Fig. 374.—Diagram of transverse section of pericardial sinus of Ædipoda cœrulescens: H, heart; s, septum; m, muscles,—the upper suspensory, the lower alary.—After Graber, from Sharp. (See also Fig. 377.)
Graber has thus explained the action of the pericardial diaphragm and chamber, as freely translated by Miall and Denny: “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, however, Miall and Denny think that 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.... 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 equalize 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.”
“The peculiar office,” says Graber, “performed by the heart has already been stated. It is nothing more than a regulator; than an organ for directing the blood in a determinate course in order that this may not wholly stagnate, or only be the plaything of a force acting in another way, as, for example, through that afforded by the body-cavity and the inner digestive canal. At regular intervals a portion of the blood is sucked through the same, and then by means of the anterior supply tube it is pushed onward into the head, whence it passes into the cavities of the tissues. The different conditions of tension under which the mass of blood stands in the different regions of the body then causes a farther circulation. Besides this, the blood passes through separate smaller pumping apparatuses, and through vessel-like modifications of cavities, also through hollow spaces between the muscles, as, for example, in the appendages where a regular backward and forward flow of the blood, especially in the limbs, wings, antennæ, and certain abdominal appendages takes place. Here and there may occasionally occur a narrow place where the flow of blood is obstructed by the accumulation of the blood corpuscles, causing a considerable stagnation.” (Graber.)
Fig. 375.—Libellula depressa, opened from the back, showing the nervous cord (b1-b3, thoracic, h1-h7, abdominal, ganglia), also the furrow-like ventral sinus closed by a muscular diaphragm.
Fig. 376.—A, part of the ventral furrow of Libellula depressa more highly magnified: a, a sternal plate (urite); c, the septum stretched over it, at s in a relaxed or collapsed state; b and d, the wing-like, sternal processes from which the muscular bundles of the diaphragm arise. B, same in Acridium.
