Fig. 34.—Muscles of Ventral Wall, with the Nerve-cord. × 5.
Fig. 35.—Muscles of Dorsal Wall, with the Heart and Pericardial Tendons. × 5.
The tergo-sternal (or expiratory) muscles (figs. 35 and 36) form vertical pairs passing from the outer part of each abdominal sternum to the corresponding tergum. Their action is to approximate the dorsal and ventral walls, and thus to reduce the capacity of the abdomen. The first tergo-sternal muscle has its ventral insertion into the stem of the postfurca, and takes an oblique course to the first abdominal tergum.
Tergal Muscles of Abdomen.—The longitudinal tergal muscles extend from the fore part of each abdominal tergum, including the first, to the same part of the tergum next behind. They are interrupted by longitudinal spaces, so that the muscular sheet is less continuous than on the ventral surface, and has a fenestrated appearance. The direction of the fibres is slightly oblique.
Oblique tergal muscles, resembling the oblique muscles of the sterna, are also present.
In the thorax the general arrangement of the muscles is greatly modified by the altered form of the dorsal and ventral plates, and by the attachment of powerful limbs.
Sternal Muscles of Thorax.—Two tubular apodemes, lying one behind the other, project into the thorax from the ventral surface (p. 59 and fig. [27]). To the foremost of these are attached three paired muscles and one median muscle. The median muscle passes to the second tubular apodeme. The anterior pair pass forwards and outwards to the base of the prothoracic leg; the next pair directly outwards to the base of the middle leg; while the posterior pair pass outwards and backwards to the arms of the medifurca. From the second tubular apodeme, in front of the metasternum, four pairs of muscles spring. Those of the anterior pass forwards and outwards to the coxa of the fore limb; the second pair directly outwards to the base of the metathoracic legs; the third pair backwards and outwards to the arms of the postfurca; the fourth pair backwards to the second abdominal sternum.
The muscles attached to the medi- and postfurca (other than those connecting them with the tubular apodemes) are:— (1) A pair passing from the posterior edge of the arms of the medifurca to the stem of the postfurca; (2) a pair which diverge from the stem of the postfurca and proceed to the fore part of the second abdominal sternum; (3) a pair passing from the posterior edge of the arms of the postfurca, these are directed inwards and backwards, and are inserted into the hinder part of the second abdominal sternum; (4) a pair already mentioned, which correspond in position and action to the tergo-sternal muscles, and spring from the stem of the postfurca, passing upwards and outwards to the sides of the first abdominal tergum.