In several of the preceding chapters we have been content to sit still and discuss various aspects of the human body. The time seems to have arrived when we should again see and examine the objects which are to be the subject of consideration. We propose then to revisit the Hunterian Museum where we may critically examine the most complete collection which has ever been brought together to illustrate the various monstrous forms which the human and also the animal body occasionally assume. Before actually surveying the gallery of the museum in which the terato-logical collection—so this series is named—is arranged it will be to our advantage to see first the specimens which illustrate the formation of normal children. Two preparations show us that as many as five children may be produced at one birth—all of which are small but normal in shape. Triplets are not uncommon; in every 7000 announcements of birth one may expect to hear of a case of triplets. Twins are common; in Ireland a twin birth has a frequency of one in seventy-two, in England about one in seventy-five, and in France about one in a hundred. In man, as in all the higher primates, one at a birth is the rule. Indeed I cannot remember any case of twins being born to either monkeys or anthropoids. We may regard the production of twins in the higher primates as an abnormality. We shall see presently that nearly all human monsters are the result of an imperfect production of twins.
On the adjoining shelves we have an opportunity of verifying the fact that there are two kinds of twins. In one form we see that each foetus is wrapped in its own membranes ; each has its own umbilical cord which conveys the blood to and from the placenta ; the two placentas may be partly or completely fused. In the other kind the two foetuses are enclosed together within the same envelope of membranes ; each has a cord but they end on the same placenta. These two varieties of twins we believe arise quite differently ; in the first kind there were two ova, both of which were fertilized; in the second there was but one ovum, which, at an early stage of development, divided into two and thus gave rise to two embryos. In the first kind the twins may be of opposite sex and with no greater resemblance to each other than a brother has to a sister, but in the second kind they are " identical twins " both being of the same sex and so alike that even the nurse finds a difficulty in telling one from the other.
It is in the production of identical twins that monsters arise. A fowl's egg which has been prepared to show the embryo of a chick at the end of the twenty-fourth hour of incubation helps us to understand the process. The chick embryo forms a small plate, spread out on the yolk. With a magnifying glass it can be seen that the posterior end of this particular embryo has undergone a process of division and that while the head end shows the rudiment of but one chick, the hind end possesses the basis for two. Another specimen shows a subdivision or duplication of both the front and hind end of the embryo, a condition which would result in twin chicks joined together by their bodies. We are now in a position to understand the various forms of human monsters shown in the teratologicl collection.
The specimens in the first case of the teratological gallery need not detain us, yet in a way they are very interesting. They show to us the inward parts of individuals in whom the viscera have been transposed. The apex of the heart is directed towards the right side, and its beat, instead of being felt during life in the fifth intercostal space of the left side of the chest, is palpable in the corresponding space of the right side. The arch of the aorta, instead of bending to the left turns to the right.
The caecum and appendix occupy the left side of the lower abdominal space in place of the right. In short we see the viscera reversed as in a mirror. Such a condition is not common although many cases have been placed on record, but no satisfactory explanation has been given of how the transposition has been produced embryologically.
The next series of specimens shows us products of human birth which none but an expert would recognize as children. They seem shapeless packages wrapped in wrinkled human skin ; no head, merely a trunk with projecting parts which simulate limbs. Some are cut open to show that within them there are a backbone, a stomach, liver and bowel, but there is no brain and the heart does not seem capable of acting as a pump. We have come to know that all of these " acardiac " or " parasitic " foetuses are never born alone ; they are the twin of a normal child. The developmental separation of the twins was almost complete ; the only junction which persisted was a union between their vessels at the placenta. One of the twins became a parasite on the other; the weaker twin, instead of maintaining its own circulation, came to trust to the heart of the larger or " host " foetus for a blood-supply, with the result that its own heart became passive and life was maintained by the blood supplied to it from the host or stronger foetus. The parasitic foetus cannot survive birth, for the moment the cord of the normal twin is tied its supply of blood is cut off and it dies. The shapeless masses of humanity show us the form and organization our bodies would assume were we to become purely parasitic or passive in our manner of life.
The monsters included in the next series are manifestly the result of imperfect separation of identical twins. In some the body is single down to the navel, but is double from that point downwards ; in other cases it is the upper part which is double—two heads and four arms—while the lower parts are those of a normal individual. In others the separation is almost complete, the bodies being united at one point only. In some monsters, as in the case of the Siamese twins, only the parts between the breastbone and the navel are united. The united twins may grow up and one may die while the other lives. In such cases the surgeon has attempted to preserve the surviving individual by severing the bond between them, but in most instances with very little success. In his well-known work on the Pathology of the Foetus Dr. J. W. Ballantyne gives an account of a remarkable case which occurred in Scotland in the reign of James IV. Two boys were born so united that they were as one individual from the waist downwards, but in their upper parts formed two individuals joined or fastened to each other back to back. These " Scottish Brothers" were excellent musicians and linguists and lived to the age of twenty-eight, when one of them died. " For which many required of the other to be merry. He answered—' How can I be merry, that have my true marrow as dead as a carrion on my back, which was wont to sing and play with me ? Therefore I pray Almighty God to deliver me out of this present life' " —a prayer we may be sure that was soon granted. In a historical case which dates back to the twelfth century a pair of twin sisters, known as the " Biddenden Maids," were born in Kent, and were found to be joined from the waist downwards but fastened together side by side. Among the specimens before us there are monsters similar in form to the " Scottish Brothers " and " Biddenden Maids," and many others which illustrate various modes of union. Such united twins are usually placed side by side or face to face. The union however may be head to head or the opposite ends of the body may be joined. All degrees of union, from a slight bond to an almost complete fusion of the two individuals, may be seen. There is one peculiar form in which the heads are fused face to face. In such cases there appear to be two faces, but when they are minutely examined it is seen that these faces are compound, the right half belonging to one head, while the left half belongs to another. Yet so similar are the halves and so accurately are they joined that they appear to form the symmetrical face of one individual, while as a matter of fact they are the united halves of two individuals.