The beings thus formed have rarely lived many years after birth, and the histories of the few that have survived are, for the most part, well known in the records of medicine. Perhaps the most remarkable is that of the Chinese A-ke, of whom and his parasite little models are to be found in most of the anatomical museums of Europe. The para site's life is, in general, only vegetative. In one of the three cases, indeed, in which it possessed all the constituent parts of a body, it moved its limbs, and appeared to have its own sensations ; but in the others, less per fectly formed, even these signs of individual life were absent ; and in only one, that of the Chinese A-ke, had the man who bore the parasite any voluntary power over its limbs. The nutrition of the parasite appears to depend entirely on the body to which it is fixed, and through which it both receives its nutritive materials and discharges its excre tions. The one increases and decreases in size with the other; and, of course, the para site dies with the individual to which it is attached. The influence which, in its turn, it exercises on its supporter is not always important. In the heteradelphs that die early, death commonly ensues from the mal formation of the main body ; if they survive, the parasite seems to do harm only, as an ordinary tumour would, by its weight, and by abstracting a certain amount of nourishment, so that those who, thus burdened, have grown up to childhood or manhood have usually been thin and delicate, like men subject to some unnatural waste. But, nevertheless, it will always be better to tolerate this evil than to risk an operation of removal, when the re sults of all the examinations yet made prove that the parasite is deeply and by important organs connected with its supporter. The only exception which I know to the correctness of my opinion is the case in which Mr. Blizard removed, with complete success, from the sacrum of a child, a congenital tumour, which seems to have been a parasite.
III. Double Monsters.
1. Anterior duplicity.
It has been already said that some of the rarer kinds of heteradelphia approximate closely to the double monsters. In all the cases that stand nearest to the transition, the parasite has been found adherent to the epi gastric region ; and the kind of duplicity which is most closely related to them, is therefore that in which the two bodies adhere by their anterior surfaces, or what we call anterior duplicity.
The most complete examples of duplicity yet known are found in this class, whose dis tinctive characters are, that two bodies, in a state of nearly equal developement, are placed exactly opposite to one another, with their sterna connected together, and with their abdominal cavities either partially or com pletely coalesced. Here, however, as in all the other classes, examples are found of gra dations towards a state of singleness. For instance, the upper parts of the body being completely double, the lower are united, so that there are but three limbs, or only two lower or posterior limbs. And, in like manner, i although in many cases the bodies are alike in size and other characters, yet there are many more in which one has so far surpassed the other, both in size and in stage of clevelope ment, as completely to fill up the series be tween this class and the decided heteradelphs. Whilst in the latter case we find close approxi mations to duplicity, there are even among the most perfect double monsters peculiarities which constantly recall to mind the parasitic attachment of the heteradelphs.
With this nearly perfect external duplicity there sometimes, but not always, corresponds an equal duplicity of organs. The umbilical cord may be single (Parsons, Otto), notwith standing the heart is perfectly double. The two umbilical arteries belong but to one child ; the umbilical vein bifurcates and enters both bodies. In another case (Cruveilhier), the single umbilical cord had two veins and four arteries, and the heart was externally single, and in its internal parts imperfectly double.
In a third double monster (Otto), the umbili cal cord had five vessels, two umbilical veins, and three arteries, and in this there were two hearts. In a fourth double monster the heart and umbilical cord were single. The bond of union, as far as the skeleton is concerned, is commonly a tough fibrous connection between the lower extremities of the sterna and the ensiform cartilages, which are set directly opposite to one another.
The rest of the sterna and the ribs are usu ally distinct, and the thoracic cavities are thereby separated. In this case there are commonly two separate and perfect hearts ; but in the cases in which the sterna are more completely fused, or (as happened in one case) entirely absent (Comm. Lilt. No riniberg.),- only a single heart, or one partially double, with, for instance, two ventricles and four auricles, or otherwise malformed, is found. But it is particularly remarkable that in these, as in all other kinds of double monsters, there is no constant relation whatever between the respective states of the external and the in ternal organs, for the condition of the two digestive canals, even in those which are ex ternally almost alike, is subject to still greater varieties than the condition of the heart. The abdominal organs are always in some degree connected ; — the two livers are usually con tinuous.
A spleen, pancreas, and stomach are com monly found in each body, and each stomach has its own duodenum, which, after some length (being continued into the jejunum) unites with the other to form a single tract of small intestine, which again divides into two canals, leading respectively to the large intestine of each body. The lungs, the urinary, and the genital systems are always double. The most remarkable example of this class was the well known Siamese twins. When exhibited, they were not exactly opposite to each other, but stood side by side, or, rather, obliquely one by the other ; but this position, there can be little doubt, was acquired by the attempts which they had instinctively made to separate from each other in walking, or in lying and sitting down, and by the extension they had thus effected in their bond of union, which was considerably more slender than in any other yet described. It was quite impossible for them to remain always face to face; there fore their bodies acquired an oblique direction, in which they also moved. The consequence of this was, that the right limbs of the one and the left of the other individual were the prin cipal organs of movement ; and that the inter mediate limbs, that is to say, the left of the one and the right of the other, remained merely passive (Dubois). The one individual was stronger than the other, and seemed to over rule him. But, nevertheless, in organic and animal relation of life they seemed to be inde pendent of each other. Each had his own circulation of blood, his own respiration, and digestive functions. There did not seem to be a large anastomosis of vessels between the two bodies. But, by analogy with the fore said cases, we may conclude that these twins are connected by the ends of their sterna, and by some of their abdominal organs. As a proof of this connection, may be adduced the result of the observations of Mr. Mayo, communicated at the Conversazione of the College of Physicians March 8th, 1831, that when either of the youths coughed, the bond of union swelled up in its whole length, proving that they had but one peritoneal cavity, of which a trans verse prolongation passed through the con necting medium. And, therefore, I should conclude, that an attempt at separation could not be made with probability of success. The probability of having to cut through a piece of liver, or a peritoneal canal, must render an operation unwarrantable, unless, indeed, after the death of one of the bodies during the healthy state of the other. The case reported by Kiinig has scarcely authority enough to support a contrary opinion.