THE EXTENSOES AND FLEXORS OF THE ABDOMEN. 99 



ride over the posterior edges of the branchiostegites. In 

 the position of extension, the overLip of the terga is great, 

 while that of the pleura of the middle somites is small. 

 As the abdomen passes from extension to flexion, the 

 overlap of the terga of course diminishes ; but an}' de- 

 crease of resistance to lateral strains which ma}' thus 

 arise, is compensated by the increasing overlap of the 

 pleura, which reaches its maximum wdien the abdomen 

 is completely flexed. 



It is obvious that longitudinal muscular fibres fixed 

 into the exoskeleton, above the axes of the joints, must 

 bring the centres of the terga of the somites closer 

 together, when they contract ; ^while muscular fibres 

 attached below the axes of the joints must approximate 

 the sterna. Hence, the former will give rise to extension, 

 and the latter to flexion, of the abdomen as a whole. 



Now there are two pairs of very considerable muscles 

 disposed in this manner. The dorsal pair, or the exten- 

 sors of the abdomen (fig. 22, e.m), are attached in front 

 to the side walls of the thorax, thence pass backwards 

 into the abdomen, and divide into bundles, which are 

 fixed to the inner surfaces of the terga of all the somites. 

 The other pair, or the flexors of the abdomen (/./») consti- 

 tute a very much larger mass of muscle, the fibres of 

 which are curiously twisted, like the strands of a rope. 

 The front end of this double rope is fixed to a series of 

 processes of the exoskeleton of the thorax, called ajjode- 

 viata, some of which roof over the sternal bh^od-sinuses 



