The pericardial diaphragm, or floor of the pericardium, is continuous, except for small oval openings scattered over its surface. It consists of loosely interwoven fibres, interspersed with elongate nuclei (connective-tissue corpuscles) and connected by a transparent membrane. Into the diaphragm are inserted pairs of muscles, which, from their shape and supposed continuity with the heart, have been named _alæ cordis_, or alary muscles.[140] These are bundles of striated muscle, about ·003 in. wide, which arise from the anterior margin of each tergum. In the middle of the abdomen every alary muscle passes inwards for about ·04 in., without breaking-up or widening, and then spreads out fanwise upon the diaphragm. The fibres unite below the heart with those of the fellow-muscle, and also join, close to the heart, those of the muscles in front and behind. The alary muscles are often said to distend the heart rhythmically by drawing its walls apart, but this cannot be true. They do not pass into the heart at all. Even if they did, a pull from opposite sides upon a flexible, cylindrical tube, would narrow and not expand its cavity. Moreover, direct observation[141] shows that the heart continues to beat after all the alary muscles have been divided, and even after it has been cut in pieces. These facts suggest that the heart of Insects is innervated by ganglia upon or within it, and indeed transparent larvæ, such as _Corethra_ or _Chironomus_, exhibit paired cells, very like simple ganglia, along the sides of the heart. [140] Lyonnet. [141] Brandt, Ueb. d. Herz der Insekten u. Muscheln. Mél. Biol. Bull. Acad. St. Petersb. Tom. VI. (1866). [Illustration: 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.