Fig. 73.—Heart, Alary Muscles, and Tracheal Arches, seen from below; to the left is a side view of the heart. T2, T3, A1, 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.

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.

Fig. 74.—Diagram to show the inter­ven­tric­ular valves and lat­eral inlets of the Heart. ML, med­ian lobe; V, valve; I, lat­eral inlet.