(a) Location
of the lung inside the turtle shell. (b) Cutaway view of the lung
showing its internal structure. Turtle lungs lie within a protective,
rigid shell. Consequently, the fixed rib cage cannot act in
ventilating the lungs. Instead, turtles have sheets of muscles within
the shell that contract and relax to force air in and out of the
lungs. Turtles also have the ability to alter air pressure within the
lungs by moving their limbs in and out of the shell. (c) In the
specialized tortoise, a diaphragmatic muscle is absent but other
respiratory muscles take its place.Within the rigid shell, the
viscera are enclosed by limiting membranes that under muscle
action alter their position during exhalation (solid line) and
inhalation (dashed line). During active exhalation, contraction of
the transversus abdominis pulls the posterior limiting membrane
up against the lung and contraction of the pectoralis draws the
shoulder girdle back into the shell, further compressing the
viscera. During active inhalation, exhalation muscles relax and
contraction of the obliquus abdominis and girdle protractor
expand the visceral cavity by pulling the posterior limiting
membrane outward and the shoulder girdle forward, respectively.