136 THE COCKROACH : 



Heart of tlie 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 



AIL 



Fig. 74. Diagram to show the interventricular valves and lateral inlets of the 

 Heart. ML, median lobe ; V, valve ; /, lateral inlet. 



conceal paired lateral inlets,* which lead from the pericardial 

 space to the hinder end of each chamber of the heart. Imme- 

 diately 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 



* It may be observed that Graber, who has paid close attention to the heart of 

 Insects, describes the inlets (c. y., 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 arrange- 

 ment in the heart of the Cockroach. 



