From the spiracles, short, wide air-tubes pass inwards, and break up into branches, which supply the walls of the body and all the viscera. Dorsal branches ascend towards the heart on the upper side of the alary muscles; each bifurcates above, and its divisions join those of the preceding and succeeding segments, thus forming loops or arches. The principal ventral branches take a transverse direction, and are usually connected by large longitudinal trunks, which pass along the sides of the body; the Cockroach, in addition to these, possesses smaller longitudinal vessels, which lie close to the middle line, on either side of the nerve-cord.[148] The ultimate branches form an intricate network of extremely delicate tubes, which penetrates or overlies every tissue. [148] The longitudinal air-tubes are characteristic of the more specialised Tracheata. In Araneidæ, many Julidæ, and Peripatus each spiracle has a separate tracheal system of its own. [Illustration: Fig. 77.--Tracheal System of Cockroach. Side view of head seen from without, introducing the chief branches of the left half. × 15.] [Illustration: Fig. 78.--Tracheal System of Cockroach. Top and front of head seen from without. × 15.] [Illustration: Fig. 79.--Tracheal System of Cockroach. Back of head, seen from the front, the fore half being removed. × 15. The letters A–J indicate corresponding branches in figs. 77, 78, and 79.] [Illustration: Fig. 80.--Tracheal System of Cockroach. The dorsal integument removed and the viscera in place. × 5.] [Illustration: Fig. 81.--Tracheal System of Cockroach. The viscera removed to show the ventral tracheal communications. × 5.] [Illustration: Fig. 82.--Tracheal System of Cockroach. The ventral integument and viscera removed to show the dorsal tracheal communications. × 5.] _Tracheal Tubes._ The accompanying figures sufficiently explain the chief features of the tracheal system of the Cockroach, so far as it can be explored by simple dissection. Leaving them to tell their own tale, we shall pass on to the minute structure of the air-tubes, the spiracles, and the physiology of Insect respiration. The tracheal wall is a folding-in of the integument, and agrees with it in general structure. Its inner lining, the intima, is chitinous, and continuous with the outer cuticle. It is secreted by an epithelium of nucleated, chitinogenous cells, and outside this is a thin and homogeneous basement membrane. The integument, the tracheal wall, and the inner layers of nearly the whole alimentary canal are continuous and equivalent structures. The lining of the larger tracheal tubes at least is shed at every moult, like that of the stomodæum and proctodæum. [Illustration: Fig. 83.--Tracheal tube with its epithelium and spiral thread. Slightly altered from a figure given by Chun (Rectal-drüsen bei den Insekten, pl. iv., fig. 1).] _Tracheal Thread._