. Annals of the South African Museum. Annale van die Suid-Afrikaanse Museum. i74 ANNALS OF THE SOUTH AFRICAN MUSEUM 1 discussion is not concerned as it has no bearing on the mechanism of the rigid joint. This lobe clearly has important hydrodynamic effects on the fin, however, and one would expect its mode of action to differ in two species in which the action of the fin as a whole differs strongly. The adductor and abductor muscles then are responsible for the movements of the pectoral fin. By contraction of the abductors other than those of the ventral lobe the antero-ventral tip of the fin-
. Annals of the South African Museum. Annale van die Suid-Afrikaanse Museum. i74 ANNALS OF THE SOUTH AFRICAN MUSEUM 1 discussion is not concerned as it has no bearing on the mechanism of the rigid joint. This lobe clearly has important hydrodynamic effects on the fin, however, and one would expect its mode of action to differ in two species in which the action of the fin as a whole differs strongly. The adductor and abductor muscles then are responsible for the movements of the pectoral fin. By contraction of the abductors other than those of the ventral lobe the antero-ventral tip of the fin-base is pulled forward and down- ward. In M. audax (pi. II) this movement causes the fin to be pulled away from the body into the extended position. This actually involves two movements, the drawing downwards of the antero-ventral tip of the fin base and the drawing forwards of the fin as a whole so that the direction of the fin tip is at right angles to the direction of the body, but both movements occur simultaneously in M. audax. In the extended position in M. audax either movement can be carried out; by twisting, the plane of the fin to the water can be altered by about io°, and the leading edge can move through any arc between the fully extended position and the folded position. v"~ Fig. 6. Range of movement of fin in Makaira indica. In M. indica (fig. 6), where the fin is permanently extended, two limited movements are possible, and are also brought about by the abductor and adductor muscles. The angle of incidence of the fin plane to the water can be altered within an arc of about 40, brought about by the pulling down by the abductors of the antero-ventral tip of the marginal ray base; the fin is also capable of an antero-posterior movement in a horizontal plane through an. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfect
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Keywords: ., bookauthorsouthafr, bookcentury1800, bookdecade1890, bookyear1898
