LATERAL REPETITION.—That the right hand and foot, and the right side of the head and trunk, are the exact counterpart of the left is a fact so obvious, that merely to assert it seems an unnecessary truism. It should, however, by no means be regarded as a matter of course. It might have been otherwise.
The human skeleton is, normally, perfectly symmetrical in all its details, and so are the skeletons of all vertebrate animals, with the exception of the Pleuronectidm, or flat fishes, noticed hereafter. The archetype or abstract ideal figure of an osseous vertebral segment, as that of Prof. Owen, at vol. iii. p. 824., is a symmetrical form. But it is doubtful whether any single bone in the skeleton should be re garded as primordially mesial and symmetrical — whether any ossific point is originally in the middle line. The ideal archetype of the above illustrious author contains three mesial azygos elements, viz. the hmmal and neural spines and centrum ; but it has always ap peared to me that each of these elements should be represented in the ideal by a pair of pieces, because each of them is occasionally represented in nature by a pair of bones: Prof. Owen, for instance, regards the two parietal bones as the neural spine of a ver tebra. Though there is no difficulty in con ceiving the coalescence of any number of pieces into one, and though it is easy to con ceive that this coalescence may have occurred before the commencement of ossification, so that two or more of the points destined to be come the centres of the ossifying process may be brought so close together as, when manifest by the earthy deposit, to appear but as one, yet it is not possible to conceive that two pieces can be developed from one ossific point. The single azygos condition may proceed from the double, but the double cannot proceed from the single ; therefore the double condition must be re garded as the primordial, and should hold place in the abstract type. There are occasionally met with certain monstrosities which seem to show in a remarkable manner that a vertebra is composed of two lateral halves that are primordially separate. Thus, in double-headed
monsters, wherein there are two vertebral columns above, which coalesce and form one below, the half of each of the two columns which is adjacent to the other seems to be lost at the point of coalescence, and the single column below this point seems to be com posed of the right half of the one and the left half' of the other of the two columns. In the skeleton of the Boa Constrictor preserved in the Hunterian Museum, there are two ver tebrm that are double on the right side and single on the left, bearing two ribs on the right side and only one on the left ; or rather there are two specimens of right halves of vertebrm, existing independently, which are anchylosed, the one to the vertebra in advance of it, the other to the one behind it. This anchylosis alone justifies the expression "vertebrm double on the right side ;" but in neither instance is it so complete as by any means to mask the real nature of the independent half vertebra. Such facts as these, especially- the existence of one half of a vertebra without the other, even seem, in contradiction to the impression stated above, to claim for each half of a vertebral segment the importance of a separate indi viduality, such as is accorded to each ver tebral segment itself. They seem to show that a vertebra is as much a compound of two lateral parts symmetrically repeating one an other, as the human spinal column is a com pound of thirty-three serial repetitions of ver tebrae. The fact of the lateral halves being reversed copies of one another, I am disposed to regard as proof' of their being parts only of a whole, and as clisentitling them to an indi viduality like that which we are accustomed to assign to unreversed serial repetitions ; it may, however, be regarded as not sufficient proof of this. The value to be given to such facts depends, here as elsewhere, upon the peculiar bias of different minds, and the asso ciations that pre-occupy them. It is not necessary in the present state of our know ledge, nay, it is not expedient, to bind the mind down to this or that view of the facts that come before it.