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Comparative
anatomy
One
problem in discussing the differences in arrangement of
muscles between the various vertebrate groups is in deciding
which muscles in each species are homologous; that is, which
have the same evolutionary and developmental origin. The
problem arises because the positionand attachment of muscles
change during evolution: a muscle lying in the same position
and attached to the same bone or cartilage in one vertebrate
may have different origins from those of another vertebrate
species. Comparison of the development of muscles in the
embryo of each species, and of their nerve supply, probably
will give the best clues. No single method may be relied
upon in all cases, and many different types of evidence
are considered before the homology is decided upon.
Vertebrate
muscles are given names derived from Latin according to
their attachments. In this system, the Latin names of the
bony points of attachment are either joined, as in the human
“sternocleidomastoid,” which runs from the sternum and clavicle
to the mastoid region of the skull, or they may be named
for their form or their gross function. There are several
standard terms that describe form and function. A muscle
may have more than one point of origin: thus itmay be described
as having, for example, two “heads” as in “biceps femoris”
(bi for two, -ceps for heads, femoris meaning “of the femur”).
It may be long, “longus,” or short, “brevis.” It may run
transversely across a body segment “transversus,” or obliquely,
“obliquus.” It may lie close to the surface, “superficialis,”
or deep, “profundus.” In describing function, flexors are
musclesthat tend to close the angle made by the two bones
to which they are attached, extensors tend to increase the
angle. Adductors pull a bone or cartilage closer to the
axis of the body, or limb, while abductors pull away from
the axis. Rotators turn one bone or cartilage with respect
to another, or turn it with respect to the midline. Pronators
turn the sole of the foot or the palm of the hand to face
the ground, while the opposite function is performed by
supinators. Constrictors and sphincters diminish the volume
of spaces or the area of structures, and dilatorsincrease
them. Because human anatomy is better understood than that
of other species, the names of muscles in humans often have
been applied to grossly equivalent muscles in animals, a
situation that often causes confusion.
