2. Lateral duplicity.
The varieties of form in anteriorly duplex monsters are closely limited by the partial nature of the union of the sterna and the nearly complete distinctness of the thoracic cavities, and hence of the whole upper part of the body. In the next class, which I call lateral duplicity, there is no such limit ; and between the highest degree of duplicity found in it and the lowest, or that in which the duplicity is most nearly reduced to singleness, there is a far more numerous series of intermediate forms than in any other of the types of double monsters. In lateral duplicity, the two bodies are not set opposite to one another, but are turned sideways from one another. They have a common thoracic cavity, for the forma tion of which (at least, in the highest degree of duplicity,) the right ribs of one body, and the left of the other, proceed towards the an terior and posterior aspects, and are there connected with an anterior and posterior sternum. The best idea of the construction of this osseous fabric may be formed by sup posing the two complete chests of two bodies to be set one against the other, and that then the anterior extremities of the right ribs of the right body, and those of the left ribs of the left body, unite with one sternum and pull it forwards, while, in tile same manner, the left ribs of the right body, and the right of the left, unite on the posterior aspect with the other sternum, and carry it back wards. The consequence is, that the two vertebral columns are turned away from one another, and that the parts above and below the thorax are double. By the formation of this common thorax, the lateral is distin guished from the anterior duplicity, in which the thoraces are commonly connected only by the points of the sterna, and, as to their cavities, are separate. And with these diffe rences of external construction, others not less important, of internal arrangement, coin cide, which fully justify the separation of the two forms, however similar the external ap pearances of many of the examples of either may be.
The numerous varieties of lateral duplicity may be divided into two principal sets. The
first begins with the complete duplicity of the whole body, and ends with its perfect sin gleness; in the second, the duplicity of the body remains, but the head gradually becomes single. The forms included herein, of which I have given ample accounts elsewhere, may be briefly summed up as follows.
I. Complete duplicity : — all the external parts and sometimes the abdominal and pelvic viscera double, —one common double-sized thoracic cavity, formed in the manner just described, and containing four lungs, and (in all cases with which I have been acquainted) only one heart. The examples of this form are very numerous, and are to be met with in all the large museums of Europe.
2. In the examples of this second group, which exhibits the first step towards single ness, one of the sterna may be traced in a succession of specimens, becoming gradually narrower, and permitting a closer approxima tion of the two corresponding upper extremi ties, till, in some examples, they are com pletely united, and there are found only three limbs above, with three or four below. The two juxtaposed scapulw, for example, are merged into one, or they remain separate, but have only one humerus between them, and this splits below, to articulate with two fore-arms ; or there is but one fore-arm, and this bears supernumerary fingers. In short by a great variety of modes there is a general tendency towards union of two of the upper extremities.
3. In the third group we have a repetition of the same series of changes in the lower limbs, as in the second was traced in the upper ; here, as there, presenting numerous varieties, in the last and lowest of which only three lower limbs, and the third of these ill formed, are found.
4. The third limb has now gradually disap peared, and, with a complete duplicity above the pelvis, there are but two limbs below it, and these well formed. In this class is placed, with many others, the Ritta-Christina monster, described by Serres, which lived to eight months, and the still more remarkable ex ample mentioned by Buchanan, of a two headed man, 28 years old, who lived in the reign of James III. of Scotland.