¡i l
T H E T R U N K .
The second division of the skeleton is subdivided into the spinal column, tho sternum, the ribs and
the pelvis.
THE SPINAL COLUMN,
In the Ichthyosaurus Chiroligostinus the spinal column is composed of about one hundred and fifty
vertebrae. E.xccpt a few bones of the tail, each vertebra has a body, which constitutes the principal part
of the bone, with a rounded anterior and posterior circumference; an arch formed by two processes for
the spinal manow, and articulatory processes witli notches above and below them for the passage of nerves
from the medullary chord. These are common characteristics, but as they differ very mucli from one
another in detail, we divide the spinal column into the dorsal and caudal regions.
T he Dorsal Vertebræ.
The atlas, the first dorsal vertebra, différa from the rest in the manner of its articulation with the
inferior occipital bone of the cranium, tho shape of its spinous process and non-possession of articulatory
surfaces, for it has no ribs. Our readera will remember that the tulierclc of the inferior occipital is large
and round; this is received into the anterior concave surface of the atlas—the posterior paries of which,
unlike those of all the other verteljrae, is fiat for synarthrodial attaclmient with the dentatus The superior
and inferior surfaces of its body are flattened, as are its sides also, the spinous process lias two scarcely
visible articulatory surfaces to meet those of the lateral occipitals, and its arch for the spinal chord is
exceedingly large; its extremity is tubercular.
The other dorsal vertebræ, distinguished by their articular surfaces for the attachment of ribs, ai'e
roi ty-foiir; tliey constitute tlie superior boundary of the thoracic and abdominal cavities. Of these the
first ten have upper and under fialtened surfaces to their bodies, as lias tlie atlas, the next twenty are
very concave both witliout and beneath after wliicli tliey become more and more flat.
Tlie antei ior doreal vertebræ to the number of twenty-two, exclusive of the atlas, may be known
from the others by two articular fossa for the ribs on each of their sides the remaining vertebræ
having but one which, instead of being rounded in a perfect circle as in the former, is degenerated into
a mere grove passing fi'om before to behind.
The spinous processes have hvo anterior and posterior transverse apophyses for articulation
with one anotlicr. Tlie median spine is oblong and thin with acute edges and a flattened head. In
heiglit tliey arc nearly equal to the long axis of tlic body of tlie vertebra to wiiicli tlicy individually belong,
and in breadtli equal to tlicir transverse diameter.
T he Caudal Verterr.u.
These, which amount to about one hundred and twelve, liave larger bodies in their sacra! province
than the dorsal vertebræ. The first ninety of tliese caudal vertebræ have transverse apophyses
instead of ilbs, wliicli articulate in concave fossæ. Tliesc spines have round lieads. long, slender, and
narrow bodies, and flat, wide and thin extremities convexly curved. They grow less aud less willi tlic
.bodies to which they belong as the tail approaches toapoint. Tlie other twenty-two vertebræ are more
square than their predecessors; the lateral processes, wliicli commenced with the pelvic portion, and the
spinous apophyses distinguish about two-thiixls of thoir number. Tliosc tliat remain liavo a simple body
with Ilattened sides and a slightly concave surface superiorly.
THE STERNUM, n «.
The sternum is composed of a median bone, with a thick rounded body, two lateral processes
anteriorly, cacli ending in a thin point, and an ensiform base, and another bone which, spreading on
both paries of the thoracic region, rests its attenuated portion upon tho anterior edge of the clavicles.
THE RIBS.
Tlic forty-four rilis on each side of tlic ab<lomcn arc attached superiorly to tho dorsal vertebrae. Tlic
anterior twenty-two have a thicker and stronger head than the othere and two convex articular surfaces
for tlie concave tubercles of the vertebrae lo which tlicy belong; they are grooved in ibc upper two-thirds
of their long diameter and have rounded anterior and posterior edges.
The remaining ribs articulate by one hollow tubercle to ttic vertebrae and have a JluUened head and
a sharpened lerminalion.
T he Sterno-costal Arcs on I nter-articular R ibs.
Tlioso arc twenty-two in number. Each arc is composed of five bones. The cc7ttral rib is round
and strong at its median portion and declines on either side until it ends in a point, which sliiles upon
a groove formed by the two external bones ;—these again support the inferior extremities of Che anterior
twenty-two ribs which, as in the former instance, slide upon the groove at tlicir superior part.
The most anterior arc rests occasionally against the posterior edge of tlic scajiular bones.
THE PELVIS,
The bones wliich constitute tlie pelvis arc six,—the ilia, ischia and pubes. They form the cavity
for the acetabulum in whicli the femur articulates.
T he Ilia. b.
The ilia arc the largest and highest in situation of the bones tliat form the pelvis. They have a
body and an e.xpanded portion. Tlie body is tlie lower part of llic ilia, and ends in an articular surface;
from tliis portion of tlie bone its expanded surface passes upwards and terminates in a semi-lunar
edge, wliicli presents an external convex and an internal concave smfacc. Tlic ilia are striated
longitudinally in (heir upper half.
Connexion. With the pubes and ischia at tlicir articular body, and with the sacral vcrtebrm by
ligament.
T he Ischia, c.
Tliese bones arc situated at the posteiior and inferior region of tlie pelvis; they liavc a body, and a
descejiding expanded portion. The body is furnished above with an articular surface which constitutes
the back part of the acetabulum. Tlic descending e.xpanded portion ends iu a convex ridge.
Cojinexion. Above witli,the ilia and pubes, below with cacli otlier and the pubes.
T he P ubes, d.
These arc also divided into a body an<l a descending expanded portion. The body is the tliicliest
]iai t of tlic bone, and is furnished with an articular extremity at its femoral part; this constitutes tlic
anterior and inner part of the acetabulum. The anterior convex edges of the descending expanded
portion compose, with those of the ilia, the anterior brim of the pelvis.
Connexion. The pubes arc attached superiorly to the ilia and ischia, and iiircriorly and anteriorly
to the ischia and one another.
T H E E X T R E M IT IE S .
The extremities form tlio third great division of the skeleton and arc called anterior and posterior.
The upper or anterior extremities are attached to the trunk by tho clavicles and scapulm. For distinctness
sake wc call the bones that enter into tlicir composition the shoulder, arm, I'ore-aim and paddle.