The proximal skeletal
elements of each pair of limbs (humeri
) are supported
by a primitively cartilaginous, pectoral, or pelvic
girdle, which lies external to the costal elements of the vertebral
skeleton. This girdle may consist of a simple cartilaginous
arc (as in the Sharks and Rays), or it may be complicated
by subdivisions and additions.
|Fig. 12. - Side-view of the pectoral arch and sternum of a Lizard
(Iguana tuberculata).- Sc, scapula; 8.8C, supra-scapula; cr,
coracoid; gl, glenoidal cavity; St, sternum; a.st,
xiphisternum; m.sc, mesoscapula; p.cr, precoracoid; m.cr, mesocoracoid; e.cr,
epicoracoid; cl, clavicle; i.cl, interclavicle.
The pectoral arch may be connected with the skull, or
with the vertebral column, by muscles, ligaments, or dermal
ossifications, though, primitively, it is perfectly free from, and
independent of, both; but it is never united with the vertebrae
by the intermediation of ribs. At first, it consists of one
continuous cartilage, on each side of the body, distinguishable
only into regions and processes, and affording an articular
surface to the bones or cartilages of the limb. But ossification
usually sets up in the cartilage, in such a way as to give
rise to a dorsal bone, called the scapula
, or shoulder-blade,
which meets, in the articular, glenoidal
cavity for the hu'
merus, with a ventral ossification, termed the coracoid
|Fig. 13. - Ventral view of the sternum and pectoral arches of Iguana tuberculata.
The letters as in Fig. 12.
By differences in the mode of ossification of the various
parts, and by other changes, that region of the primitively
cartilaginous pectoral arch which lies above the glenoidal
cavity may be ultimately divided into a scapula
and a suprascapula
; while that which lies on the ventral side may present
not only a coracoid,
but a precoracoid
and an epicoracoid
In the great majority of the Vertebrata
above fishes, the
coracoids are large, and articulate with the antero-external
margins of the primitively cartilaginous sternum
, or breastbone.
But, in most mammals, they do not reach the sternum,
and, becoming anohylosed with the scapula, they appear, in
adult life, as mere processes of that bone.
Numerous Vertebrates possess a clavicula
, or collar-bone,
which is connected with the pre-axial margin of the scapula
; but takes no part in the formation of the
glenoid cavity, and is usually, if not always, a membrane
bone. In many Vertebrata
, the inner ends of the clavicles are connected with and supported by, a median membrane bone which is closely connected with a ventral face of the sternum. This is the interclavicula,
frequently called episternum.
The pelvic, like the pectoral, arch at first consists of a
simple continuous cartilage on each side, which, in Vertebrata
higher than fishes, is divided by the acetabulum
, or articular
cavity for the reception of the head of the femur, into a dorsal
and a ventral moiety.
Three separate ossifications usually take place in this cartilage-one in the dorsal, and two in the ventral, moiety.
Hence, the pelvic arch eventually consists of a dorsal portion,
called the ilium
, and of two ventral elements, The pubis
and the- ischium
posteriorly. All these generally enter
into the composition of the acetabulum.
|Fig. 14 - Side-view of the left Os Innominatum of Man; Il, ilium; Ia, ischium; Pp, Pubic A, acetabulum; Pp, Poupart's ligament.
The ilium corresponds with the scapula. In the higher Vertebrata
the outer surface of the latter bone becomes divided
by a ridge into two fossas. The ridge, called the spine
of the scapula
, frequently ends in a prominent process termed
, and with this, in Mammalia
, the clavicle articulates.
In like manner, the outer surface of the ilium becomes divided by a ridge which grows out into a great crest
in Man and other Mammalia
, and gives attachment to muscles
The ischium corresponds very nearly with the coracoid in
the pectoral arch; the pubis with the precoracoid, and more
or less of the epicoracoid.
The pelvis possesses no osseous element corresponding
with the clavicle, but a strong ligament, the so-called Poupartes
stretches from the ilium to the pubis in many Vertebrata
and takes its place. (Fig. 14, Pp.)
On the other hand, the marsupial bones
of certain mammals,
which are ossifications of the tendons of the external
oblique muscles, seem to be unrepresented in the pectoral
arch; while there appears to be nothing clearly corresponding
with a sternum in the pelvic arch, though the precloacal cartilage
, of Lizards has much the same relation to the
isohia as the sternum has to the coracoids.
Very generally, though not universally, the ilia are closely
articulated with the modified ribs of the sacrum. The pubes
and ischia of opposite sides usually meet in a median ventral
symphysis; but in all birds, except the Ostrich, this union
does not take place.