The dorsal vessel is connected with the roof of the body by some short muscles, and is usually much surrounded by fat-body into which tracheae penetrate; by these various means it is kept in position, though only loosely attached; beneath it there is a delicate, incomplete or fenestrate, membrane, delimiting a sort of space called the pericardial chamber or sinus; connected with this membrane are some very delicate muscles, the alary muscles, extending inwards from the body wall (_b_, Fig. 72): the curtain formed by these muscles and the fenestrate membrane is called the pericardial diaphragm or septum. The alary muscles are not directly connected with the heart. [Illustration: FIG. 72.—Dorsal vessel (_c_), and alary muscles (_b_), of _Gryllotalpa_ (after Graber); _a_, aorta. _N.B._—The ventral aspect is here dorsal, and nearly the whole of the body is removed to show these parts.] [Illustration: FIG. 73.—Diagram of transverse section of pericardial sinus of _Oedipoda coerulescens_. (After Graber, _Arch. Mikr. Anat._ ix.) H, heart; _s_, septum; _m_, muscles—the upper suspensory, the lower alary.] It has been thought by some that delicate vessels exist beyond the aorta through which the fluid is distributed in definite channels, but this does not appear to be really the case, although the fluid may frequently be seen to move in definite lines at some distance from the heart. There is still much uncertainty as to some of the details of the action of the heart, and more especially as to the influence of the alary muscles. The effect of the contraction of these must be to increase the area of the pericardial chamber by rendering {135}its floor or septum less arched, as shown in our diagram (Fig. 73), representing a transverse section through the pericardial chamber, H being the dorsal vessel with _m_ its suspensory muscles, and _s_ its septum, with _m_ the alary muscles. The contraction of these latter would draw the septum into the position of the dotted line, thus increasing the area of the sinus above; but as this floor or septum is a fenestrated structure, its contraction allows fluid to pass through it to the chamber above; thus this arrangement may be looked on as a means of keeping up a supply of fluid to the dorsal vessel, the perforated septum, when it contracts, exerting pressure on the tissues below; these are saturated with fluid, which passes through the apertures to the enlarged pericardial chamber.