[138] See, for a full account of this discussion, MacLeod sur la Structure des Trachées, et la Circulation Péritrachéenne (1880). The peritracheal circulation was refuted by Joly (Ann. Sci. Nat., 1849). [Illustration: Fig. 73.--Heart, Alary Muscles, and Tracheal Arches, seen from below; to the left is a side view of the heart. _T^2_, _T^3_, _A^1_, alary muscles attached to the second thoracic, third thoracic, and first abdominal terga. × 6. Fig. 35 (p. 74) is not quite correct as to the details of the heart. The thoracic portion should be chambered, and additional chambers and alary muscles represented at the end of the abdomen. These omissions are rectified in the present figure.] _Heart of the Cockroach._ [Illustration: Fig. 74.--Diagram to show the interventricular valves and lateral inlets of the Heart. _ML_, median lobe; _V_, valve; _I_, lateral inlet.] The heart of the Cockroach is a long, narrow tube, lying immediately beneath the middle line of the thorax and abdomen. It consists of thirteen segments (fig. 73), which correspond to three thoracic and ten abdominal somites. Each segment, as a rule, ends behind in a conspicuous fold which projects backwards from the dorsal surface; immediately in front of this are two lateral lobes. The median lobe passes into the angle between two adjacent terga, and is continuous with the dorsal wall of the segment next behind, from which it is separated only by a deep constriction, while the lateral folds conceal paired lateral inlets,[139] which lead from the pericardial space to the hinder end of each chamber of the heart. Immediately in front of each constriction is the interventricular valve, a pear-shaped mass of nucleated cells, hanging down from the upper wall of the heart, and inclining forward below. The position of this valve indicates that during systole it closes upon the constricted boundary between two chambers, thus shutting off at once the inlets and the passage into the chambers behind. In this way the progressive and rhythmical contraction of the chambers impels a steady forward current of blood, allowing an intermittent stream to enter from the pericardial space, but preventing regurgitation. [139] It may be observed that Graber, who has paid close attention to the heart of Insects, describes the inlets (_e. g._, in _Dytiscus_) as situated, not at the hinder end, but in the middle of each segment. We have not been able to discover such an arrangement in the heart of the Cockroach. [Illustration: Fig. 75.--Junction of two chambers of the Heart, seen from above. _ML_, median lobe; _I_, lateral inlet.]