Of the muscles of the stem or axis, those of the neck and tail are
well-developed and specialized, while those of the lower back are more
or less reduced, or even completely degenerated owing to the rigidity of
this region, brought about by the great antero-posterior extent of the
pelvis.
The muscles of the limbs show a great amount of specialization, away
from the fundamental reptilian and mammalian conditions. The muscles of
the fore limbs are most aberrant, but at the same time more uniformly
developed than those of the hinder extremities. The reasons are obvious.
The whole wing is a unique modification, deeply affecting the skeletal,
muscular and tegumentary structures, but fluttering, skimming, sailing,
soaring are motions much more akin to one another than climbing and
grasping, running, scratching, paddling and wading. The modifications of
the hind-limbs are in fact many times greater (such as extremely long
legs, with four, three or only two toes; very short legs, almost
incapable of walking, with all four toes directed forwards, or two or
one backwards, and two or more connected and therefore bound to act
together, in various ways). Thus it has come to pass that the muscles
of the hind limbs are, like their framework, more easily compared with
those of reptiles and mammals than are the wings, whilst within the
class of birds they show an enormous amount of variation in direct
correlation with their manifold requirements. The only really aberrant
modifications of the wing-muscles are found in the Ratitae, where they
are, however, all easily explained by reduction, and in the penguins,
where the wings are greatly specialized into blades for rowing with
screw-like motions.
The wing of the bird is folded in a unique way, namely, the radius
parallel with the humerus, and the whole wrist and hand with their
ulnar side against the ulna; upper and forearm in a state of
supination, the hand in that of strong abduction. Dorsal and ventral
bending, even in the extended wing, is almost impossible. Consequently
only a few of the original extensor muscles have been preserved, but
these are much modified into very independent organs, notably the
_extensor metacarpi radialis longus_, the _ext. metac. ulnaris_ and
the two _radio_- and _ulnari-metacarpi_ muscles, all of which are
inserted upon the metacarpus by means of long tendons. The chief
muscular mass, arising from the sternum in the shape of a U, is the
_pectoralis_ muscle; its fibres converge into a strong tendon, which
is inserted upon the greater tubercle and upper crest of the humerus,
which it depresses and slightly rotates forwards during the
downstroke. This great muscle covers completely the
_supracoracoideus_, generally described as the second pectoral, or
_subclavius_ muscle, in reality homologous with the mammalian
_supraspinatus_ muscle. This arises mostly from the angle formed by
the keel with the body of the sternum, passes by a strong tendon
through the _foramen triosseum_, and is inserted upon the upper
tubercle of the humeral crest, which it rotates and abducts. The
extent of the origin of this muscle from the sternum, on which it
leaves converging, parallel or diverging impressions, is of some
taxonomic value.
[Illustration: FIG. 14.--Wing muscles of a Goose. Bi, Biceps; Elast.
sec., elastic vinculum and Exp.sec., _expansor secundariorum_; Pt.br
and Pt.lg, short and long propatagial muscles; Tri, triceps.