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Tetrapod
musculature
In
the living urodeles (the newts and salamanders) of the class
Amphibia, the axial muscles are most important for propulsion.
The limbs of urodeles are quite weak and tend to be carried
forward passively with the undulations of the body. As the
primary propulsive force is provided by the muscles of the
trunk, urodeles retain large axial muscles. The axial muscles
are still segmented, separated by myocommata, although the
myomeres run vertically and without the elaborate folding
seen in jawed fishes. The epaxial muscles, given the name
“dorsalis trunci” in tetrapods, are little changed, although
some modification has taken place to promote a facility
for dorsoventral bending of the spine that occurs in tetrapods
but rarely in fishes.
The
anurans (frogs and toads) have rather similar, but considerably
reduced, epaxial muscles. There is, however, a trend in
tetrapods toward finer control of muscular action with increasing
complexity. In the reptiles the epaxial muscles, although
still retaining a semisegmental structure, are divided into
several structural and functional units. The deepest set
of muscles, the transversospinalis group, are short and
run obliquely forward, over one to four vertebrae, from
the transverse process of one vertebra to the lamina (the
flat plate of bone at the base of the vertebral spine) of
a more anterior vertebra. The transversospinalis group is
particularly responsible for rotatory movements of the spine.
Superficial to transversospinalis lies longissimus, with
much longer fibres, which is important in extension of the
back. More superficially still, and lateral to these muscle
blocks, iliocostalis is a flat, sheetlike muscle that runs
from the pelvic girdle upward and laterally to attach to
the ribs. It is particularly important in lateral flexion
(bending) of the spine. This general pattern is further
complicated in snakes, which have secondarily returned to
the propulsive use of the axial muscles. In birds the vertebral
column of the trunk region undergoes much fusion, and this
complexity is reduced,as indeed it is in chelonians (turtles
and tortoises). Mammals retain the broad pattern of the
reptile epaxial musculature but (with the exception of the
innervation of the musculature) have greatly reduced the
segmentation that is present in reptiles.
In
the tails of urodeles the hypaxial muscles are also largely
unchanged. As with all land vertebrates, however, the demands
of support of the viscera when living in an air environment
have brought about major modifications of the hypaxial musculature
of the trunk. In typical tetrapods a strong series of ribs
has developed for the same reason. Although urodeles have
secondarily reduced their ribs, they show many of the typically
tetrapod features of the hypaxial musculature. The muscles
fall into three groups. A group of subvertebral muscles
forms ventral to the vertebrae, in the region of their joints
with the ribs at the transverse processes. It acts in ventral
and lateral flexion (bending) of the spine. A rectus abdominis
muscle runs longitudinally along the ventral aspect of the
body wall between the pectoral and pelvic girdles, and laterally
this is associated with the third group, the lateral hypaxial
muscles. This consists of three major layers of muscle whose
fibres are oriented in differing directions, a feature that
gives additional strength to the body wall. Superficially
lies the external oblique muscle, with fibres running longitudinally
but somewhat ventrally; deep to this lies the internal oblique,
with fibres running longitudinally and somewhat dorsally;
and deepest lies the transversus muscle, whose fibres run
dorsoventrally.
In
the higher tetrapods the external and internal obliques
tend to become further divided into layers in the abdominal
region. The thoracic representatives of these muscles tend
to become divided into discontinuous, rather thin muscle
layers between the ribs (external and internal intercostals),
superficial to the ribs (supercostals), and deep to the
ribs (subcostals). While only the rectus abdominis tends
to retain visible evidence of segmental origin, in its tendinous
intersections (which are present even in humans) the segmental
innervation of the hypaxial muscles is retained in all tetrapods.
In
tetrapods, unlike fishes, the pectoral girdle does not have
a solid bony connection to the axialskeleton but rather
is supported by a series of muscles derived from the outer
layer of hypaxial trunk muscles. This is no doubt another
adaptation to life in an air environment, where the cushioning
effect of water has been lost. These muscular slings are
not readily demonstrated in the living amphibians, which
are either skeletally degenerate as in urodeles or highly
specialized toward leaping, as in the anurans (frogs and
toads). In more typical tetrapods,there are two major derivatives
of the external oblique attaching the scapula (shoulder
blade) to the body: first, the serratus, made up of numerous
fingerlike slips running from the scapula tothe neighbouring
ribs; second, levator scapulae, which are fused with serratus
along its caudal (tail-end) border. Levator scapulae consist
of fibres running more anteriorly to ribs or transverseprocesses
of the neck. Mammals, and some reptiles, have a third such
muscle, attaching the pectoral girdle to the region of the
spine, called rhomboideus. The mammals also have utilized
part of the hypaxial musculature to form a muscular septum
between the region of the lungs and heart (the thoracic
cavity) and the region of the digestive and reproductive
viscera (the abdominal cavity). This is the diaphragm, which
is the most important respiratory muscle in themammalian
body.
The
six axially derived eye muscles of fishes undergo only small
modifications in tetrapods. Eye movements are changed, partly
according to changes in the orientation of the orbit such
as the trend toward orbital frontality that is typical in
the primates. Additional eye muscles may be derived by splitting
some of these six muscles. An example of this is the retractor
bulbi muscle, which is derived from the lateral rectus muscle.
In amphibians and some reptiles it pulls the eyeball deeper
into the orbit for protection, and in amphibians it is an
aid in swallowing. Another example is the levator palpebrae
superioris, derived from the superior rectus, which elevates
the upper eyelid to open the eye.
The
limb muscles of typical tetrapods are derived from the dorsal
and ventral muscle blocks of the paired fins of fishes.
In tetrapod development, this pattern of derivation from
dorsal and ventral muscle blocks is repeated. As a consequence,
the homologies of the muscles of the typical tetrapod limb
often can be traced by considering the source of innervation
of each muscle from the nerves of the dorsal (or extensor)
compartment or the ventral (or flexor) compartment.
In
the pectoral limb, the dorsal, extensor group of muscles
includes several that appear consistently and with similar
roles. Beginning with the muscles that act on the humerus
(the proximal bone of the limb), all tetrapods have a large,
sheetlike muscle, known as the latissimus dorsi, which runs
from the side of the trunk to the humerus. The latissimus
dorsi muscle retracts the humerus and thus propels the body
forward. Acting to rotate, flex, or adductthe humerus, depending
on limb posture, is a muscle known as subcoracoscapularis
in amphibians, reptiles, and birds and subscapularis in
mammals. It runs from the deep surface of the shoulder girdle
to the humerus. In amphibians, the dorsalis scapulae arise
from the anterior edge of the scapula. The same muscle is
known as the deltoideus in reptiles and mammals, andin the
latter, part of its origin moves from the scapula to the
clavicle (collar bone). It is a major abductor of the shoulder
in most tetrapods. At the elbow joint, all tetrapods have
a muscle called triceps as the major extensor. It arises
in several heads from the shoulder girdle and humerus. There
are always a variable number of extensor muscles for the
wrist and digits (fingers and toes) arising from the region
of the elbow joint, on the lateral aspect of the humerus.
On
the ventral, flexor aspect of the pectoral limb, the pectoralis
is found in all tetrapods. The pectoralis runs from the
chest wall to the humerus, on which it acts to pull the
humerus downward and backward. This muscle is not only important
in providing forward thrust in quadrupedal locomotion but
is the chief depressor of the forelimb in birds and bats.
The major elevator of the wing in birds, supracoracoideus,
is present in all tetrapods. In mammals, the supracoracoideus
retains its attachment to the humerus, but its previous
point of origin (the coracoid plate) disappears and the
muscle now appears as two separate blocks of muscle arising
on either side of the spine of the scapula as an abductor
muscle (supraspinatus) and a rotator and flexor (infraspinatus).
Coracobrachialis and (except in amphibians) biceps arise
from the tip of the coracoid and act to flex the elbow.
In this they are aided by the brachialis muscle, which arises
from the humerus. As on the extensor aspect, there are always
a number of flexors of the wrist and digits. These arise
on the medial side of the distal humerus.
The
muscles of the pelvic limb cannot be readily compared beyond
the reptiles and mammals. Even in these cases, changes in
limb posture have led to major changes in the arrangement
and function of muscles. On the dorsal aspect, a single,
large muscle in reptiles, puboischiofemoralis, runs from
the bones of the pelvis to the femur (the proximal bone
of the hind limb). This reptilian muscle appears to be represented
by three mammalian hip muscles: psoas, iliacus, and pectineus.
Iliofemoralis acts as an abductor of the hip in reptiles
and appearsto be represented by the gluteal muscles in mammals,
but the function of the gluteal musclesis different. More
similar in reptiles and mammals is the quadriceps or quadratus
femoris, which consists of multiple heads (four in mammals)
that arise from the pelvic girdle and femur and insert by
a common tendon into the tibia (the larger bone of the distal
pectoral limb). It is the sole extensor of the knee joint
in both the reptiles and the mammals. The extensors of the
ankle and digits in both reptiles and mammals are not dissimilar
to those of the pectoral limb and take origin from the lateral
and anterior surfaces of the two distal bones of the pelvic
limb. On the ventral aspect of the hind limb, small, deep
muscles run from the internal and external pelvis to the
head of the femur and help in adduction and rotation. Of
these, the puboischiofemoralis externus of reptiles appears
to be represented by the obturator externus of mammals,
and similarly the ischiotrochantericus of reptiles appears
to be the homologue of the obturator internus of mammals.
Again, the major adductor of the hip of reptiles, adductor
femoris, appearsto be homologous with some of the muscles
called the adductors in mammals. There seem to be some homologies
between the major flexors of the hip and thigh in reptiles,
such as puboischiotibialis, and two deeper muscles, flexor
tibialis externus and internus, and some functionally equivalent
muscles in mammals: the gracilis, semimembranosus, and semitendinosus.
In reptiles, the axial muscle of the tail is strong, and
the caudifemoralis, a powerful flexor of the thigh which
originates in the tail, is consequently large. The tail
in mammals, although usually present, is much more gracile,
and as a result caudofemoralis is represented by only a
few small muscles. Another major change is in the flexors
of ankle and digits. In reptiles, these insert by long tendons
passing below the ankle joint, much as in the forelimb.
In mammals, however, the equivalent long flexor, gastrocnemius,
inserts on a new bony process, the calcaneal tuberosity
or heel bone, which gives more efficient leverage.
The
hypobranchial muscles of tetrapods are both reduced and
modified in comparison with those of jawed fishes. In tetrapods
these straplike muscles still arise from elements of the
pectoral girdle but now pass to the new derivatives of the
gill arches of fishes: the hyoid bone and laryngeal cartilages.
They act primarily in the gross movements of these structures
in swallowing and the production of sound—for example, as
depressors of the hyoid (sternohyoid, omohyoid) or of the
larynx (sternothyroid). Fibres of the hypobranchial muscles
in the region of the hyoid are utilized to form the internal
musculature of the tongue.
The
branchial musculature is also modified in tetrapods from
the condition seen in jawed fishes.The development of a
shoulder muscle, the trapezius, from the levator muscles
of the gill arches of fishes, as previously discussed, is
taken further in tetrapods, by the separation of further
slips of muscle to form muscles such as sternocleidomastoid,
a muscle important for humans in movements of the head and
in breathing. In mammals that lose the clavicle, these slips
may further be modified to form muscles running from the
head to the pectoral limb. Tetrapods, with the exception
of mammals, utilize part of the constrictor muscle of the
hyoid arch to form the depressor mandibulae, which replaces
the hypobranchial muscles as the major jaw-opening muscle.
The restructuring of the posterior jaw in mammals leads
to the further replacement of this new muscle by the digastric,
which is a compound muscle made upof parts of the constrictors
of the first and second branchial arches. Thus it is partly
innervated by the mandibular division of the fifth cranial
nerve (as is the case with other jaw muscles and the tensor
tympani, one of the muscles of the ear), and partly by the
seventh cranial nerve, the facial nerve (which also supplies
an ear muscle associated with the stapes, an ear bone derived
from the hyoid arch). The levator palatoquadrati, which
elevates the upper jaw in jawed fishes, is retained as a
jaw muscle in birds and in some reptiles, as they share
the ability of fishes to move the upper jaw. The adductor
mandibulae is much altered in tetrapods, although its overall
function is retained. During the course of tetrapod evolution
it becomes a superficial muscle, and in mammals it splits
into several functional units arising from the undersurface
and side of the skull and attaching to various points on
the mandible. These are: the lateral pterygoid, which pulls
the jaw forward; the medial pterygoid and its partner, the
masseter, which close the jaw and move it from side to side;
and the temporalis, which closes the jaw and pulls it backward.
All are innervated by the first-arch cranial nerve, the
fifth nerve. The intermandibularis of jawed fishes is retained
as the mylohyoid of tetrapods, which is an elevatorof the
tongue.
Finally,
the constrictor muscle of the hyoid arch, which in bony
fishes is used to control the operculum, is remodeled in
tetrapods as a sheathing superficial muscle of the neck,
the sphincter colli. It derives its innervation from the
nerve of the hyoid arch, the seventh, or facial, nerve.
This cranial nerve is named from the further adaptation
of the sphincter colli muscle in mammals, particularly in
higher primates, as the many small muscles of facial expression,
which allow people to smile, laugh, and frown.
