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).
We owe to Owen the first clear distinction between "homologous" and
"analogous" organs; it was he who first proposed the terms "homologue"
and "analogue," which he defined as follows:--"_Analogue_. A part or
organ in one animal which has the same function as another part or organ
in a different animal." "_Homologue_. The same organ in different
animals under every variety of form and function."[165]
He introduced also useful distinctions between Special, General, and
Serial Homology. "The relations of homology," he writes, "are of three
kinds: the first is that above defined, viz., the correspondency of a
part or organ, determined by its relative position and connections, with
a part or organ in a different animal; the determination of which
homology indicates that such animals are constructed on a common type;
when, for example, the correspondence of the basilar process of the
human occipital bone with the distinct bone called 'basi-occipital' in a
fish or crocodile is shown, the _special homology_ of that process is
determined. A higher relation of homology is that in which a part or
series of parts stands to the fundamental or general type, and its
enunciation involves and implies a knowledge of the type on which a
natural group of animals, the Vertebrate, for example, is constructed.
Thus when the basilar process of the human occipital bone is determined
to be the 'centrum' or 'body' of the last cranial vertebra, its _general
homology_ is enunciated.
"If it be admitted that the general type of the vertebrate endoskeleton
is rightly represented by the idea of a series of essentially similar
segments succeeding each other longitudinally from one end of the body
to the other, such segments being for the most part composed of pieces
similar in number and arrangement, and though sometimes extremely
modified for special functions, yet never so as to wholly mask their
typical character--then any given part of one segment may be repeated in
the rest of the series, just as one bone may be reproduced in the
skeletons of different species, and this kind of repetition or
representative relation in the segments of the same skeleton I call
'serial homology'" (p. 7). As an example of serial homology we might
take the centra of the vertebrae--the vomer, the presphenoid, the
basisphenoid, the basioccipital and the series of centra in the spinal
column. Such serially repeated parts are called _homotypes_ (p. 8).
Not all the bones of the vertebrate skeleton are included in the
archetype as constituents of the vertebrae. Thus the branchial and
pharyngeal arches are accounted part of the splanchnoskeleton, as
belonging to the same category as the heart bone of some ruminants, and
the ossicles of the stomach in the lobster (p. 70). The ossicles of the
ear in mammals are "peculiar mammalian productions in relation to the
exalted functions of a special organ of sense" (p. 140, f.n.). This
recognition of a possible development of new organs to meet new
functions shows unmistakably the influence of Cuvier. Owen was indeed
well aware of the importance of the functional aspect of living things,
and he often adopted the teleological point of view. As a true
morphologist, however, he held that the principle of adaptation does not
suffice to explain the existence of special homologies. The ossification
of the bones of the skull from separate centres may be purposive in
Eutheria, in that it prevents injury to the skull at birth; but how
explain on teleological principles the similar ossification from
separate centres in marsupials, birds and reptiles? How explain above
all the fact that the centres are the same in number and relative
position in all these groups? Surely we must accept the idea of an
archetype "on which it has pleased the divine Architect to build up
certain of his diversified living works" (p. 73).
In his study of centres of ossification, Owen made in point of theory a
distinct advance on his predecessors. We saw that Geoffroy recognised
the importance of studying the ossification of the skeleton, and that
Cuvier accepted such embryological evidence as an aid in determining
homologies. Owen pointed out that it was necessary to distinguish
between centres of ossification which were teleological in import and
such as were purely indicative of homological relationships. Many bones,
single in the adult, arise from separate centres of ossification, but we
must distinguish between "those centres of ossification that have
homological relations, and those that have only teleological ones;
_i.e._, between the separate points of ossification of a human bone
which typify vertebral elements, often permanently distinct bones in the
lower animals; and the separate points which, without such
signification, facilitate the progress of osteogeny, and have for their
obvious final cause the well-being of the growing animal" (p. 105).
There is, for example, a teleological reason why in mammals and leaping
Amphibia (_e.g._, frogs), the long bones should ossify first at their
ends, for the brain is thus protected from concussion; in reptiles that
creep there is less danger of concussion, and the long bones ossify in
the middle (p. 105). But there is no teleological reason why the
coracoid process of the scapula should in all mammals develop from a
separate centre. The coracoid is however a real vertebral element
(haemapophysis), and in monotremes, birds and reptiles it is in the adult
a large and separate bone. Its ossification from a separate centre in
mammals has therefore a homological significance. The scapula in mammals
is an example of what Owen calls a "homologically compound" bone. All
those bones which are formed by a coalescence of parts answering to
distinct elements of the typical vertebra are "homologically compound"
(p. 105). On the other hand, "All those bones which represent single
vertebral elements are 'teleologically compound' when developed from
more than one centre, whether such centres subsequently coalesce, or
remain distinct, or even become the subject of individual adaptive
modifications, with special joints, muscles, etc., for particular
offices" (p. 106). The limb-skeleton, corresponding as it does to a
single bone of the archetype, is the typical example of a teleologically
compound bone. Owen in his definition of teleological compoundness has
combined two kinds of adaptation--(1) temporary adaptation of bones to
the exigencies of development, birth and growth (_e.g._, development of
long bones from separate centres); (2) definitive adaptation of a
skeletal part to the functions which it has to perform (_e.g._,
teleological structure of limbs). Such adaptations are, so to speak,
grafted on the archetype.
Owen's general views on the nature of living things merit some
attention. Organic forms, according to Owen, result from the
antagonistic working of two principles, of which one brings about a
vegetative repetition of structure, while the other, a teleological
principle, shapes the living thing to its functions. The former
principle is illustrated in the archetype of the vertebrate skeleton, in
the segmentation of the Articulates, in the almost mathematical symmetry
of Echinoderms, and the actually crystalline spicules of sponges. It is
the same principle which causes repetition of the forms of crystals in
the inorganic world. "The repetition of similar segments in a vertebral
column, and of similar elements in a vertebral segment, is analogous to
the repetition of similar crystals as the result of polarising force in
the growth of an inorganic body" (p. 171). This "general polarising
force" it is which mainly produces the similarity of forms, the
repetition of parts, and generally the signs of the unity of
organisation. The adaptive or "special organising force" or [Greek:
idea], on the other hand, produces the diversity of organic beings. In
every species these two forces are at work, and the extent to which the
general polarising or "vegetative-repetition-force" is subdued by the
teleological is an index of the grade of the species.
This view is analogous to the Geoffroyan conception that the diversity
of form is limited by the unity of plan. Owen thus ranges himself with
Geoffroy against Cuvier, who considered that diversity of form is
limited only by the principle of the adaptation of parts.
[164] Owen introduced most of the names of bones now
current.
[165] _Lectures on Invertebrate Animals_, pp. 374, 379,
1843.