Fig. 76.—Heart and Pericardial Diaphragm. On the right, as seen from above; on the left, as seen from below; the bottom figure represents a transverse section. Ht, heart; PD, pericardial diaphragm; AM, alary muscle; Tr, tracheal tube; PC, pericardial fat-cells; PC′, multinucleate fat-cells.
Scattered over the upper surface of the pericardial diaphragm are groups of cells, similar to the fat-masses of the perivisceral space. Over the fan-like expansions of the alary muscles are different fat-cells, which form branched and multinucleate 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 by the dorsal tracheæ 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.[142] 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 fenestræ 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 respiratory character. The diaphragm appears to give mechanical support to the heart, resisting pressure from a distended alimentary 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.
Circulation of the Cockroach.
The pulsations of the heart are rhythmical and usually frequent, the number of beats in a given time varying with the species, the age, and especially with the degree of activity or excitement of the Insect observed.[143]
Cornelius[144] watched the pulsations in a white Cockroach immediately after its change of skin, and reckoned them at eighty per minute; but he remarks that the Insect was restless, and that the beats were probably accelerated in consequence.