5. The union proceeding, and this simpli city of the lower part of the body being re tained, examples come next in which the upper parts also are united ; the two super fluous upper limbs being united into one, pre senting a single upper-arm, with a double fore-arm and hand, or a single upper-arm, fore-arm, and hand with ten fingers, or only a malformed limb, or a mere projection, occu pying the place of the superfluous limbs.
6. Even this last indication of duplicity of the upper parts ceases, a scapula only remains, or this also is absent ; and next, one of the sterna having disappeared, and the vertebral columns having been connected on the corre sponding side by their respective ribs united into single arches, these now become gradually shorter, and the columns approach each other more and more nearly, till they are connected by only a cartilaginous substance in the place of ribs, or are at some part fused together.
7. In this next group both the upper and lower parts of the body are single ; the ver tebral column is single below the cervical region, or exhibits only a trace of duplicity (to which something similar is often presented by the sternum), but at the cervical region becomes double, and on each portion bears a head. Of this also I have published many examples.
8. In the eighth group the extent of that cervical part of the column which is double becomes less and less.
9. In the ninth group, the two heads are seated on an apparently single neck, in which all the cervical vertebrm are single, or only bear traces of duplicity, except the first two, or the first alone.
10. Hitherto the duplicity of the head was perfect ; in this group the two heads also begin to coalesce, and, in a considerable num ber of cases, gradations are traced in which the adjacent ears are very closely approxi mated, and the heads are united behind. Then there are cases in which one ear only is placed between the adjacent surfaces of the two heads, and this disappears gradually, and at last totally. Next in order are the cases in which the adjacent ears being lost, the two adjacent and middle eyes first become very close, and then occupying one orbit finally coalesce. Next come those cases in which there is such a union of the heads, that the two upper jaws are articulated with one lower jaw; and, lastly, those in which the head is doubled only in individual parts or in which there is one perfect heart, with some imperfect part or parts of another attached to it.
11. The eleventh group of this division includes the cases of lateral duplicity, in which the body is single in the middle, but doubled above and below (or in brutes ante riorly and posteriorly). In these, which are
of rarer kinds, a single neck bears two more or less completely separated heads. The vertebral column is for a considerable length single, but at its lower part again divides, and bears two sets of lower extremities.
12. In the twelfth group the body is single above and doubled below.
13. In this there is a tendency towards singleness, or even complete singleness, of the head, but all parts of the trunk and all the limbs are doubled, an arrangement by which, as al ready stated, these form a series entirely dis tinct from the rest. In some of these cases the two heads are found coalesced below ; in others, to which the name of janiceps has been given, one face is directed backwards and the other forwards, the remainders of the two heads being merged into one ; in others, one face is perfectly, the other very deficiently, developed ; in others there are only the indis tinct traces of a second head presented in the existence of one or two ears on the posterior aspect of the more perfect one ; in others, this trace of duplicity is still less evident ; and, lastly, in the remainder of the group, it is entirely lost, and one head only, which may be well or ill formed, is found upon the double body.
3. Inferior duplicity.
My third division of double monsters in cludes the cases in which there are two com plete bodies with the lower portions of their respective trunks united, so that there is a head with upper extremities both above and below (the bodies being placed in the same straight line) and on either side of the part at which they meet two lower limbs. One may best conceive this arrangement by sup posing two children stuck together by their buttocks, and so fixed with wide-spreading lower limbs, as may be seen in fig. 146. of the second volume of this Cyclopmdia. A com mon body is thus formed with a head at each end, with two upper limbs both above and below, and with two lower limbs, one belong ing to each fetus on the right and two on the left of the united portions. A few cases only of this remarkable monstrosity are recorded ; and in these the duplicity was not always complete, but exhibited in some the same tendency towards singleness as was noticed in the others. Thus in some there were but three lower extremities; in others there were but two, or two with a third ill-developed on the other side: and, again, in other groups there were those which have a perfect head at one end of the trunk, but an imperfect one or none at all at the other.