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'etudes medicales_), published in 1803, Burdin assimilated the skull to the vertebral column.
Oken, in an inaugural dissertation (Programm) _Ueber die Bedeutung der Schadelknochen_,[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 Dumeril read a paper to the Academie 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 vertebrae.
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 vertebrae. Bojanus in his _Anatome testudinis europaeae_ (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 vertebrae; it was quite another to demonstrate the relation of the separate bones of the skull to the supposed vertebrae.
Upon this much uncertainty reigned; there was not even unanimity as to the number of vertebrae to be distinguished. Goethe found six vertebrae in the skull; Spix, and at first Oken, three only, Geoffroy seven; the accepted orthodox number seems to have been four (Bojanus, Oken, Owen).
As an example of the method of treatment adopted we may take Oken's matured account of the composition of the cranial vertebrae, as given in the English translation of his _Lehrbuch_. "To a perfect vertebra," he says, "belong at least five pieces, namely, the body, in front the two ribs, behind the two arches or spinous processes" (p. 370). In the cervical vertebrae the transverse processes represent the ribs. The skull consists of four vertebrae, the occipital, the parietal, the frontal and the nasal, or, named after the sense with which each is associated, the auditory, the lingual, the ocular and the olfactory. The "bodies" of these vertebrae are the body of the occipital (basioccipital), the two bodies of the sphenoid (basi- and pre-sphenoid), and the vomer. The transverse processes of each are the condyles of the occipitals (exoccipitals), the alae of the two sphenoids (alisphenoids and orbitosphenoids) and the lateral surfaces of the vomer. The arches or spinous processes are the occipital crest, the parietals, the frontals, and the nasals.
The cranium is thus composed of four rings of bone, each composed of the typical elements of a vertebra.
The arbitrary nature of the comparison is obvious enough. As Cuvier pointed out in the posthumous edition of his _Lecons_, it is only the occipital segment that shows any real analogy with a vertebra--an analogy which Cuvier ascribed to similarity of function. He admitted a faint resemblance of the parietal segment to a vertebra:--"The body of the sphenoid does indeed look like a repetition of the basioccipital, but having a different function it takes on another form, especially above, by reason of its posterior clinoid apophyses."[157] He denied the resemblance of the frontal and nasal "vertebrae" to true vertebrae, pointing out that both parietals and frontals are bones specially developed for the purpose of roofing over and protecting the cerebrum.
A very curious development was given to the vertebral theory by K. G.
Carus, who seems to have taken as his text a saying of Oken's, that the whole skeleton is only a repeated vertebra.[158] His system is worthy of some consideration, for he tries to work out a geometry of the skeleton.[160]
His method of deduction is a good example of pure _Naturphilosophie_.
Life, he says, is the development of something determinate from something indeterminate. A finite indeterminate thing, that is, a liquid, must take a spherical form if it is to exist as an individual.
Hence the sphere is the prototype of every organic body. Development takes place by antagonism, by polarity, typically by the division and multiplication of the sphere. In the course of development the sphere may change, by expansion into an egg-shaped body, or by contraction into a crystalline form, the changes due to expansion being typical of living things, those due to contraction being typical of dead. At the surface of the primitive living sphere is developed the protective _dermatoskeleton_, which naturally takes the shape of a hollow sphere; round the digestive cavity which is formed in the living sphere is developed the _splanchnoskeleton_; round the nervous system (which is, as it were, the animal within the animal) is developed the _neuroskeleton_. All skeletal formations belong to one or other of these systems.
Carus defines his aim to be the discovery of the inner law which presides over the formation of the skeleton throughout the animal kingdom; he desires to know "how such and such a formation is realised in virtue of the eternal laws of reason" (iii., p. 93). Here we touch the kernel of _Naturphilosophie_--the search for rational laws which are active in Nature; the discontent with merely empirical laws.
The thesis which Carus sustains is that all forms of skeleton, whether of dermatoskeleton, splanchnoskeleton, or neuroskeleton, can be deduced from the hollow sphere, which is the primary form of any skeleton whatsoever (p. 95). That means, put empirically, that every skeleton can be represented schematically by a number of hollow spheres, suitably modified in shape, and suitably arranged. The chief modification in shape exhibited by bones is one which is intermediate between the organic and the crystalline series of modifications of the sphere. The organic modifications are bounded by curved lines, the crystalline by straight; the intermediate partly by curved and partly by straight lines. They are the dicone (the shape of a diabolo) and the cylinder.
These forms must necessarily be of importance for the skeleton, which is intermediate between the organic and the inorganic. "The dicone embodies the real significance of the bone," writes Carus. Each dicone and cylinder composing the skeleton is called by Carus a vertebra.
We may expect then all skeletons to be composed of spheres, cylinders and dicones in diverse arrangements. Nature being infinite, all the possible types of arrangement of these elements must exist in the test or skeleton of some animal, living, fossil, or to come (p. 127). One conceives easily what the main types of skeleton must be. In some animals, _e.g._, sea-urchins, the skeleton is a simple sphere; in others, _e.g._, starfish, secondary rows of spheres radiate out from a central sphere or ring; in annulate animals the skeleton consists of a row of partially fused spheres.
In Vertebrates the arrangement is more complex. There are first the protovertebral rings of the dermatoskeleton, these being principally the ribs, limb-girdles, and jaws. Round the central nervous system are developed the deutovertebral rings of the neuroskeleton (vertebrae in the ordinary sense). The apophyses and bodies of the vertebrae, and the bones of the members[160] are composed of columns of tritovertebrae, or vertebrae of the third order. Thus the whole vertebrate skeleton is a particular arrangement of vertebrae, which in their turn are modifications of the primary hollow sphere.
The German transcendentalists were more or less contemporary with E.
Geoffroy, and no doubt influenced him, especially in his later years, as they certainly did his follower Serres. Oken indeed wrote, in a note[161]
appended to Geoffroy's paper on the vertebral column of insects, that "Mr Geoffroy [_sic_] is without a doubt the first to introduce in France _Naturphilosophie_ into comparative anatomy, that is to say, that philosophy one of whose doctrines it is to seek after the _signification_ of organs in the scale of organised beings." This is, however, an exaggeration, for Geoffroy was primarily a morphologist, whereas the morphology of the German transcendentalists was only a side-issue of their _Naturphilosophie_.
Geoffroy, on his part, exercised some influence on the transcendentalists. He asserts[162] indeed that Spix got some of the ideas published in the _Cephalogenesis_ (1815) from attending his course of lectures in 1809. It is certainly the case that Spix published before Geoffroy the view that the opercular bones are homologous with the ear-ossicles, adopting, however, a different homology for the separate bones.[163]
Some speculations seem to have been common to both schools--for instance, the law of Meckel-Serres, the vertebral theory of the skull, and the recognition of serial homology in the appendages of Arthropods (Savigny, Oken). Latreille and Duges, as well as Serres, clearly show in their theoretical views the influence of Oken and the other transcendentalists. Geoffroy's principle of connections and law of compensation were recognised by some at least of the Germans.
But whatever his actual historical relations may have been with the German school, Geoffroy was vastly their superior in the matter of pure morphology. He alone brought to clear consciousness the principles on which a pure morphology could be based: the Germans were transcendental philosophers first, and morphologists after.
One understands from this how J. F. Meckel, who was in some ways the leading comparative anatomist in Germany at this time, could be at once a transcendentalist and an opponent of Geoffroy. Meckel had a curiously eclectic mind. A disciple of Cuvier, having studied in 1804-6 the rich collections at the Museum in Paris, the translator of Cuvier's _Lecons d'anatomie comparee_, he earned for himself the title of the "German Cuvier," partly through the publication of his comprehensive textbook (_System der vergl. Anatomie_, 5 vols.), partly by his extensive and many-sided research work, partly by his authoritative teaching. His _System_ shows in almost every page of its theoretical part the influence of Cuvier; and it is through having assimilated Cuvier's teaching as to the importance of function that Meckel combats Geoffroy's law of connections, at least in its rigorous form. He submits that the connections of bones and muscles must change in relation to functional requirements. He rejects Geoffroy's theory of the vertebrate nature of Articulates. Generally throughout his work the functional point of view is well to the fore.
Yet at heart Meckel was a transcendentalist of the German school. His vagaries on the subject of "homologues" leave no doubt about that, and, in spite of Cuvier, he believed, though not very firmly, in the existence of one single type of structure.
A Cuverian by training, his lack of morphological sense threw him into the ranks of the transcendentalists, to whom perhaps he belonged by nature.
[141] For a full account, see Kohlbrugge, _Zool. Annalen_, xxxviii., 1911.
[142] _Rede uber das Verhaltnis der organischen Krafte_, Stuttgart u. Tubingen, 1793 (1814). See Radl, _loc.
cit._, i., p. 261; ii., p. 57.
[143] _Supplem. ad historiam embryonis_, Tubingen, 1797.
[144] _Lehrbuch der Naturphilosophie_, Eng. trans., p.
491, 1847.
[145] _Ueber Entwickelungsgeschichte der Thiere_, i., p.
xvii., 1828.
[146] _Zoologie_, Landshut, i., 1808.
[147] _Anatomie u. Bildungsgeschichte des Gehirns im Fotus des Menschen_, Nurnberg, 1816.
[148] _Beytrage zur vergleichende Anatomie_, Leipzig, i., 1808-9, ii., 1811-2.
[149] Cetacea were generally considered at this time to be mammals of low organisation.
[150] From the French trans., which appeared under the title _Traite gen. d'Anat. comparee_, i., p. 449, 1828.
[151] _Cf._ Geoffroy (_supra_, p. 70).
[152] _Beytrage_, ii., 2, 1812. Also in his _System d.
vergl. Anat._, i., 1821.
[153] In J. F. Meckel's _Beytrage_, ii.
[154] _Zur Morphologie_, i., 2, p. 250, 1820; and ii., 2, pp. 122-4, 1824.
[155] See translation, giving the gist of this paper, in Huxley's _Lectures on the Elements of Comparative Anatomy_, pp. 282-6, London, 1864.
[156] Reil's _Archiv. f. Physiol._, vii., 1807.
[157] _Lecons d'anatomie comparee_, 3rd ed., Brussels reprint, i., p. 414, 1836.
[158] In his Programm, _U. d. Bedeut. d. Schadelknochen_, 1807.
[159] _Traite elementaire d'anatomie comparee_ (French trans.), vol. iii., Paris, 1835. First developed in his volume _Von den Ur-Theilen des Knochen und Schalen-Gerustes_, Leipzig, 1828.
[160] Dutrochet in 1821 had tried to prove that the bones of the members belong to the type of the vertebra--the dicone.
[161] _Isis_, pp. 552-9, 1820 (2).
[162] _Mem. Mus. d'Hist. nat._, ix., 1822.
[163] Cuvier and Valenciennes, _Hist. nat. Poissons_, i., p. 311, f.n.