The Origin and Evolution of Mammals - Moodle

104 THE ORIGIN AND EVOLUTION OF MAMMALS
the back part of the humerus head inwards and
forwards, which imparted the anterior shift of the
head of the humerus in the glenoid and therefore
the posterior retraction movement of the shaft of
the bone. These two muscles were primarily
responsible for the power stroke of the forelimb.
The accompanying rotation of the humerus about
its long axis was caused by the exact points of insertion
of the muscles. The pectoralis inserts in front of
the line of the axis of the humerus; the subcoracoscapularis
inserts behind that line. Both therefore
impart an anticlockwise rotation, as viewed from
the left. The final component of the power stroke,
extension of the limb at the elbow, was the result of
contraction of the triceps muscle that ran from the
girdle and dorsal surface of the humerus to an
insertion on the olecranon process of the ulna.
The recovery phase of the limb cycle required flexion
of the elbow by means of the biceps muscle from
the coracoid to the flexor surface of the radius and
ulna. Elevation of the humerus resulted from the
contraction of several muscles running dorsally from
the humerus. The deltoideus connected the clavicle
and dorsal edge of the scapula to the inner end of
the delto-pectoral crest of the humerus, and the latissimus
dorsi was a broad sheet of muscle from the
tissue facia of the sides and back of the animal to a
transverse ridge distal to the head of the humerus.
The main protractor muscle was the supracoracoideus,
originating from the lateral face of the coracoid
and lower part of the scapula. Its insertion was
on the anterior part of the head of the humerus and
by imparting an inwardly directed force to the head,
it caused the head to move posteriorly and the shaft
to protract anteriorly, exactly the reverse of the effect
of subcoraco-scapularis during retraction. Again, the
rotation of the humerus about its long axis, this time
clockwise as viewed from the left, resulted from the
points of attachment of the recovery-phase muscles
relative to the line of axis of the bone.
Hindlimb. The ilium, pubis, and ischium are comparable
to one another in size and together constitute a
broad, plate-like pelvis with the acetabulum occupying
the middle (Fig. 4.6(a)). This is in the form of
a simple, quite shallow concavity into which the convex
articulating surface on the proximal end of the
femur fits. The femur (Fig. 4.6(c)) was constrained
to a mainly horizontal plane, but within that, its
movement was quite free. It could undergo different
combinations of protraction and retraction, elevation
and depression, and rotation about its long axis.
As with the equivalent forelimb bones, the tibia and
fibula of the hindlimb participated in four joints to
control and accommodate the various relative movements
between the distal end of the femur and
the foot, although the details are quite different. The
knee joint (Fig. 4.6(c)–(e)) was a very strongly built
hinge joint, by virtue of the wide articulating head
of the tibia connecting to both of the two articulating
condyles on the ventral side of the end of the femur.
In contrast, the fibula is a far more slender bone
whose head articulates only with the side of one of
the femoral condyles. Most of the hinging action at
the ankle was controlled by the articulation of the
lower end of the fibula with both the proximal ankle
bones, calcaneum, and astragalus. The tibia, so
broadly expanded proximally, has a relatively slender
distal end, which only articulates with the side
of the astragalus. The relative rotational movement
between the end of the femur and the foot occurred
by rotation between both the tibia and the fibula
independently of each other on their articulations
with the underside of the femur (Fig. 4.6(d)). The
importance of this arrangement may have been to
restrict the ankle joint exclusively to a hinging
action rather than allowing rotation as well, which
was better suited to transmitting large forces to the
ground during locomotion. The pes is interpreted as
digitigrade, with the large astragalus and calcaneum
held vertically and the several tarsal bones
and digits spreading the weight of the animal over
the ground (Fig. 4.6(e)). As with the forefoot, there
was considerable flexibility between the bones, and
the primitive hind foot digital formula of 2-3-4-5-4
was retained.
The musculature of the hindlimb (Fig. 4.6(b)) was
quite simple in principle, with four major muscles
for each of the four major movements of the femur
at the hip joint. The pubo-ischio-femoralis externus
ran from the outer surface of the pubo-ischiadic
plate to the ventral side of the femur and caused
adduction. The huge caudi femoralis muscle was
inserted on to the fourth trochanter on the underside
of the femur, and extended the entire length of
the tail. This, the largest muscle in the primitive
tetrapod’s body, caused the powerful retraction of
the femur that provided the main locomotory force.