as a con sequence the lungs and stomach are also dou bled. When in like manner the stomach is double, each has its spleen and pancreas ; but the state of the liver is very variable ; some times there are two, sometimes but one with a single or double gall-bladder ; and these differences often occur in the same form of external duplicity.
Parts placed on the surface of the body are more liable to multiplication than the internal organs, and duplicity of a single part is there fore much less rare than the formation of a complete double body. The upper half of the body is more frequently doubled than the lower, probably in consequence of its earlier developement and the admitted preponderance of the upper parts of the body. The union of the two bodies takes place only between similar parts. The more each of the bodies is developed, the less is the bond of union between them, as the examples of the Siamese twins and the Hungarian sisters sufficiently prove. And with this law is connected an other, namely, that the probability of grow ing up is greater in the same proportion as the bond of union is smaller, and the coin cident fusion of internal organs less, as these two and other double monsters prove. So also the further the several organs are from the situation at which the bodies are united, the more perfect they are, — one body is almost always less developed than the other : in the heteradelphs this is al ways the case, and in others the difference between the two bodies, though less evident, is scarcely less constant than in them. There are not commonly any signs of a double monster having been at first two individuals. For except in the cases of posterior and supe rior duplicity, and some singular examples of attachment of the umbilical cord of one foetus to the head or the body of the other, there is never more than one placenta and one cord, and the latter usually contains only a single set of vessels, which divide when they reach the abdomen. And even in the posterior and superior varieties of duplicity it is not yet certain that there are two placentm ; in some cases the placenta was positively single, and in the remainder it has very rarely been ex amined. A last general rule is that in double monsters the twins are of the same sex. There is no well proved exception to this important rule.