THE SKELETON OF THE NEWT (_Molge cristata_).
I. EXOSKELETON.
The skin of the Newt is quite devoid of any exoskeletal structures.
The only exoskeletal structures that the animal possesses are the
teeth, and these are most conveniently described with the
endoskeleton.
II. ENDOSKELETON.
The endoskeleton of the Newt, though ossified to a considerable
extent, is more cartilaginous than is that of the frog. It is
divisible into an =axial portion= including the vertebral column,
skull, ribs, and sternum, and an =appendicular portion= including the
skeleton of the limbs and their girdles.
1. THE AXIAL SKELETON.
A. THE VERTEBRAL COLUMN.
This consists of about fifty vertebrae arranged in a regular
continuous series. The first vertebra differs a good deal from any of
the others; the seventeenth or sacral vertebra and the eighteenth or
first caudal also present peculiarities of their own. The remaining
vertebrae are divided by the sacrum into an anterior series of =trunk=
vertebrae which bear fairly large ribs, and a posterior series of
=caudal= vertebrae, all of which except the first few are ribless.
THE TRUNK VERTEBRAE.
Any vertebra from the second to the sixteenth may be taken as a type
of the trunk vertebrae.
The general form is elongated and somewhat hour-glass shaped, and the
=centra= are convex in front and concave behind; an opisthocoelous
condition such as this is quite exceptional in Anura. The =notochord=
may persist intervertebrally[54], but in the centre of each vertebra
it becomes greatly constricted or altogether obliterated, and replaced
by marrow. The superficial portion of the centrum is ossified, while
the articular surfaces are cartilaginous. The =neural arches= are low
and articulate together by means of =zygapophyses= borne on short
diverging processes. The anterior zygapophyses look upwards, the
posterior downwards. Each neural arch is drawn out dorsally into a
very slight cartilaginous =neural spine=.
On each centrum, at a little behind the middle line, there arise a
pair of short backwardly-directed =transverse processes=; each of
which becomes divided into two slightly divergent portions, a dorsal
portion which meets the tubercular process of the rib and is derived
from the neural arch, and a ventral portion which meets the capitular
process of the rib and is derived from the ventral or haemal arch. The
division between these two parts of the transverse processes can be
traced back as far as the sacrum.
The =first vertebra= as already mentioned differs much from all the
others. It has no ribs, and presents anteriorly two slightly divergent
concave surfaces which articulate with the occipital condyles of the
skull. Between these surfaces the dorsal portion of the anterior face
of the centrum is drawn out into a prominent =odontoid process=, the
occurrence of which renders it probable that the first vertebra of
the newt is really the axis, and that the atlas with the exception of
the odontoid process has become fused with the skull. The sacral
vertebra or =sacrum= differs from the vertebrae immediately in front
of it only in the fact that its transverse processes are stouter and
more obviously divided into dorsal and ventral portions.
THE CAUDAL VERTEBRAE.
The =caudal vertebrae= are about twenty-four in number. The anterior
ones have hour-glass shaped centra, and short backwardly-directed
transverse processes. The middle and posterior ones have rather
shorter centra, and are without transverse processes. The neural
arches resemble those of the trunk vertebrae, but each is drawn out
into a rather high cartilaginous neural spine abruptly truncated
anteriorly. All the caudal vertebrae except the first have also a
haemal arch, which is very similar to the neural arch, and is drawn
out into a haemal spine quite similar to the neural spine. Both neural
and haemal arches are ossified continuously with the centra.
B. THE SKULL.
The skull of the newt is divisible into three principal parts:--
(1) an axial part, the =cranium proper=, which encloses the brain and
to which
(2) the =capsules= of the =auditory and olfactory sense organs= are
fused;
(3) the skeleton of the =jaws and hyoid apparatus=. The skull is much
flattened and expanded, though not so much as in the frog.
(1) THE CRANIUM PROPER.
The =cranium proper= or =brain case= is an unsegmented tube which
remains partly cartilaginous, and is partly converted into cartilage
bone, partly sheathed by membrane bone. The roof and floor of the
cartilaginous cranium are, as is the case also in the frog, pierced
by holes or fontanelles, and these are so large that the main part of
the roof and floor comes to be formed by membrane bone.
Two pairs of large ossifications take place in the cranial walls. Of
these the more posterior on each side represents the =exoccipital= and
all three =periotic= bones. It bears a small convex patch of cartilage
for articulation with the atlas, and with its fellow forms the
boundary of the foramen magnum.
Two foramina pierce the exoccipital just in front of the occipital
condyle and transmit respectively the glossopharyngeal and
pneumogastric (fig. 21, X) nerves. Lying laterally to these nerve
openings is seen a patch of cartilage, the =stapes=, which is
homologous with the stapes or proximal element of the columellar chain
in the frog. Further forward in front of the stapes is the small
opening for the exit of the facial nerve, and seen in a lateral view
close to the orbitosphenoid, that for the trigeminal (fig. 21, C, 5).
In front of these large bones the lateral parts of the cranial walls
remain cartilaginous for a short distance, and then there follow two
elongated bones, the =orbitosphenoids= (fig. 21, B and C, 11), pierced
by the foramina for the exit of the optic nerves. These bones partly
correspond to the sphenethmoid of the frog.
The _membrane bones_ connected with the cranium are the _parietals_,
_frontals_ and _prefronto-lachrymals_ on the dorsal surface, and the
_parasphenoid_ on the ventral surface.
The _parietals_ (fig. 21, A and C, 6) roof over the posterior part of
the great dorsal fontanelle and overlap the exoccipito-periotics. They
meet one another along a sinuous suture in the middle line, as do also
the _frontals_ which overlap them in front. The _frontals_ and
_parietals_ both extend for a short distance down the sides of the
cranium and meet the orbitosphenoids. The _prefronto-lachrymals_ (fig.
21, A and C, 7) connect the frontals with the maxillae.
[Illustration FIG. 21. A DORSAL, B VENTRAL, AND C LATERAL VIEWS OF THE
SKULL OF A NEWT (_Molge cristata_) × 2-1/2 (after PARKER).
The cartilage is dotted, the cartilage bones are marked with dots and
dashes, the membrane bones are left white.
1. premaxillae.
2. anterior nares.
3. posterior nares.
4. nasal.
5. frontal.
6. parietal.
7. prefronto-lachrymal.
8. maxillae.
9. vomero-palatine.
10. parasphenoid.
11. orbitosphenoid.
12. pterygoid.
13. squamosal.
14. pro-otic region of exoccipito-periotic.
15. quadrate.
16. quadrate cartilage.
17. exoccipital region of exoccipito-periotic.
18. articular.
19. articular cartilage.
20. dentary.
21. splenial.
22. middle narial passage.
II. V. VII. IX. X. foramina for the exit of cranial nerves.]
On the ventral surface is the large _parasphenoid_ (fig. 21, B, 10),
which is widest behind and overlapped anteriorly by the
vomero-palatines.
(2) THE SENSE CAPSULES.
The =auditory capsules= become almost completely ossified continuously
with the exoccipitals; they have been already described.
The =nasal capsules= are large and quite unossified though they are
overlain by membrane bone. They appear on the dorsal surface between
the anterior nares and the nasal process of the premaxillae. They
enclose the nasal organs, bound the inner side of the anterior narial
opening, and are connected with one another posteriorly by a
cartilaginous area.
Developed in connection with the nasal capsules are a pair of rather
large _nasals_ (fig. 21, A and C, 4), which lie on the dorsal surface
immediately in front of the frontals. Each forms part of the posterior
boundary of one of the anterior nares, and the two are separated from
one another in the middle line by the nasal process of the premaxillae
(fig. 21, A, 1), and the opening of the =middle narial passage= (fig.
21, A and B, 22), which passes right through the skull.
On the ventral surface of the skull and forming the greater part of
the boundary of the posterior nares are two large bones, the
_vomero-palatines_ (fig. 21, B and C, 9). Each consists of a wide
anterior portion, partly separated from its fellow in the middle line
by the ventral opening of the middle narial passage, and of a long
pointed posterior portion which is separated from its fellow by the
_parasphenoid_, and bears a row of small pointed teeth formed of
dentine capped with enamel.
(3) THE JAWS.
The =upper jaw= of the newt is a discontinuous structure divided into
two parts, an anterior part which consists of membrane bones, the
_maxillae_ and _premaxillae_, and a posterior part which remains mainly
cartilaginous.
The _premaxillae_ are united, forming a single bone, which in a
ventral view is seen to meet the maxillae and vomero-palatines, and in
a dorsal view to send back a nasal process (fig. 21, A, 1) between the
nasals.
The _maxillae_ are large bones, each terminating in a point
posteriorly. A single row of teeth similar to those on the
vomero-palatines runs along the outer margin of the maxillae and
premaxillae.
The posterior part of the upper jaw forms a mass of cartilage which
extends forwards towards the maxillae as a long pointed process whose
ventral surface and sides are overlapped by a membrane bone, the
_pterygoid_ (fig. 21, 12).
The suspensorial bones include the =quadrate= and _squamosal_. The
=quadrate= (fig. 21, 15) which forms the true =suspensorium= is
directed forwards and outwards, and is terminated by a patch of
cartilage with which the mandible articulates.
The lower jaw or mandible remains partly cartilaginous, while its
ossifications include two membrane bones and one cartilage bone. The
cartilage bone is the =articular= (fig. 21, C, 18), it forms the
posterior part of the ramus, extends forwards for some distance along
its inner side, and is terminated posteriorly by a patch of cartilage
which articulates with the quadrate. The _dentary_ (fig. 21, C, 20) is
a large bone which forms the anterior part and nearly all the outer
half of each ramus, and bears teeth similar to those of the upper jaw.
Attached to its inner face is a long slender _splenial_ (fig. 21, C,
21).
THE HYOID APPARATUS.
This consists of the hyoid arch and part of the first two branchial
arches.
The =hyoid arch= (fig. 29, A, 2) consists of a pair of =cornua=, each
of which is divided into two halves. The dorsal half forming the
=cerato-hyal= is mainly ossified though tipped with cartilage, and is
connected by ligament with the suspensorium. The ventral half
(=hypo-hyal=) is cartilaginous, and is connected with the
basibranchial.
The =branchial arches= consist of a median piece, the =basibranchial=,
which is ossified in the centre and cartilaginous at either end, and
of two pairs of =cerato-branchials= which are attached to the
cartilaginous part (fig. 29, A, 8) of the basibranchial. The first
cerato-branchial is chiefly ossified, the second (fig. 29, A, 4) is a
good deal smaller and is cartilaginous. Both are united dorsally to a
single =epi-branchial=, which is terminated by a small cartilaginous
area at the free end but is elsewhere well ossified.
C. THE RIBS.
The ribs are short imperfectly ossified structures, bifid at their
proximal end where they articulate with the transverse processes, and
tipped both proximally and distally with cartilage. The dorsal portion
of the proximal end corresponds to the =tuberculum= of the ribs of
higher animals, and the ventral portion to the =capitulum=. Some of
the anterior ribs have a step-like notch on their dorsal surfaces.
The second to twelfth ribs are fairly equal in size, but further back
they decrease slightly. The ribs which connect the sacral vertebrae
with the ilia are however large. The short ribs borne on the anterior
caudal vertebrae are cartilaginous.
D. THE STERNUM.
The sternum (fig. 22, A, 6) is a rather broad plate of cartilage,
drawn out posteriorly into a median process marked by a prominent
ridge. On its antero-lateral margins it bears surfaces for
articulation with the pectoral girdle.
2. THE APPENDICULAR SKELETON.
A. THE PECTORAL GIRDLE.
This is of a very simple character, and remains throughout life in an
imperfectly ossified condition. It consists of a dorsal =scapular
portion=, and a ventral =coracoid portion= partially divided into an
anterior part, the precoracoid, and a posterior part, the =coracoid=.
[Illustration FIG. 22. A VENTRAL, AND B LATERAL VIEW OF THE SHOULDER
GIRDLE AND STERNUM OF AN OLD MALE CRESTED NEWT (_Molge cristata_) × 3
(after PARKER).
1. scapula.
2. suprascapula.
3. coracoid.
4. glenoid cavity.
5. precoracoid.
6. sternum.]
The =scapular portion= is a slightly curved oblong plate; its proximal
third the =scapula= (fig. 22, 1) is ossified and bounds part of the
well-marked =glenoid cavity= (fig. 22, 4); its distal portion forms a
large oblong cartilaginous plate, the =suprascapula= (fig. 22, 2).
The =precoracoid= (fig. 22, 5) forms a small forwardly-directed
cartilaginous plate. The =coracoid= (fig. 22, 3) forms a much larger
plate, the greater part of which is unossified and overlaps its fellow
in the middle line, the two being overlapped by the sternum. Around
the glenoid cavity is an area which is mainly ossified and is
continuous with the scapula.
B. THE ANTERIOR LIMB.
This is divisible into three parts, the =upper arm= or =brachium=, the
=fore-arm= or =antibrachium=, and the =manus=.
The =upper arm= includes a single bone, the =humerus=.
The =humerus= is a slender bone cylindrical in the middle and expanded
at either end, the proximal part forms a rounded =head= which
articulates with the glenoid cavity. Along the proximal part of the
anterior or pre-axial surface runs a strong =deltoid ridge=. The
proximal part of the postaxial surface also bears a small outgrowth.
The =fore-arm= contains two bones, the =radius= and =ulna=, both of
which are small and imperfectly ossified at their terminations.
The =radius= (fig. 23, B, 11) or pre-axial bone is rather the larger
of the two, and is considerably expanded at its proximal end. The
=ulna= or postaxial bone is somewhat expanded distally, but is not
drawn out proximally into an olecranon process.
The =manus= consists of two parts, a group of small bones forming the
=carpus= or =wrist=, and the =hand=.
The =carpus= is in a very simple unmodified condition as compared with
that of the Frog. It consists of a proximal row of two bones and a
distal row of four, with one, the =centrale=, interposed between. All
these bones are small and polygonal and are imbedded in a plate of
cartilage.
The bones of the proximal row are a smaller pre-axial bone, the
=radiale= (fig. 23, B, 13), and a larger postaxial bone, which
represents the fused =ulnare= and =intermedium= of the very simple
carpus described on pp. 26 and 27.
The four bones of the distal row are respectively =carpalia= 2, 3, 4
and 5.
The =hand= consists of four digits, that corresponding to the thumb of
the human hand, judging from the analogy of the frog probably being
the one that is absent.
[Illustration FIG. 23. A RIGHT POSTERIOR, AND B RIGHT ANTERIOR LIMB OF
A NEWT × 1-1/2 (_Molge cristata_).
1. femur.
2. tibia.
3. fibula.
4. tibiale.
5. intermedium.
6. fibulare.
7. centrale of tarsus.
8. tarsale 1.
9. tarsalia 4 and 5 fused.
10. humerus.
11. radius.
12. ulna.
13. radiale.
14. intermedium and ulnare fused.
15. centrale of carpus, the pointing line passes across carpale 2.
16. carpale 3.
17. carpale 5.
I. II. III. IV. V. digits.]
Each digit consists of a somewhat elongated =metacarpal= and of two or
three phalanges. The metacarpals are contracted in the middle and
expanded at either end. They are connected with the carpus by
cartilage, and the articulations between the several phalanges, and
between the metacarpals and phalanges are also cartilaginous. The
second, third, and fifth digits have two phalanges apiece, the fourth,
which is the longest, has three. The second metacarpal in the
specimen examined and figured articulates partly with carpale 2,
partly with carpale 3.
C. THE PELVIC GIRDLE.
The pelvic girdle of the Newt is in a much less modified condition
than is that of the Frog (see p. 165). It consists of a dorsal
element, the =ilium=, a posterior ventral element, the =ischium=, and
an anterior ventral element, the =pubis=, to which is attached an
=epipubis=.
The =ilium= is a somewhat cylindrical bone which at its ventral end
meets the ischium, and forms part of the =acetabulum=. It is then
directed upwards and slightly backwards, and is attached to the ribs
of the sacral vertebra.
The =ischia= are a pair of somewhat square bones which meet one
another in the middle line; they form part of the acetabulum, and are
united to the ilia above.
In front of the ischia is a narrow cartilaginous area which represents
the =pubes=. Projecting forwards from it is a bifid cartilaginous
=epipubis=.
D. THE POSTERIOR LIMB.
This is divisible into a proximal portion, the =thigh=, a middle
portion, the =crus= or =shin=, and a distal portion, the =pes=.
The =thigh= consists of a single bone, the =femur= (fig. 23, A, 1),
which has a thin shaft and expanded ends. The anterior part of the
pre-axial border and posterior part of the postaxial border bear
slight outgrowths.
The =crus= or =shin= includes two short bones, the =tibia= and
=fibula=, which are nearly equal in length. The pre-axial bone or
tibia is a straight bone thickest at its proximal end, the postaxial
bone or =fibula= (fig. 23, A, 3) is a rather stouter curved bone of
nearly equal diameter throughout.
The =pes= includes the =tarsus= or =ankle=, and the =foot=.
The =tarsus= consists of eight small bones arranged in a proximal row
of three, the =tibiale=, =intermedium= and =fibulare=, and a distal
row of four =tarsalia=, with one bone, the =centrale= (fig. 23, A, 7),
interposed between the two rows. In the specimen examined, the
=tibiale=, is a small bone articulating with the tibia, the
=intermedium= (fig. 23, A, 5) is larger and articulates with both
tibia and fibula, the =fibulare= is the largest of the three and
articulates with the fibula.
The bones of the distal row are =tarsalia 1=, =2=, =3=, and a bone
representing =4= and =5= fused. In the specimen examined tarsale 1 is
pushed away dorsally (fig. 23, A, 8), so as to lie between the tibiale
and tarsale 2. All the tarsal bones are small and somewhat polygonal,
and are connected with one another, and with the tibia and fibula on
the one hand, and with the metatarsals on the other by a thin layer of
cartilage.
The five =digits= of the foot each consist of a =metatarsal= and of a
certain number of =phalanges=. In the specimen examined, owing to the
shifting of tarsale 1, the first metatarsal as well as the second
articulates with tarsale 2, while the fifth metatarsal articulates
partially with the bone representing the fused tarsalia 4 and 5,
partially with the fibulare. All the bones of the digits except the
distal phalanges are terminated at each end by cartilaginous
epiphyses, the distal phalanx of each digit has a cartilaginous
epiphysis only on its proximal end.
The first, second, and fifth digits have two phalanges apiece, the
third and fourth have three.
Figure 31 B, showing a Newt's tarsus copied from Gegenbaur, has
precisely the arrangement generally regarded as primitive for the
higher vertebrates, except that tarsalia 4 and 5 are fused.
FOOTNOTES:
[54] i.e. between one vertebra and the next.