Fig. 36.—Muscles of lateral wall, &c. × 5.
Fig. 37.—Muscles of left mesothoracic leg, seen from behind. The muscles are—Adductor and abductor of the coxa; extensor and flexor of femoral joint; flexor and extensor of tibial joint; flexor of tarsus; and a retractor tarsi, which swings the tarsus backwards, so that it points away from the head. It is opposed by another muscle, which moves the tarsus forwards. Both muscles parallelise the tarsus to the axis of the body, but in opposite directions.
The muscles attached to the arms of each furca pass to other structures in or near the middle line of the body. The pull of such muscles must alter the slope of the two steps in the ventral floor of the thorax (p. [58], and fig. [3], p. 12). When the furca is drawn forwards, the step is rendered vertical or even inclined forward, the sterna being approximated; while, on the other hand, a backward pull brings the step into a horizontal position, and separates the sterna.
Tergal Muscles of Thorax.—The longitudinal tergal muscles are much reduced in width when compared with those of the abdomen. Sets of obliquely placed muscles, which may be called the lateral thoracic muscles, arise from near the middle of each tergum, and converge to tendinous insertions on the fore edge of each succeeding tergum, close to the lateral wall of the body.
The principal muscles of the legs are figured and named, and their action can readily be inferred from the names assigned to them.