Fig. 75.—Junction of two chambers of the Heart, seen from above. ML, median lobe; I, lateral inlet.
The wall of the heart includes several distinct layers. There are (1) a transparent, structureless intima, only visible when thrown into folds; (2) a partial endocardium, of scattered, nucleated cells, which passes into the interventricular valves; (3) a muscular layer, consisting of close-set annular, and distant longitudinal fibres. The annular muscles are slightly interrupted at regular and frequent intervals, and are imperfectly joined along the middle line above and below, so as to indicate (what has been independently proved) that the heart arises as two half-tubes, which afterwards join along the middle. Elongate nuclei are to be seen here and there among the muscles. The adventitia (4), or connective tissue layer, is but slightly developed in the adult Cockroach.
Within the muscular layer is a structure which we have failed to make out to our own satisfaction. It presents the appearance of regular but imperfect rings, which do not extend over the upper third of the heart. They probably meet in a ventral suture, but this and other details are hard to make out, owing to the transparency of the parts. The rings stain with difficulty, and we have not observed nuclei belonging to them. Each extends over more than one bundle of annular muscles.
The difficulty of investigating a structure so minute and delicate as the heart of an Insect may explain a good deal of the discrepancy noted on comparing various published descriptions. Perhaps the most obvious peculiarity which distinguishes the heart of the Cockroach, is the subdivision of the thoracic portions into three chambers, which, though less prominent in side-view than the abdominal chambers, are, nevertheless, perfectly distinct. The number of abdominal chambers is also unusually high; but it is so easy to overlook the small chambers at the posterior end of the abdomen, that the number given in some of the species may have been under-estimated.
Pericardial Diaphragm and Space.
The heart lies in a pericardial chamber, which is bounded above by the terga and the longitudinal tergal muscles; below by a fenestrated membrane, the pericardial diaphragm. The intermediate space, which is of inconsiderable depth, is nearly filled by a cellular mass laden with fat, and resembling the fat-body.
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.