MUSCULAR SYSTEM. 151 



and pellucid, in fishes, where their feebleness is compensated 

 for by the great number which co-operate in the same move- 

 ment. The progression of fishes being effected chiefly by 

 the lateral motions of the tail and of the trunk, the vertebral 

 column is expanded vertically upwards and downwards, and 

 the great muscles of the trunk are disposed in transverse 

 strata along its sides, like those which move the segments of 

 a worm or an insect. The feeble irritability of the muscular 

 fibres in fishes, their softness, and their colourless transpa- 

 rency, correspond with their common conditions in the inver- 

 tebrata, and in the embryos of the higher vertebrated classes, 

 and they accord with the still soft condition of the bones into 

 which they are inserted. Aided in their ascent and descent 

 in the water by the compression and expansion of their air- 

 sac, and with very imperfectly developed arms and legs, the 

 active movements of fishes are but little varied, their mus- 

 cles seldom divide into fasciculi, or terminate in narrow ten- 

 dons to be attached to small points of their feeble skeleton. 

 The fibres which compose the great lateral muscles of the 

 trunk run in a longitudinal direction, are divided into nu- 

 merous transverse oblique strata by intervening tendinous 

 aponeuroses, and are disposed chiefly in four series of oblique 

 muscles, as seen in the perch, (Fig. /O. a, b, c, d.) The in- 



FIG. 70. 



tervening white tendinous bands which connect these strata 

 of muscles have a crescentic form, are disposed vertically 

 along the sides of the vertebrae and the ribs, and are attached 

 externally by their zig-zag margin to the inner surface of the 

 skin. The upper series (,) have their thin white tendons 



