SKELETAI^ MUSCUI.ATURE OF THE KING SALMON. 



31 



length of the fish. In the caudal region, for example, this extent is so great that the 

 contraction of a single myomere, should it occur, would bend the skeleton toward that 

 side through an extent of several segments. The longitudinal extent of a myomere in 

 the caudal region, opposite the anal fin, is 12 centimeters, i. e., 15 myomeres, of the 

 muscle. The alternate cone-like folds of the septa mutually support each other. It 



Fig. I. — Diagrams to illustrate the mechanical relations of the muscle fibers and tendonous septa of the lateral muscles. The 

 diagrams should be considered in comparison with the dissections presented in plates i and ii. A, position in rest; B, position- 

 during contraction of the left side. 



The figures are drawn to represent a composite view of an idealized transverse plane that world cut the individual myomeres 

 and septa through the greatest longitudinal extent. This plane cuts the anterior fold in the median line and the posterior fold 

 through a plane somewhat dorsal to the median line. The posterior folds are less obUque to the skeletal axis than are the anterior 

 folds. More anteriorly the septa will be less obUque, posteriorly more oblique than shown (see pi. i). This diagram is con- 

 structed for the region under the dorsal fin. Note that during contraction of one side the individual fibers on the opposite side 

 are stretched slightly, a condition favorable to the expenditure of contractile energy. Note also that the muscle fibers retain 

 their relatively parallel position with reference to the adjacent skeletal axis. The anterior folds of the septa of the anterior sur- 

 faces of the myomeres act as anchors against the posterior folds (dorsal and ventral) of the septa of the posterior surface. 

 As both are inelastic they serve as admirable tendons. Considering the depth of the septa it is obvious that flexion will increase 

 the thickness of the mass of muscle slightly. But the anchoring is such that during flexion parallel septa move or shear over 

 each other in such a way as to produce a maximal amount of movement of the trunk of the salmon by a relatively small amount 

 of muscle fiber shortening, a most advantageous physiological justification of a complex anatomical mechanical relation. 



is obvious that the successive septa are very close together and that the fibers from 

 one to the other run very obliquely. In other words, when a contraction occurs every 

 individual fiber is in the best mechanical position to expend all its energy in a much 

 more direct pull on the septal sheet and on the skeleton than would be the case if the 

 myocommata were simple vertical septa placed at right angles to the axis of the fish. 



