Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

90 ANATOMY.
Compara- chpterus, Jlstularia, &c. have indeed no vestige of rib.
tive But in the sturgeon, balista, eel, uranoscopus, pleuronectes.
Anatomy. sea-wolf, and dory, they are in the shape of short rudi-
mental processes ; in the trigla and loricaria their sides are
horizontal; in the perch, carp, pike, and chetodon, they
encompass nearly the whole upper region of the abdomi¬
nal cavity ; and, lastly, in the silver-fish (zeus vomer), the
herring, rhomboidal salmon, &c. they are united to a
sternum. In the little animal named sea-horse (syngna-
thus hippocampus), several series of osseous tubercles of
the skin, surrounding the body like belts, are supposed to
represent false r'bs. The sternum is limited to a small
number of Fishes. Besides those already mentioned, in
the dory there is a series of minute flat bones disseminat¬
ed along the lower edge of the belly, which is supposed
to represent a rudimental sternum.
In size and number the ribs vary, though in the silurus,
carp, and chetodon they are of largest proportional size ;
in the herring they are as fine as hairs.
The head. The head in the finny tribes is more an object of zoolo-
Cramum. gical than anatomical description. The chief points to be
remarked are, that the cranium forms but a small part of
the head; that the orbits are separated by a septum, some¬
times membranous, occasionally, as in the wolf-fish, bony ;
and that there is on each side a large movable bone, cor¬
responding to the quadrilateral of Birds, not square,
however, but oblong, which supports not only the lower
jaw and palatine arches, but the gill-cover. In the car¬
tilaginous fishes the sutures are early obliterated, and the
cranium consists of an inseparable mass of cartilage. In
the bony fishes the cranium is separable into numerous
pieces, and in the perch they amount to 80. In the cra¬
nium of fishes the anatomist recognises more distinctly
than in the superior orders the formation according to the
vertebral type. Small in proportion to the whole head,
the cranium appears like a direct continuation of the ver¬
tebral column. In the osseous division of the class es¬
pecially, the cranium may be distinguished into the occi¬
pital or posterior vertebra, the spheno-parietal or middle,
and the frontal or facial vertebra. The cavity thus formed
is very small; yet small as it is, it is not exactly filled by
the brain, between which and the bones there is inter¬
posed a pellucid fluid, contained in fine cellular tissue.
The cranium of the osseous fishes also is widest between
the ears, because the organ of hearing is contained within
its cavity with the brain. In the cartilaginous it is quite
different.
i.oa»ino- Though Fishes are destitute of extremities similar to
live mem- those possessed by the other three classes of the Verte-
bersorfins. BRATa, they are not, however, without locomotive mem¬
bers. The thoracic extremities are represented by tbe
pectoral fins, and the pelvic by the ventral. In short,
it may be said that the bones of the thoracic and abdomi¬
nal extremities are converted into osseous rays in the
finny tribes.
Pectoral In the Ray genus, in which the wing-like disposition of
fins. the pectoral fins gives the body a rhomboidal shape, they
consist of numerous radiating cartilaginous lines, all at¬
tached to a cartilage parallel to the spine, divisible into
two or three others, and articulated above to another ad¬
herent to the spine. Below there is a strong transverse
bar common to the cartilages of both fins, and separat¬
ing at once the sternum and clavicle. This transverse
bar is also seen in the shark tribe; but their pectoral
fins, which are much smaller, are not articulated with the
spine.
In the osseous fishes, and in many others usually re¬
ferred to the cartilaginous division, e. g. the balista, the
pectoral fins are fixed to an osseous belt, which sur¬
rounds the body behind the gills, and which supports the
posterior margin of their aperture. This belt consists of Compara.
a single bone on each side, articulated to the posterior- tive
superior angle of the cranium, and uniting below the^™ny
breast with that of the opposite side. This bone, which peclora]
may be regarded as a scapula, varies in shape and the^nS4
ano-le which it forms with its fellow in different species.
In fishes flattened vertically, the angle of union is acute;
in those which are depressed, the angle is so obtuse as to
form nearly a straight line. In many fishes, especially
those of the order Thoracici, e.g. pleuronectes, coitus, zeus,
chetodon, perch, &c., in the small unicorn (Sa/fste), and
others, the superior part forms a large spine,, which de¬
scends immediately behind the fin, and to which the ad¬
ductor muscles are attached. This spine, which is mov¬
able, has been improperly named a clavicle.
The rays by which the membrane is supported are not
directly articulated to this belt, but are connected by a
row of minute flat bones, which may be compared to the
carpus in the other three classes. When the fiist ray of
the pectoral fin, however, is thorny, as in the harness-fish
{loricaria), and some species of silurus, it is articulated di¬
rectly with an osseous belt; and it is remarkable that some
fishes, as the silurus and stickle-back, have the power of
retaining this spinous ray erected against the body as a
means of defence. This is effected by a cylindrical tu¬
bercle, on which the spinous ray is articulated by a hoi-,
low, bounded before and behind by an elevated process.
When the spine is erected, the anterior process, entering
a hole in the cylindrical tubercle, is locked in it by the
spine revolving slightly on its axis, so that it cannot be
inflected unless by the spine revolving in the opposite di¬
rection.
The pectoral fins are so long that they answer the pur- Pterygoid
pose of wings in several species of trigla, the trigla Ai-pectoral
rundo, the flying gurnard {trigla volitans), the springinghns-
gurnard {trigla evolans,) in the scorpmna volitans, the
tropical flying fish {exoccetus volitans), and some others.
Their situation also is liable to vary. In the exoccetus
they are near the gills, but in tbe blennius and others they
are remote. Lastly, they are totally wanting in a small
number only, as the lamprey {petromyzon), the hag-fish
{myxine, Lin.; gastrobranchus), the murcena, the eel genus,
the sphagobranchus, &c.
The abdominal or ventral fins, which correspond to the Abdominal
pelvic extremities of the other classes, are so denominated or ventral
because in the majority of fishes they are situate below hns-
the belly, and nearer the anal outlet than the pectoral. By
this circumstance a numerous order are distinguished by
tbe name of Abdominal Fishes (Abdominales).
In a small number of fishes, comprehending the gadus,
blennius, hurtus, callionymus, trachinus, and uranosco¬
pus, the ventral fins are placed under the throat, below
the aperture of the gills, and before the pectoral fins.
This order is therefore distinguished by the name of Ju-
GULARES.
In the most numerous order of all, the ventral fins are
situate behind and below the pectoral fins. These have
therefore been denominated Thoracic Fishes (Phora-
cici).
The ventral fins consist of two parts—one formed of
rays covered by a double membrane, apparent externally,
and constituting the proper ventral fin ; the other internal,
representing the coxal bones of the pelvis, always support¬
ing the pinna! rays, and often articulated with the bones
of the trunk. It is never articulated, however, with the
spine, nor does it form an osseous belt round the abdo¬
men. The bones of which it consists are generally flat¬
tened, varying in shape, and in mutual contact by the in¬
ternal margin. In the shark and ray genera only is there
a single transverse bone, nearly cylindrical, to the extre-