SYMMETRY. 



845 



fluid, but must look upon it merely as present 

 in excess in the diseases above alluded to. 

 (G. Owen Rccs.} 



SYMMETRY (W ptrpov). In its general 

 acceptation this word means a just and har- 

 monious proportionment of parts to one 

 another and to their whole ; in anatomy, how- 

 ever, it has a different and more restricted 

 meaning. With its anatomical signification 

 alone I have now to deal, and that may be 

 defined as follows : Symmetry is a word 

 used to express an idea that would be more 

 correctly represented by a verb than a noun, 

 for it is the idea of not a thing but a fact 

 the fact, namely, that one half of an animal is 

 usually an exact reversed copy of the other 

 the right side is a reversed copy, or repetition, 

 of the left. To this there are numerous ex- 

 ceptions, even in the human subject ; of which 

 hereafter. 



That unreversed serial copying or repetition 

 which is observable, for instance, between the 

 scapular and pelvic limbs, is enunciated by the 

 analogous expression serial homology. The 

 point on which a distinction may be made 

 between symmetry and homology, is that of 

 the reversing of the copy or repetition as 

 characteristic of the latter. This characteristic 

 seems to impress one with the notion that the 

 two halves are parts of a whole, whereas an 

 unreversed serial recurrence of similar parts 

 inclines one to accord a kind of separate in- 

 dividuality to each repetition. A clear dis- 

 tinction between these two styles of repetition 

 ought undoubtedly to be firmly impressed and 

 maintained on the mind. I have, however, 

 for want of a convenient inflection of the 

 word under consideration, at the risk of some 

 confusion, been long accustomed to use the 

 expression lateral homology in reference to 

 symmetrical repetition and in that sense I 

 shall have to use it in this article. 



Whether the word symmetry should be 

 applied to that antero-posterior repetition 

 which is met with in caudal vertebrae of fishes, 

 for instance, is not yet determined by usage, 

 and it will be sufficient for me hereafter simply 

 to make my remarks upon the apparent exist- 

 ence of it. In so doing I shall use the ex- 

 pression antei'o-posterior homology in a sense 

 precisely parallel to that of lateral homology. 



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 Pleuronectidae, 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 haemal 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- 

 tebras 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 vertebrae, 

 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 "vertebrae 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 disentitling them to an indi- 

 viduality like that which we are accustomed 

 to assign to unreversed serial repetitions; it 



