Muscular hydrostat

A muscular hydrostat is a biological structure found in animals. It is used to manipulate items (including food) or to move its host about and consists mainly of muscles with no skeletal support. It performs its hydraulic movement without fluid in a separate compartment, as in a hydrostatic skeleton.

A muscular hydrostat, like a hydrostatic skeleton, relies on the fact that water is effectively incompressible at physiological pressures. In contrast to a hydrostatic skeleton, where muscle surrounds a fluid-filled cavity, a muscular hydrostat is composed mainly of muscle tissue. Since muscle tissue itself is mainly made of water and is also effectively incompressible, similar principles apply.

Muscles provide the force to move a muscular hydrostat. Since muscles are only able to produce force by contracting and becoming shorter, different groups of muscles have to work against each other, with one group relaxing and lengthening as the other group provides the force by contracting. Such complementary muscle groups are termed antagonist muscles.

The muscle fibers in a muscular hydrostat are oriented in three different directions: parallel to the long axis, perpendicular to the long axis, and wrapped obliquely around the long axis.

The muscles parallel to the long axis are arranged in longitudinal bundles. The more peripherally these are located, the more elaborate bending movements are possible. A more peripheral distribution is found in mammalian tongues, octopus arms, nautilus tentacles, and elephant proboscides. Tongues that are adapted for protrusion typically have centrally located longitudinal fibers. These are found in snake tongues, many lizard tongues, and the mammalian anteaters.

The muscles perpendicular to the long axis may be arranged in a transverse, circular, or radial pattern. A transverse arrangement involves sheets of muscle fibers running perpendicular to the long axis, usually alternating between horizontal and vertical orientations. This arrangement is found in the arms and tentacles of squid, octopuses, and in most mammalian tongues. A radial arrangement involves fibers radiating out in all directions from the center of the organ. This is found in the tentacles of the chambered nautilus and in the elephant proboscis (trunk). A circular arrangement has rings of contractive fibers around the long axis. This is found in many mammalian and lizard tongues along with squid tentacles.

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