Form and Function: A Contribution to the History of Animal Morphology

by E. S. Russell

Under the "theory of the repetition or multiplication of parts within the organism" may be included, first, generalisations on the serial homology of parts, and second, more or less confused attempts to demonstrate that the whole organisation is repeated in certain of the parts. The recognition of serial homologies constituted a real advance in morphology; the "philosophical" idea of the repetition of the whole in the parts led to many absurdities. It led Oken to assert that in the head the whole trunk is repeated, that the upper jaw corresponds to the arms, the lower to the legs, that in each jaw the same bony divisions exist as in the limbs, the teeth, for instance, corresponding to the claws (loc. cit., p. 408). It led him to distinguish "two animals" in every body—the cephalic and the sexual animal. Each of these has its own organs; thus "in the perfect animal there are two intestinal systems thoroughly distinct from each other, two intestines which belong to two different animals, the sexual and cephalic animal, or the plant and the animal" (p. 382). The intestine of the sexual animal is the large intestine; the lungs of the sexual animal are the kidneys, its glottis is the urethra, its mouth the anus. So, too, the mouth is the stomach of the head. On another line of thought the sternum is a ventral vertebral column. Limbs are connate ribs, the digits indicating the number of ribs included (cf. Dugès, supra, p. 88).

J. F. Meckel[152] discusses "homologies" of this kind in the thorough and pedestrian way so characteristic of him. Not only, he says, are the right and left halves of the body comparable with one another, but also the upper and the lower, the dividing line being drawn at the level of the diaphragm. The lumbar complex corresponds to the skull, the anus to the mouth, the urino-genital opening to the nasal opening; in general, the urino-genital system corresponds to the respiratory, the kidneys to the lungs, the ureters to bronchi, the testes and ovaries to the thymus (he had observed the physiological relation between the development of the thymus and the state of the genital organs), the prostate and the uterus to the thyroid gland, and the penis and clitoris to the tongue. The fore-limbs and girdle correspond in detail with the hind limbs and the pelvis—a point already worked out by Vicq d'Azyr; the dorsal and ventral halves of the body are likewise comparable in some respects, the sternum, for example, answering in the arrangement of its bones, muscles and arteries to the vertebral column. The skeleton of each member is in some respects a repetition of the vertebral column.

His brother, D. A. Meckel,[153] worked out an elaborate comparison between the alimentary canal and the genital organs, basing the legitimacy of the comparison upon early embryological relations and upon the state of things in Cœlentera, where genital and digestive organs occupy the same cavity. In his view the uterus corresponded to the stomach, the vagina to the œsophagus, the fallopian tubes to the intestine, and so on.

The vertebral theory of the skull took its origin from the same habit of thought. As part of the wider idea of the metameric repetition of parts it had some scientific worth, but the theory was pushed too far, and the facts were twisted to suit it. Among annulate animals the theory of repetition found ample scope; Oken was able to compare with justice the jaws of crabs and insects with their other limbs, as Savigny did later in a more scientific way. Among Vertebrates the application of the theory of serial repetition was not so obvious, except in the case of the vertebræ. Goethe seems to have been the first to hit upon the idea that the skull is composed of a number of vertebræ, serially homologous with those of the vertebral column. He tells us that the idea flashed into his mind when contemplating in the Jewish cemetery at Venice a dried sheep's skull. The discovery was made in 1790, but not published till 1820.[154]

The idea seems to have been taught by Kielmeyer, one of the earliest of the "philosophers of nature," but it was not published by him.

In a book (Cours d'Études médicales), published in 1803, Burdin assimilated the skull to the vertebral column.

Oken, in an inaugural dissertation (Programm) Ueber die Bedeutung der Schädelknochen,[155] published in 1807, gave to the theory its necessary development. Autenrieth, also in 1807,[156] distinguishing separate ganglia in the brain, was not far from the hypothesis that each of these ganglia must have its separate vertebra.

In 1808 Duméril read a paper to the Académie des Sciences in which he compared the skull to a gigantic vertebra, basing his hypothesis on the similarity existing between the crests and depressions on the hinder part of the skull and those on the posterior surfaces of the vertebræ.

After Oken's work the vertebral theory was taken up generally by both the German and the French anatomists. Spix published in 1815 a large volume on the skull, entitled Cephalogenesis, distinguishing (as Oken did at first) three cranial vertebræ. Bojanus in his Anatome testudinis europæae (1819), and in a series of papers in Isis (1817-1819, and 1821) established the existence of a fourth cranial vertebra, and this was accepted by Oken in the later editions of his Lehrbuch. Meckel and Carus among the Germans, de Blainville and E. Geoffroy among the French, contributed to the development of the theory. In England the theory was championed particularly by Richard Owen.

It was one thing to assert in a moment of inspiration that the skull was composed of modified vertebræ; it was quite another to demonstrate the relation of the separate bones of the skull to the supposed vertebræ. Upon this much uncertainty reigned; there was not even unanimity as to the number of vertebræ to be distinguished. Goethe found six vertebræ in the skull; Spix, and at first Oken, three only, Geoffroy seven; the accepted orthodox number seems to have been four (Bojanus, Oken, Owen).