THE THORACIC EXO-SKELETON. 175 



median thoraco-abdominal muscles arise. The posterior ex- 

 tremity of the metafurca is pointed, and dips down between the 

 posterior coxas ; the anterior extremity rests upon the mesofurca. 



The Dorsal Valve. If an incision be made through the 

 anterior part of the scutum horizontally backwards until it 

 meets the anterior extremities of the dorso-pleural syndesmosis, 

 the syndesmotic membrane is easily divided, after cutting off 

 the wings close to their roots, as far back as the anterior 

 external angles of the scutellurn. Here the dorsum articulates 

 with the post-scutellum by a diarthrosis, the posterior dorso- 

 pleural joint. With a little pains the articulation can be broken 

 through and the remainder of the syndesmotic membrane be- 

 tween the lower surface of the scutellurn and the post-scutellum 

 divided. In this way the thorax can be separated into a ventro- 

 pleural and a dorsal valve by the natural articulation, which 

 permits these parts to move upon each other by the action of 

 the powerful sterno-dorsal and longitudinal thoracic muscles. 

 The dissection should be made in an artificial exuvium (see 

 Appendix to chapter). 



The Under Surface of the Dorsal Valve. The dorsal valve con- 

 sists of part of the prescutum, of the scutum, and the scutellum. 



The scutum and prescutum when seen from their under 

 surface present an oval fossa, each lateral margin of which 

 exhibits a rim or ridge in front projecting towards the middle 

 line, the dorso-pleural costa, and a strong process behind, the 

 great alar apophysis. 



The great alar apophysis (Plate VII., J) is an inflection of the 

 posterior angle of the scutum, and is separated from the dorso- 

 pleural costa by a deep fissure. It terminates in front in an 

 articular head, which supports the anterior wing-root. Its inner 

 edge is nearly parallel with the inner edge of the dorso-pleural 

 costa. 



The scutellum, seen from its under surface, has the form of a 

 conical pouch, the anterior margin of its inferior wall being 

 deeply crescentic, with a thickened rim, the scutellar rim. 

 The anterior extremities of the rim each give off two divergent 

 processes, at the common root of which there is a circular 



