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.
Muscular anatomy
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 antagonistic pairs.
The muscle fibers in a muscular hydrostat are oriented in three 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 tetrapod tongues, octopus arms, nautilus tentacles, and elephant trunks. 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.
Helical or oblique fibers around the long axis are generally present in two layers with opposite chirality and wrap around the central core of musculature.
Mechanism of operation
In a muscular hydrostat, the musculature itself both creates movement and provides skeletal support for that movement. It can provide this support because it is composed primarily of an incompressible “liquid" and is thus constant in volume. The most important biomechanical feature of a muscular hydrostat is its constant volume. Muscle is composed primarily of an aqueous liquid that is essentially incompressible at physiological pressures. In a muscular hydrostat or any other structure of constant volume, a decrease in one dimension will cause a compensatory increase in at least one other dimension. The mechanisms of elongation, bending and torsion in muscular hydrostats all depend on constancy of volume to effect shape changes in the absence of stiff skeletal attachments. Since muscular hydrostats are under constant volume when the diameter increases or decreases, the length must also decrease or increase, respectively. When looking at a cylinder the volume is: V=πr²l. When the radius is differentiated with respect to the length: dr/dl=-r/(2l). From this, if a diameter decreases by 25%, the length will increase by approximately 80% which may produce a large amount of force depending on what the animal is trying to do.
Elongation and shortening
Elongation in hydrostats is caused by the contraction of transverse or helical musculature arrangements. Given the constant volume of muscular hydrostats, these contractions cause an elongation of the longitudinal muscles. Change in length is proportional to the square of the decrease in diameter. Therefore, contractions of muscles perpendicular to the long axis will cause a decrease in diameter while keeping a constant volume will elongate the organ length-wise. Shortening, on the other hand, can be caused by contraction of the muscles parallel to the long axis resulting in the organ increasing in diameter as well as shortening in length.