THE LATERAL MUSCULATURE AND THE SWIMMING OF FISH

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Abstract
SUMMARY: The lateral musculature of the fish‐like chordates has been examined in an effort to find reasons for the peculiar pattern of the myomeres. There are three distinct types of myomeres: amphioxine, cyclostomine and piscine. All have in common the function of bending their possessors' bodies from side to side to provide locomotor force. In all cases pull is applied obliquely to the long axis of the body. The myomeres pull at once latero‐craniad and laterocaudad to bend the body from its position of equilibrium, or rest, into concavity. If continued, this movement would lead to a condition of instability, in which the vectors of the forces applied would pass across the axis of bending and the body would be distorted. However, the length of the body and its resistance to compression prevents this. Also, during locomotion, restorative forces are developed on the convex side of the body to supplant the original bending movement and the body passes through the equilibrium position to become concave on the opposite side. These reactions follow one another through myomere after myomere from head to tail along the body of the animal; the result is lateral waves passing rapidly down the body, the reaction of which with the water provides locomotion. Each myomere covers several vertebral segments and succeeding myomeres overlap. This allows the bending to progress smoothly along the body as each myomere affects not only its own section of the body but also those of its neighbours.The myomeres gain attachment to vertebral centra, neural and haemal spines (or ribs) and the connective tissue septa between these structures. Secondary attachments by means of longitudinal ligaments, formed by the apposition of deep myosepta, often occur, especially among the Chondrichthyes.The mechanical advantage of a segment of the lateral musculature is very small, but it approaches unity among the tunas, In these fish the muscle is concentrated in the body region, while the caudal peduncle supports long tendons running to the caudal fin.Intermuscular bones appear in the myosepta of many species of fish. These appear to have been developed in response to stresses and strains set up by the musculature.

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