Text-book of comparative anatomy . muscles being inserted in each. The pair of ventral or sternal muscles (sm)are attached anteriorly to the ventral side ofthe cephalo-thorax to a row of processes ofthe exoskeleton directed inwards, and partlybound together by transverse ridges, whichroof over the thoracic portion of the ventralchord and the sub-neural vessel. Posteriorlythe sternal longitudinal muscles are succes-sively inserted on the inner sides of the sternalskeleton of the abdominal segments. Thefibres of this muscle are twisted like thestrands of a rope. If the pair of sternal or flexor
Text-book of comparative anatomy . muscles being inserted in each. The pair of ventral or sternal muscles (sm)are attached anteriorly to the ventral side ofthe cephalo-thorax to a row of processes ofthe exoskeleton directed inwards, and partlybound together by transverse ridges, whichroof over the thoracic portion of the ventralchord and the sub-neural vessel. Posteriorlythe sternal longitudinal muscles are succes-sively inserted on the inner sides of the sternalskeleton of the abdominal segments. Thefibres of this muscle are twisted like thestrands of a rope. If the pair of sternal or flexor muscles con-tracts, the abdominal row of segments cor-responding with our diagram bends ventrally(Fig. 231, £) till, as is the case in swimming,the telson touches the ventral side of thecephalo-thorax. While in this position theintersternal membranes of the abdomen arefolded, the whole tergal integument isstretched, and the tergal articular facets andinterarticular membranes are drawn out fromunder the terga which cover them, and. FIG. 232.—Astacus fluviatilis. Longi-tudinal section to represent the mostimportant muscles and their relation tothe exoskeleton (after Huxley), em, Ex-tensor ; fm, flexor of the abdomen ; u<lm,adductor of the mandiblesthorax; ab]-abg, abdominaltelson; 1-19, limbs, 1-13, of the cephalo-thorax, 14-19, of the abdomen. ctJi, cephalo- come freely to the surface. If now the sternalsegments; t, muscle slackens, and the pair of tergal orextensor muscles (Fig. 231, A ; Fig. 232) con-tracts, the abdomen is straightened, the tergapass under one another like the tiles of a roof, and the intersternal membranes arestretched. A dorsal bending of the body is impossible, first because the intersternalmembranes do not allow of further stretching, and secondly, because the terga canonly be pushed one beneath the other as far as the posterior limit of their articularfacets. This is best exemplified by the illustration (Fig. 231, A}. CR USTACEA—MUSCULATURE 335 Th
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
