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Muscles
of the trunk
The
consequences of an upright posture for the support of both
the thoracic and abdominal viscera are profound, but the
muscular modifications in the trunk are few. Whereas in
pronograde animals the abdominal viscera are supported by
the ventral abdominal wall, in the orthograde posture most
support comes from the pelvis. This inevitably places greater
strain onthe passage through the muscles of the anterior
abdominal wall, the inguinal canal, which marks the route
taken by the descending testicle in the male. Weakness in
the canal can result in herniation.
Differences
are also seen in the musculature (the levator ani) that
supports the floor of the pelvis and that also controls
the passage of feces. The loss of a tail in all the apes
has led to a major rearrangement of this muscle. There is
more overlap and fusion between the various parts of the
levator ani in modern humans than in the apes, and the muscular
sling that comprises the puborectalis in humans is more
substantial than in the apes.
