Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

ANATOMY.
Compara- From the specimens of the Ichthyosaurus hitherto dis-
tivc covered, it appears that the number of vertebra? varies from
80 to 90 or more; in one entire specimen they amounted
Osteoiorri- t0 104, (Conybeare and De la Beche); that they are flat-
cal pecuh- tened, with the transverse diameter greater than the longi-
arities of tudinal, and the two articulating surfaces of the bodies
the Ich- calycoid or cup-shaped as in Fishes. Though the annular
thyosauruspart js distinct from the body, it is united to its sides. The
spinous processes, which are long and prominent, form a
continuous ridge above the spine, and are connected to
each other by a process from the front of the one spine,
which is inserted into a pit in the back of the other. In¬
stead of proper transverse process, a certain number of
the vertebrae are provided with two tubercles on each
side of the body, of which the superior, convex, is articu¬
lated to the tubercle of the rib, while the other, which is
concave, receives the head. In the inferior part of the
vertebral column, these two tubercles, after approximat¬
ing, are eventually identified into one.
The ribs, which are numerous, and extend from the
occiput to the pelvis, are slender and trilateral in shape,
bifurcated above, and attached to the vertebrae by a head
and tubercle. In the perfect specimen of Mr de la Beche
they amount to 31, and of these 17 appear to be cervical
or anterior false ribs, with single tubercles; thus affording
another mark of resemblance to the Saurial family in
osteological characters.
The bones of the head, distinguished by the extraordi¬
nary size of the orbit, are similar to those of the Saurial
Reptiles. The sternum, collar-bone, and scapula, though
also similar to those of this family, bear a much closer re¬
semblance to the figure of these parts in the Echidna and
Ornithorhyncus. The humerus is short, thick, and sinuat-
ed; the bones of the fore arm flat, and probably constitut¬
ing part of the fore or thoracic fin. The Carpus consists
of three rows, the first containing three bones, the other
two, four each. These are followed by five or six rows of
flattened, irregularly cuboidal bones, gradually diminishing
in size and number to the tips, and which represent at once
the metacarpus and phalanges of the fore paw, used appa¬
rently chiefly as a fin or paddle. The pelvic extremities
appear to have been less strong and perfectly construct¬
ed than the thoracic. The femur is smaller and shorter
than the humerus ; the tibia and fibula are flattened like
the idna and radius ; the tarsus consists of two rows only,
the first containing three, and the second five bones ; and
this in like manner terminates in five ranges of flattened
bones, gradually diminishing in size, and which represent
the metatarsus and metatarsal phalanges of the hind paw
or paddle.
From the specimens hitherto discovered of the Plesiosau¬
rus, it appears that the total number of vertebrae amounts
to 90, of which 35 appear to be cervical, while the other
55 are dorsal and caudal, the regions of which are propor¬
tionally short. The head of this animal also is small and
compressed, nor has it the large orbit of the Ichthyosau¬
rus. Each rib consists of a vertebral and sternal portion,
united at an obtuse angle, the former articulated by a
single head to the transverse process, and the latter con¬
nected with its fellow by a transverse slip, so that the
lower or abdominal ribs appear to have surrounded the
abdomen with a complete cincture. The anterior part of
the chest is occupied by two trilateral bones uniting in
the middle, which, from t.hpir r,nnnp/->f-!nn uritR
89
num and the transverse bone the clavicles; and it is Compara-
orthy of remark, that not only this bone, but the middle tive
piece, closely resembles in figure and disposition those 0f Anatom-v-
the Echidna and Ornithorhyncus. The pelvis consists of^^
three bones, a vertebra or superior, corresponding to the
ilium narrow and slightly incurvated ; an anterior, ascend¬
ing forwards, and broad, separating the pubis; and a pos¬
terior, short, forming the ischium. The humerus and femur
are longer than in the Ichthyosaurus. There is a very
short radius and ulna, and tibia and fibula, articulated
with five carpal and tarsal bones; and the rest of both
paddles consists of successive rows of flattened but lon£
bones, contracted in the middle, and expanded at the ex¬
tremities, representing the metacarpal and metatarsal di¬
gital phalanges. (Home, Phil. Trans. 1816, 1818, 1819,
1820; De la Beche and Conybeare, Geological Transac¬
tions, vol. v. p. 559 ; and Cuvier, Ossemens Eossiles, vol. v.
part ii.)
SECT.
and Ple¬
siosaurus.
the middle, which, from their connection with the scapula,
are believed to be the coracoid bones; and above these
is a transverse piece, with a middle notch and lateral
sinuated elevations, which is regarded as the sternum;
while the scapula extends on each side like a buttress be¬
tween the two. It is not improbable, nevertheless, that
the middle portions named coracoid bones are the ster-
vol. m.
IV. OSTEOLOGY OF THE FISHES.
The Serpentine or Ophiad Reptiles present in their
osteological characters an approximative transition to
those of Fishes. While in the former order the skeleton
is reduced to the spinal column, ribs, and head, in the
latter class the spine and head only are left; and in some
tribes the transition is still more distinctly marked by
the presence of ribs.
The vertebra of a fish is distinguished from that of any Vertebra;,
other animal by the shape of its body. The cephalic and
caudal, or anterior and posterior surfaces, are hollow cup¬
like cones, so that the union of each two vertebrae forms
a double conical cavity, united by the base, containing a
substance composed of concentric fibro-cartilaginous layers,
with intermediate albuminous or gelatinous matter. By
this cartilage the vertebral bodies are united; and on this
the motions of the spine are effected. This motion, how¬
ever, is chiefly lateral; for the spinous processes are so
long, and the articulation so complex, that antero-poste-
rior inflection or extension is nearly impracticable.
In the cartilaginous fishes, for instance the shark, stur¬
geon, and lamprey, the vertebral bodies form simple tubes,
which, from the extreme elasticity of the constituent car-
tilage, propel the contained fluid to a considerable dis¬
tance. Thus Sir E. Home saw the fluid projected to the
height of four feet from the intervertebral cavities of the
shark. {Phil. Trans. 1809.) In this order, also, the
spine is infinitely more flexible, and its resilient power,
when bent by the muscles, is almost incredible. On each
side, also, the vertebrae are excavated, to form a canal for
lodging the large blood-vessels.
Ihe vertebrae of fishes are numerous, and not easily
distinguished into classes. They may, however, be dis¬
tinguished into two, according as the spinous process is
above only, or above and below at once. Those with the
dorsal spine only are denominated dorsal or abdominal verte¬
bra, and have commonly at the sides transverse processes
for the attachment of the ribs. Those with the dorsal
and ventral spines are distinguished as the caudal verte¬
bra. Ihe last caudal vertebra is generally trilateral, flat¬
tened in the vertical direction; and its tip is marked with
articular pits, which indicate the attachment of the small
elongated bones which sustain the caudal fins.
Ihe number of vertebrae varies. In the uranoscopus
or star-gazer there are only 25, in the balista 17, and in
the four-spined trunk-fish (ostracioii) only 13; while in
the sturgeon the number is 84, in the eel 115, and in the
shark 207.
Though Fishes have no chest, and require none, since
their respiratory organs are gills, all of them are not void
of ribs. The ray, shark, syngnathus, tetraodon, diodon, cy~
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