THE THORACIC EXO-SKELETON. 187 



the cornua of the metapleura ; it lies on the lower edge of the post-scutum, 

 and is connected'by syndesmosis with the dorsal arch of the first abdominal 



segment. 



The Thorax as a whole. The numerous complex sclerites of 

 the thoracic wall are united into three groups ; those of each 

 possess little or no movement on each other, and are united 

 by symphysis. 



The whole dorsum forms one portion of the thoracic wall, 

 the sterna and pleura a second, and the post-dorsum, or post- 

 scutellum, with the tympanic bullae, a third. 



The sterno-pleural portion of the thorax somewhat resembles 

 an old Spanish ship-of-war. I term it the ' carina.' The post- 

 dorsal portion is firmly attached to the carina and forms a 

 kind of poop, the ' dolium.' The dorsum is attached to 

 the carina by the elastic prescutum in front, and rests upon 

 the four diarthroses. Between the anterior and posterior diar- 

 throses the dorsum and the pleurae are united by syndesmoses, 

 and a loose syndesmosis connects the scutellum with the dolium 

 behind the posterior diarthroses. The tympanic fissures are 

 oblique extensions of the dorso-pleural syndesmoses, and open 

 and close like the gussets of a bellows. This permits of an 

 increase or diminution of the convexity of the dorsum. 



The mesopleural syndesmosis also allows of some variation 

 in the convexity of the carina. 



The great muscles which affect the magnitude of the thoracic 

 cavity are the dorsales and sterno-dorsales (Plate XL). 



The contraction of the latter diminishes the convexity of 



/ 



both the dorsum and the carina, closes the tympanic fissure, 

 and increases the breadth of the thorax ; whilst that of the 

 former increases the convexity of the dorsum and carina, 

 closes the mesopleural suture, opens the tympanic fissure, and 

 shortens the longitudinal, but increases the vertical and trans- 

 verse diameters of the thorax. 



Increased convexity of the dorsum and carina renders the 

 dorso-pleural syndesmoses tense, whilst diminished convexity 

 relaxes them. The effect of these movements will be con- 

 sidered hereafter. 



