TERMINOLOGY AND TOPOGRAPHY OF THE SKELETON.
In order to readily comprehend the subsequent account of the
modifications of the skeleton in the various sub-classes and groups of
Fishes, the student has to acquaint himself with the terms used for the
numerous bones of the fish skeleton, as well as with their relative
position. The skeleton of any of the more common kinds of osseous fish
may serve for this purpose; that of the Perch is chosen here.
The series of bones constituting the axis of the body, and destined to
protect the spinal chord and some large longitudinal blood-vessels, is
called the _vertebral_ or _spinal column_; the single bones are the
_vertebræ_. The _skull_ consists of the bones surrounding the brain
and organs of sense, and of a number of arches suspended from it, to
support the commencement of the alimentary canal and the respiratory
organs.
The _vertebra_ (Fig. 22) consists of a body or _centrum_ (_c_), with
a concave anterior and posterior surface, and generally of several
_processes_ or _apophyses_, as--1. Two _neurapophyses_ (_na_), which,
on the dorsal side, rising upwards, form the _neural arch_ over the
canal, in which the spinal chord is lodged. 2. Two _parapophyses_
(_pa_) usually projecting from the lower part of the sides of the body,
or two _hæmapophyses_ (_ha_) which actually coalesce to form on the
ventral side the hæmal canal for a large trunk of the vascular system.
3. A _neural spine_ (_ns_), which crowns the neurapophyses, or is
interposed between their tips. 4. A _hæmal spine_ (_hs_), having the
same relation to the hæmapophyses. 5. Two _pleurapophyses_ or floating
_ribs_, suspended from, or from the base of, the parapophyses. 6. In
most fishes the neural arches are connected together by articular or
oblique processes, _zygapophyses_ (_za_), which are developed from the
base of each neurapophysis.
[Illustration: Fig. 22.--Side and Front view of Fish-vertebra.]
The vertebræ are either _abdominal_ or _caudal_ vertebræ, the
coalescence of the parapophyses into a complete hæmal ring, and the
suspension of the anal fin generally forming a sufficiently well-marked
boundary between abdominal and caudal regions (Fig. 23). In the Perch
there are twenty-one abdominal and as many caudal vertebræ. The centrum
of the first vertebra or atlas is very short, with the apophyses
scarcely indicated, and lacking ribs like the succeeding vertebra. All
the other abdominal vertebræ, with the exception of the last or two
last, are provided with ribs, many of which are bifid (72). A series of
flat spines (74), called _interneurals_, to which the spines and rays
of the dorsal fins are articulated, are supported by the neural spines,
the strength of the neurals and interneurals corresponding to that of
the _dermal_ spines (75). The caudal vertebræ differ from the abdominal
in having the hæmapophyseal elements converted into spines similar to
the neurals, the anterior being likewise destined to support a series
of _interhæmals_ (79), to which the anal rays are articulated. The
last and smallest caudal vertebra articulates with the _hypural_ (70),
a fan-like bone, which, together with the dilated hindmost neural and
hæmal elements, supports the caudal rays.
Looking at a perch’s _skull_ from the side (Fig. 24), the most
superficial bones will be found to be those of the jaws, a chain of
thin bones round the lower half of the eye, and the opercles.
The anterior margin of the upper jaw is formed by the _intermaxillary_
or _premaxillary_ (17) which bears teeth, terminates in a pedicle
above, to allow of a forward sliding motion of the jaw, and is dilated
into a flat triangular process behind, on which leans the second
bone of the upper jaw, the _maxillary_ (18). This bone is toothless,
articulates with the vomer and palatine bone, and is greatly dilated
towards its distal extremity. Both the maxillary and intermaxillary
lie and move parallel to each other, being connected by a narrow
membrane; in many other fishes their relative position is very
different.
The _mandible_ or lower jaw consists of a right and left ramus;
their union by a ligament in front is called _symphysis_. Each ramus
is formed of several pieces; that which, by a sigmoid concavity
articulates with the quadrate, is the _articulary_ bone (35); it sends
upwards a coronoid process, to which a ligament from the maxillary
and the masticatory muscles are attached; and forwards a long-pointed
process, to be sheathed in the deep notch of the anterior piece. A
small separate piece (36) at the lower posterior angle of the mandible
is termed _angular_. The largest piece (34) is tooth-bearing, and hence
termed _dentary_; at its inner surface it is always deeply excavated,
to receive a cylindrical cartilage, called _Meckel’s cartilage_, the
remains of an embryonic condition of the jaw, the articulary and
angular being but ossified parts of it. In other _Teleostei_ this
number is still more increased by a _splenial_ and other bones.
The _infraorbital_ ring of bones (Fig. 23, ^{19}) consists of several
(four) pieces, of which the anterior is the largest, and distinguished
as _præorbital_.
The so-called _præoperculum_ (30) belongs rather to the bones of the
suspensorium of the mandible, presently to be described, than to the
opercles proper. It is narrow, strong, angularly bent, so as to consist
of a vertical and horizontal limb, with an incompletely closed canal
running along both limbs. As it is quite a superficial bone, and
frequently armed with various spines, its form and configuration form
an important item in the descriptive details of many fishes.
The principal piece of the gill-cover is the _operculum_ (28),
triangular in shape, situated behind, and movably united with, the
vertical limb of the præoperculum. There is an articulary cavity at
its upper anterior angle for its junction with the hyomandibular. The
oblong lamella below the operculum is the _sub-operculum_ (32), and
the one in front of this latter, below the horizontal limb of the
præoperculum, is the _interoperculum_ (33), which is connected by
ligament with the angular piece of the lower jaw, and is also attached
to the outer face of the hyoid, so that the gill-covers cannot open or
shut without the hyoid apparatus executing a corresponding movement.
The chain of flat bones which, after the removal of the temporal
muscles, appear arranged within the inner concavity of the præoperculum
(Fig. 24), are comprised with the latter under the common name of
_mandibulary suspensorium_. They connect the mandible with the
cranium. The uppermost, the _epitympanic_ or _hyomandibular_ (23), is
articulated by a double articulary head with the mastoid and posterior
frontal. Another articulary head is destined for the opercular joint.
The _mesotympanic_ or _symplectic_ (31) appears as a styliform
prolongation of the lower part of the hyomandibular; is entirely
cartilaginous in the young, but nearly entirely ossified in the adult.
The position of this bone is noteworthy, because, directly inwards of
its cartilaginous junction with the hyomandibular, there is situated
the uppermost piece of the hyoid arch, the stylohyal. The next bone of
the series is the _pretympanic_ or _metapterygoid_ (27), a flat bone
forming a bridge towards the pterygoid, and not rarely absent in the
teleosteous sub-class. Finally, the large triangular _hypo-tympanic_ or
_quadrate_ (26) has a large condyle for the mandibulary joint.
The palatine arch (Fig. 26) connects the suspensorium with the
anterior extremity of the skull, and is formed by three bones: the
_entopterygoid_ (25), an oblong and thin bone attached to the inner
border of the palatine and pterygoid, and increasing the surface of
the bony roof of the mouth towards the median line; it constitutes
also the floor of the orbit. The _pterygoid_ (24) (or _os transversum_)
starts from the quadrate, and is joined by suture to the _palatine_,
which is toothed, and reaches to the vomer and anterior frontal.
In the occipital region there are distinguished the _basi-occipital_
(5), readily recognised by the conical excavation corresponding and
similar to that of the atlas, with which it is articulated through
the intervention of a capsule filled with a gelatinous substance (the
remains of the notochord); the _exoccipitals_ (10), articulated, one on
each side, to the basi-occipital, and expanding on the upper surface of
that bone, so as to meet and support the spinal column; a superficial
thin lamella (13), suturally connected with the exoccipitals,
not constant in fishes, and erroneously believed by Cuvier to be
the _petrosal_ (_os petrosum_) of higher animals; further, the
_paroccipitals_ (9), which are wedged in between the exoccipitals and
_supraoccipital_. This last bone (8) forms the key of the arch over the
occipital foramen, and raises a strong high crest from the whole length
of its mesial line; a transverse supraoccipital ridge, coming from each
side of the base of this spine runs outwards laterally to the external
angles of the bone. The supraoccipital separates the parietals, and
forms a suture with the frontals.
In front of the basi-occipital the base of the skull is formed by the
_basisphenoid_ (_parasphenoid_ of Huxley) (6). This very long and
narrow bone extends from the basi-occipital beyond the brain-capsule to
between the orbits, where it forms the support of the fibro-membranous
interorbital septum. Anteriorly it is connate with another long
hammer-shaped bone (16), the _vomer_, the head of which marks the
anterior end of the palate, and is beset with teeth. The _alisphenoids_
(11) are short broad bones, rising from the basisphenoid; their
posterior margins are suturally connected with the anterior of the
basi- and exoccipitals.
[Illustration: Fig. 23.--Skeleton of the Perch.]
[Illustration: Fig. 24.--Skeleton of a Perch’s Skull.]
[Illustration: Fig. 25.--Hyoid arch, branchial apparatus, and
scapulary arch of the Perch.]
[Illustration: Fig. 26.--Lower view of Skull of Perch.]
[Illustration: Fig. 27.--Hyoid bone of the Perch.]
The formation of the posterior part of the side of the skull is
completed by the _mastoid_ and _parietal_ bones. The former (12)
projects outwards and backwards farther than the paroccipital, forming
the outer strong process of the side of the cranium. This process
lodges on its upper surface one of the main ducts of the muciferous
system, and affords the base of articulation to a part of the
hyomandibular. Its extremity gives attachment to the strong tendon of
the dorso-lateral muscles of the trunk. The _parietals_ (7) are flat
bones, of comparatively much smaller extent than in higher Vertebrates,
and separated from each other by the anterior prolongation of the
supraoccipital.
The anterior wall of the brain-capsule (or the posterior of the orbit)
is formed by the _orbitosphenoids_ (14), between which, superiorly,
the olfactory nerves, and inferiorly, the optic, pass out of the
cranium. In addition to this paired bone, the Perch and many other
fishes possess another single bone (15),--the _os sphenoideum anterius_
of Cuvier, _ethmoid_ of Owen, and _basisphenoid_ of Huxley; it is
Y-shaped, each lateral branch being connected with an orbito-sphenoid,
whilst the lower branch rests upon the long basal bone.
A cartilage, the substance of which is thickest above the vomer,
and which extends as a narrow stripe along the interorbital septum,
represents the _ethmoid_ of higher Vertebrata; the olfactory nerves run
along, and finally perforate it.
There remain, finally, the bones distinguishable on the upper surface
of the skull; the largest, extending from the nasal cavities to the
occipital, are the _frontal_ bones (1), which also form the upper
margin of the orbit. The _postfrontals_ (4) are small bones placed
on the supero-posterior angle of the orbit, and serving as the point
from which the infraorbital ring is suspended. The _pre-frontals_ (2),
also small, occupy the anterior margin of the orbit. A pair of small
tubiform bones (20), the _turbinals_, occupy the foremost part of the
snout, in front of the frontals, and are separated from each other by
intervening cartilage.
After removal of the gill-cover and mandibulary suspensorium, the hyoid
arch, which encloses the branchial apparatus, and farther behind,
the humeral arch are laid open to view (Fig. 25). These parts can be
readily separated from the cranium proper.
The _hyoid arch_ is suspended by a slender styliform bone, the
_stylohyal_ (29), from the hyomandibulars; it consists of three
segments, the _epihyal_ (37), _ceratohyal_ (38), which is the longest
and strongest piece, and the _basihyal_, which is formed by two
juxtaposed pieces (39, 40). Between the latter there is a median
styliform ossicle (41), extending forwards into the substance of the
tongue, called _glossohyal_ or _os linguale_; and below the junction of
the two hyoid branches there is a vertical single bone (42), expanded
along its lower edge, which, connected by ligament with the anterior
extremity of the humeral arch, forms the _isthmus_ separating the
two gill-openings. This bone is called the _urohyal_. Articulated or
attached by ligaments to the epihyal and ceratohyal are a number of
sword-shaped bones or rays (43), the _branchiostegals_, between which
the branchiostegal membrane is extended.
The _branchial arches_ (Figs. 25 and 27) are enclosed within the hyoid
arch, with which they are closely connected at the base. They are five
in number, of which four bear gills, whilst the fifth (56) remains
dwarfed, is beset with teeth, and called the _lower pharyngeal_ bone.
The arches adhere by their lower extremities to a chain of ossicles
(53, 54, 55), _basibranchials_, and, curving as they ascend, nearly
meet at the base of the cranium, to which they are attached by a
layer of ligamentous and cellular tissue. Each of the first three
branchial arches consists of four pieces movably connected with one
another. The lowest is the _hypobranchial_ (57), the next much longer
one (58) the _cerato-branchial_, and, above this, a slender and a
short irregularly-shaped _epibranchial_ (61). In the fourth arch
the hypobranchial is absent. The uppermost of these segments (62),
especially of the fourth arch, are dilated, and more or less confluent;
they are beset with fine teeth, and generally distinguished as the
_upper pharyngeal bones_. Only the cerato-branchial is represented in
the fifth arch or lower pharyngeal. On their outer convex side the
branchial segments are grooved for the reception of large blood-vessels
and nerves; on the inner side they support horny processes (63), called
the _gill-rakers_, which do not form part of the skeleton.
The _scapular_ or _humeral arch_ is suspended from the skull by the
(_suprascapula_) _post-temporal_ (46), which, in the Perch, is attached
by a triple prong to the occipital and mastoid bones. Then follows the
(_scapula_) _supraclavicula_ (47), and the arch is completed below
by the union of the large (_coracoid_) _clavicula_ (48) with its
fellow. Two flat bones (51, 52), each with a vacuity, attached to the
clavicle have been determined as the (_radius_ and _ulna_) _coracoid_
and _scapula_ of higher vertebrates, and the two series of small
bones (53) intervening between the forearm and the fin as _carpals_
and _metacarpals_. A two-jointed appendage the (_epicoracoid_)
_postclavicula_, is attached to the clavicle: its upper piece (49) is
broad and lamelliform, its lower (50) styliform and pointed.
The ventral fins are articulated to a pair of flat triangular bones,
the _pubic_ bones (80).
The bones of the skull of the fish have received so many different
interpretations that no two accounts agree in their nomenclature,
so that their study is a matter of considerable difficulty to the
beginner. The following synonymic table will tend to overcome
difficulties arising from this cause; it contains the terms used
by Cuvier, those introduced by Owen, and finally the nomenclature
of Stannius, Huxley, and Parker. Those adopted in the present work
are printed in italics. The numbers refer to the figures in the
accompanying woodcuts (Figs. 23–27).
_Cuvier._ _Owen._ _Stannius._ _Huxley, Parker,
etc._
1. Frontal principal _Frontal_ Os frontale
2. Frontal antérieur _Prefrontal_ Os frontale Lateral ethmoid
anterius (Parker)
3. _Ethmoid_ Nasal Os ethmoideum
4. Frontal _Postfrontal_ Os frontale Sphenotic
postérieur posterius (Parker)
5. Basilaire _Basioccipital_ Os basilare
6. Sphénoide _Basisphenoid_ Os sphenoideum Sometimes referred
basilare to as “_Basal_”
7. Pariétal _Parietal_ Os parietale
8. Interpariétal or _Supraoccipital_ Os occipitale
occipital superius
supérieure
9. Occipital externe _Paroccipital_ Os occipitale Epioticum
externum (Huxley)
10. Occipital lateral _Exoccipital_ Os occipitale
laterale
11. Grande aile du _Alisphenoid_ Ala temporalis Prooticum
sphénoide (_Huxley_)
12. Mastoidien _Mastoid_ Os mastoideum {
+ os extrascapulare {Opisthoticum[5]
{ +_Squamosal_
13. Rocher Petrosal and Oberflächliche { (Huxley)
Otosteal Knochen-lamelle {
14. Aile orbitaire _Orbitosphenoid_ Ala orbitalis Alisphenoid
(Huxley)
15. Sphenoide Ethmoid and Os sphenoideum _Basisphenoid_
antérieur Ethmoturbinal anterius (Huxley)
16. _Vomer_ Vomer Vomer
17. Intermaxillaire _Inter- or Pre- Os intermaxillare
maxillary_
18. Maxillaire _Maxillary_ Os maxillare
supérieur
19. Sousorbitaires _Infraorbital Ossa
ring_ infraorbitalia
20. Nasal _Turbinal_ Os terminale
22. Palatine _Palatin_ Os palatinum
23. Temporal Epitympanic Os temporale _Hyomandibular_
(Huxley)
24. Transverse _Pterygoid_ Os transversum
s. pterygoideum
externum
25. Ptérygoidien _Entopterygoid_ Os pterygoideum Mesopterygoid
interne (Parker)
26. Jugal Hypotympanic Os quadratojugale _Quadrate_
(Huxley)
27. Tympanal Pretympanic Os tympanicum _Metapterygoid_
(Huxley)
28. Operculaire _Operculum_ Operculum
29. Styloide _Stylohyal_ Os styloideum
30. Préopercule _Præoperculum_ Præoperculum
31. Symplectique Mesotympanic _Os symplecticum_
32. Sousopercule _Suboperculum_ Suboperculum
33. Interopercule _Interoperculum_ Interoperculum
34. Dentaire _Dentary_ Os dentale
35. Articulaire _Articulary_ Os articulare
36. Angulaire _Angular_ Os angulare
37. } Grandes pièces _Epihyal_ }
38. } latérales _Ceratohyal_ } Segmente der
} Zungenbein-
39. } Petites pièces _Basihyal_ } Schenkel
40. } laterales }
41. Os lingual _Glossohyal_ Os linguale s.
entoglossum
42. Queue de l’os hyoide _Urohyal_ Basibranchiostegal
(Parker)
43. Rayon branchiostège _Branchiostegal_ Radii branchio-
stegi
46. Surscapulaire Suprascapula Omolita _Post-temporal_
(Parker)
47. Scapulaire Scapula Scapula _Supraclavicula_
(Parker)
48. Humeral Coracoid Clavicula _Clavicula_
(Parker)
49. } Coracoid Epicoracoid _Postclavicula_
50. } (Parker)
51. Cubital Radius } Ossa carpi _Coracoid_ (Parker)
52. Radial Ulna } _Scapula_ (Parker)
53. Os du carpe _Carpals_ Ossa metacarpi { _Basalia_ (Huxley),
{ Brachials (Parker)
53 bis. }
54. } Chaine intermédiaire _Basibranchials_ Copula
55. }
56. Pharyngiens _Lower_ Ossa pharyngea
inférieurs _Pharyngeals_ inferiora
57. Pièce interne de _Hypobranchial_ }
partie inférieure }
de l’arceau }
branchiale }
58. Pièce externe „ _Ceratobranchial_ } Segmente der
59. Stylet de prémière _Upper epibranchial_ } Kiemenbogen-
arceau branchiale _of first branchial_ } Schenkel
_arch_ }
61. Partie supérieure _Epibranchials_ }
de l’arceau }
branchiale }
62. Os pharyngian Pharyngo- Os pharyngeum _Upper pharyngeals_
supérieur branchial superius
63. _Gill-rakers_
65. Rayons de la _Pectoral rays_ Brustflossen-
pectorale Strahlen
67, 68. Vertèbres _Abdominal Bauchwirbel
abdominales vertebræ_
69. Vertèbres caudales _Caudal vertebræ_ Schwanzwirbel
70. Plaque triangulaire [Aggregated Verticale Platte _Hypural_(Huxley)
et verticale interhæmals]
71. _Caudal rays_ Schwanzflossen
Strahlen
72. Côte _Rib_ Rippen
73. Appendices or _Epipleural Muskel-Gräthen
stylets spines_
74. Interépineux _Interneural Ossa interspinalia
spines_ s. obere
Flossentræger
75. Épines et rayons _Dorsal rays and_ Rückenflossen-Strahlen
dorsales _spines_ u. Stacheln
76. _First
interneural_
78. _Rudimentary_
_caudal rays_
79. Apophyses épineuses _Interhæmal Untere Flossentræger
inférieures spines_
80. _Pubic_ Becken
81. _Ventral spine_ Bauchflossen-Stachel