An architectural analysis is offered of the trunk muscles in fish, which are arranged in a longitudinal series of geometrically complex myomeres. The myomeres are separated by myosepta, collagenous sheets with complex fibre patterns. The muscle fibres in the myomeres are also arranged in complex three-dimensional patterns. Previously, it has been proposed that the muscle fibre arrangement allows for a uniform strain distribution within the muscle. Physical constraints limit the range of shapes that fibre-reinforced materials such as muscles can adopt, irrespective of their genetic profile. The three-dimensional shapes of myosepta are predicted by mechanical modelling from the requirements for mechanical stability and prescribed muscle fibre arrangements. The model can also be used to study the force transmission and likely locations of ligaments and bones in the myosepta. The model shows that the dorsal and ventral fins are located such that unfavourable mechanical interactions with the trunk muscles are avoided. In bony fish, extensive muscular deformations (notably in the region of the horizontal septum) that would not contribute to bending are avoided by the mechanical support of the skin, intramuscular bones and ribs. In sharks, the skin plays a more prominent role in avoiding such deformations because of the absence of bony elements.

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