THE MUSCLES ; THE FAT-BODY AND CCELOM. 75 



The oblique sternal muscles (fig. 34), which are very short, 

 connect the adjacent edges of the sterna (2-3, 3-4, 4-5, 5-6, 

 6-7). They extend inwards nearly to the middle line, but, like 

 the longitudinal sternal muscles, they are not developed beneath 

 the nerve-cord. Acting together, the oblique sternal muscles 

 would antagonise the longitudinal, but it is probable that they 

 are chiefly used to effect lateral flexion of the abdomen, and 

 that only the muscles of one side of the abdomen contract at 

 once. 



The tergo-sternal (or expiratory) muscles (figs. 35 and 36) 

 form vertical pairs passing from the outer part of each abdo- 

 minal 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 out- 



