Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

ANATOMY.
i)l
Compara- mities of which the fins are attached. The direction of
dve the pelvic plane to the walls of the abdomen varies ac-
Anatomy. cor(]ing to the shape of the body of the fish. In the flat
vCntra?^ ^shes th0)' are directed obliquely, and their inner margin
gns forms the keel of the belly. In fishes with a broad or cy¬
lindrical belly they form a plane more or less horizontal.
In the Jugular and Thoracic Fishes, the pelvic
bones are always articulated with the base of the belt
which sustains the pectoral fins; and they vary much in
shape and situation.
In the trachinus, uranoscopus, coitus, sciama, chetodon,
and perch, these two bones are united by their inner mar¬
gin. In the cuckoo-gurnard, in which they are united by
the posterior tip only of their internal margin, they are
broad, flat, and oval. In the sole and flounder genus (pleu-
ronectes), in which the fins are attached to their anterior
tip, they are united in a quadrangular pyramid, the apex
of which is directed backwards and upwards, and the
base forwards. In some of the stickle-backs these bones
once, and perhaps intermediate between the two, this ab- Compara-
breviation is carried perhaps to its greatest possible degree, tive
in leaving the articular ends only of the four locomotive Anatom.v-
extremities. Lastly this reduction is merely prepara-
tory to that exhibited in the whole class of Fishes, in
which the three longitudinal bones so conspicuous in the
higher classes of animals are completely obliterated, and
those representing the hand or forepaw and foot are arti¬
culated directly to the shoulder and pelvic bones.
Besides the bones already mentioned as constituting
the skeleton, there are observed in the osseous fishes
minute bones, generally fork-like in shape, disseminated
through all the muscular parts of the body. The purpose
of these bones, which, as being totally insulated from the
other parts of the skeleton, are denominated ossicula mus¬
culorum, is chiefly to afford points of support; and they
are probably to be regarded as rudimental representatives
of osseous parts, more completely developed in the higher
animals.
are altogether separate, and being long, receive in their
middle a movable spine, which supplies the place of the
ventral fin. In the dory (zeus faber, L.) they are flat and
triangular, in mutual contact by their whole surface. In
the silver-fish (zeus vomer) they are small and cylindrical.
In the whole of the Abdominal order, on the contrary,
the pelvic bones are equally unconnected with the bones
of the shoulder and with the osseous belt of the pectoral
fins, and are confined to the middle-inferior part of the
belly, not far from the anus. In general these two bones
are separate from each other, and are retained in their si¬
tuation by ligaments. In the carp, in which they are
elongated, they touch only by their posterior third. In the
herring, in which they are small and approximated, they
are continuous with the minute bones of the sternum. In
the pike they are broad and trilateral, approximated by
their anterior tips, separate behind where the fin is at¬
tached. In the silurus, in which they are united, they
form a round and often spinous shield before, while the
fins are attached to the exterior-posterior margin. Last¬
ly, in the cuirassier or harness-fish (loricaria), the pelvic
bones are united in one piece, the posterior notch of which
forms the anal aperture, while the fins are attached to its
external margin.
The proper fin consists of a certain number of osseous
rays, simple or bifid, supported by one or two rows of mi¬
nute bones placed between them and the pelvic bones.
On these small bones the constituent rays move, diverging
or converging like the rods of a fan, while the whole fin
may be inflected or extended by the minute bones moving
on the pelvic, so as to adduct or abduct the fin.
In the cartilaginous fishes the structure is different. To
the tip of each pelvic bone are articulated two principal
cartilages, one external, forming a kind of toe with seven
or eight joints; the other internal, supporting all the other
rays of the fin, which often exceed thirty in number.
Analogy or If we suppose these bones, like the minute ones of the
nrincM ■ Pectora^ fin> to represent the tarsus of the other three
i orgamsa-11 c^asses’ ^ must follow that, in the locomotive extremities,
tion. humerus, with the ulna and radius, and the femur,
with the tibia and fibula, are obliterated. It is not un¬
important to observe, that the general structure of the
Vertebrata tends through various transitions to this ter¬
mination. In the Amphibia the long bones of the extre¬
mities are shortened by removing the diaphysis, and leaving
their articulating ends only. In the Cetacea the pelvic
extremities are removed altogether. In the Cheloniad
and Saurial Reptiles the same long bones of the extre¬
mities are much abridged; and in the Ichthyoid Rep-
iiles, now extinct, but sharing by their structure a form
of animal existence partaking of the reptile and fish at
It is further a curious circumstance, that the skeleton, Violation
which is so symmetrical in all the other classes and orders, of the law
begins to exhibit a deviation from this first in the skele-°* s.ymrne-
ton of the finny tribes. In the Sole genus {Pleuronectes) tlT-
this deviation is very conspicuous. Both eyes are placed
on the same side of the mesial plane; and the side on
which the eyes are placed is broader than the opposite
one. The former is bounded by a convex margin, the lat¬
ter by a concave one. Jbe orbit towards the former is
large, the other small and imperfect. Conversely, it is to
be observed, that in the latter the maxillary and intermax¬
illary bones are larger than in the former. The sides of
the inferior jaw are less discordant; and though in the
Sole and Plaice those of the eyeless side are more straight
and elongated than those of the other, in the Turbot (Pleu-
ronectes maximus) they are nearly symmetrical.
CHAP. II.—COMPARATIVE MYOLOGY.
Though this is the proper place to consider the pecu¬
liarities of the muscular system of animals, the limits as¬
signed to this sketch will not allow us to enter into de¬
tails. We shall merely, therefore, take a cursory view of
those points in which the myology of the lower animals
differs from that of the human subject.
In general, in the lower animals, especially the Mam¬
malia, Birds, and Reptiles, the muscles correspond in
situation to those of the human subject; and whatever
modifications they undergo consist in changes of figure,
and in some few instances in changes in attachment. The
former kind of changes may be in all cases pretty accu¬
rately estimated by the osteological characters of the class,
order, or genus; for when the position, shape, or direc¬
tion of a bone is altered, in the same proportion nearly
are the attached muscles altered in their attributes.
Though in the lower animals, however, the zootomist Deficiency
traces muscles in general quite analogous to those of the in number,
human subject, in several instances this analogy ceases to
be observed. In general the muscles of the lower animals
are less numerous than those of the human subject; and
this deficiency in number, though not much observed in
the Quadrumana, is very remarkable in all the inferior
orders of the Mammalia, and still more in the Birds
and Reptiles. In general, also, these variations are most
conspicuous in the locomotive extremities. Thus the
small pectoral muscle, which is present in the Quadru¬
mana, is wanting in the Carnivora and the whole of
the Ungulated Animals and the Reptiles. The short
supinator is present in the Canine and Feline genera, but
the long is wanting; and both are absent in the Chirop-
TERA, RoDENTIA, PACHYDERMATA, RuMINANTIA, and