140 THE COCKROACH : 



different fat-cells, which form branched and raultinucleate lobes, 

 and radiate in the same direction as the underlying muscles. 



Tracheal trunks, arising close to the stigmata, ascend upon 

 the tergal wall towards the heart. They overlie the alary 

 muscles, and end near the heart by bifurcation, sending one 

 branch forward and another backward to meet corresponding 

 branches of adjacent trunks. A series of arches is thus formed 

 bv the dorsal trachea) on each side of the heart. Occasionally 



/ 



an arch is subdivided into two smaller parallel tubes. A few 

 branches of distribution are given off to the fat-cells of the 

 pericardium. 



Graber has explained the action of the pericardial diaphragm 

 and chamber in the following way.* When the alary muscles 

 contract, they depress the diaphragm, which is arched upwards 

 when at rest. A rush of blood towards the heart is thereby set 

 up, and the blood streams through the perforated diaphragm 

 into the pericardial chamber. Here it bathes a spongy or 

 cavernous tissue (the fat-cells), which is largely supplied with 

 air tubes, and having been thus aerated, passes immediately 

 forwards to the heart, entering it at the moment of diastole, 

 which is simultaneous with the sinking of the diaphragm. 



In the Cockroach the facts of structure do not altogether 

 justify this explanation. The fenestnc of the diaphragm are 

 mere openings without valves. The descent of a perforated non- 

 valvular plate can bring no pressure to bear upon the blood, 

 for it is not contended that the alary muscles are powerful 

 enough to change the figure of the abdominal rings. Moreover, 

 we find comparatively few tracheal tubes in the pericardial 

 chamber, and can discover no proof that in the Cockroach 

 the fat-cells adjacent to the heart have any special respi- 

 ratory character. The diaphragm appears to give mechanical 

 support to the heart, resisting pressure from a distended ali- 

 mentary canal, while the sheets of fat-cells, in addition to their 

 proper physiological office, may equalise small local pressures, 

 and prevent displacement. The movement of the blood towards 

 the heart must (we think) depend, not upon the alary muscles, 

 but upon the far more powerful muscles of the abdominal wall, 

 and upon the pumping action of the heart itself. 



* Arch. f. mikr. Anat., Bd. IX. (1872) ; Insekten, ch. x. 



