Form and Function

E >> E. S. (Edward Stuart) Russell >> Form and Function

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33

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 Coelentera, where genital and digestive organs occupy the same
cavity. In his view the uterus corresponded to the stomach, the vagina
to the oesophagus, 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 vertebrae. Goethe seems to have been the first to hit upon the idea
that the skull is composed of a number of vertebrae, 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'Etudes medicales_), published in 1803, Burdin
assimilated the skull to the vertebral column.

Oken, in an inaugural dissertation (Programm) _Ueber die Bedeutung der
Schaedelknochen_,[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 ueber das Verhaeltnis der organischen Kraefte_,
Stuttgart u. Tuebingen, 1793 (1814). See Radl, _loc.
cit._, i., p. 261; ii., p. 57.

[143] _Supplem. ad historiam embryonis_, Tuebingen, 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 Foetus
des Menschen_, Nuernberg, 1816.

[148] _Beytraege 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] _Beytraege_, ii., 2, 1812. Also in his _System d.
vergl. Anat._, i., 1821.

[153] In J. F. Meckel's _Beytraege_, 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. Schaedelknochen_,
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.

CHAPTER VIII

TRANSCENDENTAL ANATOMY IN ENGLAND--RICHARD OWEN

Richard Owen is the epigonos of transcendental morphology; in him its
guiding ideas find clear expression, and in his writings are no
half-truths struggling for utterance. But he was, though a staunch
transcendentalist, an eclectic of the older ideas current in his time;
for he picked out what was best in the older systems--Cuvier's
teleology, Geoffroy's principle of connections, Oken's idea of the
serial repetition of parts. In particular, he assimilated the teaching
of Cuvier, the great opponent of the transcendentalists, and reconciled
it in part with his own transcendentalism. His main theoretical views
are to be found in his volume _On the Archetype and Homologies of the
Vertebrate Skeleton_ (London, 1848). The master-idea of the book is that
the vertebrate skeleton consists of a series of comparable segments,
each of which Owen calls a vertebra. His definition of a vertebra is,
"one of those segments of the endo-skeleton which constitute the axis of
the body, and the protecting canals of the nervous and vascular trunks"
(p. 81). The parts of a typical vertebra are shown in Fig. 4, which is
copied from Owen's Fig. 14.

|||
zygapophysis ||| -- neural spine
\ |||
*//^\\*
diapophysis // \\ -- neurapohysis
\ // o \\
===== --- =====
/ \
===== |CENTRUM| O ===== -- peiurapophysis
\ /
===== --- =====
/ \\ //
parapophysis *\\v//*
/ |||
zygapophysis ||| -- haemal spine
|||

FIG. 4.--Ideal Typical Vertebra. (After Owen.)

In Fig. 5 (page 103) is shown an actual vertebra, as Owen conceives it,
the "vertebra" being that of a bird.

[Illustration: FIG. 5.--Natural Typical Vertebra; Thorax of a Bird.
(After Owen.)]

A segment of sternum is included as the "haemal spine" of the vertebra
(_hs_); the vertebral rib is the "pleurapophysis" (_pl_); the sternal
rib the "haemapophysis" (_h_); the uncinate process of the vertebral rib
is known as the "diverging appendage" (_a_). The whole vertebrate
skeleton is composed of a series of vertebrae which show these typical
parts. We arrive thus at the conception of an "Archetype" of the
vertebrate skeleton, such as is represented in Fig. 6.

The archetype is only a scheme of what is usually constant in the
vertebrate skeleton, and both the number and the arrangement of the
bones in any real Vertebrate are subject to variation. "It has been
abundantly proved," Owen writes, towards the end of his volume, "that
the idea of a natural segment (vertebra) of the endoskeleton does not
necessarily involve the presence of a particular number of pieces, or
even a determinate and unchangeable arrangement of them. The great
object of my present labour has been to deduce ... the relative value
and constancy of the different vertebral elements, and to trace the kind
and extent of their variations within the limits of a plain and obvious
maintenance of a typical character" (p. 146).

It goes without saying that Owen considered the skull to be formed of
vertebrae--the vertebral theory of the skull was, in his system, a
deduction from the vertebral theory of the skeleton. He recognised four
cranial vertebrae; the arrangement of them, and the relation of their
constituent bones to the parts of the typical vertebra are shown in the
table appearing on page 106. So far as their first three elements are
concerned, these vertebrae are practically identical with the vertebrae
distinguished in the classical vertebral theory of the skull, as
enunciated by Oken. A divergence appears with the determination of the
other elements of the vertebrae. The upper and lower jaws are associated
with the nasal and frontal vertebrae respectively, not however as limbs
of the head, but as constituent elements of these vertebrae. In the same
way the hyoid apparatus is part and parcel of the parietal vertebra, and
the pectoral girdle and fore-limbs part of the occipital vertebra.

[Illustration: FIG. 6.--The Archetype of the Vertebrate Skeleton. (After
Owen.)]

Cranial Vertebrae.[164] (After Owen, 1848, p. 165.)

+---------------+---------------+----------------+---------------+-------------+
| Vertebrae. | Occipital. | Parietal. | Frontal. | Nasal. |
+===============+===============+================+===============+=============+
|Centra. |Basioccipital. |Basisphenoid. |Presphenoid. |Vomer. |
+---------------+---------------+----------------+---------------+-------------+
|Neurapophyses. |Exoccipital. |Alisphenoid. |Orbitosphenoid.|Prefrontal. |
+---------------+---------------+----------------+---------------+-------------+
|Neural Spines. |Supraoccipital.|Parietal. |Frontal. |Nasal. |
+---------------+---------------+----------------+---------------+-------------+
|Parapophyses. |Paroccipital. |Mastoid. |Postfrontal. |None. |
+---------------+---------------+----------------+---------------+-------------+
|Pleurapophyses.|Scapular. |Stylohyal. |Tympanic. |Palatal. |
+---------------+---------------+----------------+---------------+-------------+
|Haemapophyses. |Coracoid. |Ceratohyal. |Articular. |Maxillary. |
+---------------+---------------+----------------+---------------+-------------+
|Haemal Spines. |Episternum. |Basihyal. |Dentary. |Premaxillary.|
+---------------+---------------+----------------+---------------+-------------+
| Diverging |Fore-limb or |Branchiostegals.|Operculum. |Pterygoid and|
| Appendage. | Fin. | | | Zygoma. |
+---------------+---------------+----------------+---------------+-------------+

Owen's reasons for considering the pectoral girdle and the fore-limb
part of the occipital vertebra are as follows. In fish the pectoral
girdle is slung to the skull by means of the post-temporal bone
(supra-scapula, according to Owen) which abuts on the occipital arch. In
_Lepidosiren_, whose skeleton resembles the archetype in many ways, the
pectoral girdle is likewise attached to the occipital segment.

In most other Vertebrates the pectoral girdle has shifted backwards
along the vertebral column, by a "metastasis" (Geoffroy) similar to that
by which the pelvic fins in many fish have shifted up close to the
pectoral girdle. The scapula (with supra-scapula) is the pleurapophysis,
the coracoid the haemapophysis, of the occipital vertebra. The clavicle
is homologised with the slender bone in fish now known as the
post-clavicle, which shows a connection with the first or atlas vertebra
of the vertebral column, forming, according to Owen, the haemapophysis of
the atlas. Owen considers it no objection to this view that in other
Vertebrates the clavicle is anterior to the coracoid--"its anterior
position to the coracoid in the air-breathing Vertebrata is no valid
argument against the determination, since in these we have shown that
the true scapular arch is displaced backwards" (_On the Nature of
Limbs_, p. 63, London, 1849). In the pelvic girdle the ilium corresponds
to the scapula, the ischium to the coracoid, the pubis to the clavicle.
Hence the ilium is a pleurapophysis, the ischium and pubis are both
haemapophyses. The fore-limb is the developed "appendage" of the
occipital vertebra, the hind-limb the developed "appendage" of the
pelvic vertebra. They are serially homologous with, for example, the
uncinate processes of the ribs in birds (see Figs. 5 and 6). The
fore-limb is a simple filament in _Lepidosiren_, and presents few joints
in _Proteus_ and _Amphiuma_; in other air-breathing Vertebrates it shows
a more complete development, the humerus, radius and ulna, and the bones
of the wrist and hand becoming differentiated out.

As the fore-limb is equivalent to a single bone of the archetype, it is
said to be, in its developed state, "teleologically compound" (p. 103).

Since in the archetype every vertebra has its appendage, more than two
pairs of locomotory limbs might have been developed. "Any given
appendage might have been the seat of such developments as convert that
of the pelvic arch into a locomotive limb; and the true insight into the
general homology of limbs leads us to recognise many potential pairs in
the typical endoskeleton. The possible and conceivable modifications of
the vertebrate archetype are far from having been exhausted in the forms
which have hitherto been recognised, from the primaeval fishes of the
palaeozoic ocean of this planet up to the present time" (p. 102). It is
not of the essence of the vertebrate type to be tetrapodal.

In determining homologies Owen remained true to Geoffroy's principle of
connections. Speaking of an attempt which had been made to determine
homologies by the mode of development, he writes, "There exists
doubtless a close general resemblance in the mode of development of
homologous parts; but this is subject to modification, like the forms,
proportions, functions, and very substance of such parts, without their
essential homological relationships being thereby obliterated. These
relationships are mainly, if not wholly, determined by the relative
position and connection of the parts, and may exist independently of
form, proportions, substance, function and similarity of development.
But the connections must be sought for at every period of development,
and the changes of relative position, if any, during growth, must be
compared with the connections which the part presents in the classes
where vegetative repetition is greatest and adaptive modification least"
(p. 6). It is interesting to note that in Owen's opinion comparative
anatomy explains embryology. Thus the scapula, which is the
pleurapophysis of the occipital vertebra, is vertical on its first
appearance in the embryo of tetrapoda, and lies close up to the head
(_On the Nature of Limbs_, p. 49)--the embryo shows a greater
resemblance to the archetype than the adult. "We perceive a return to
it, as it were, in the early phases of development of the highest
organised of the actually existing species, or we ought rather to say
that development starts from the old point; and thus, in regard to the
scapula, we can explain the constancy of its first appearance close to
the head, whether in the human embryo or in that of the swan, also its
vertical position to the axis of the spinal column, by its general
homology as the rib or 'pleurapophysis' of the occipital
vertebra" (_Limbs_, p. 56).

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33