Blood-System 



55 



ML 



of the heart. These inlets, as well as the passage 

 from chamber to chamber, can be closed by a valve 

 which hangs from the upper wall, and allows a 

 forward, but not a backward flow of blood (fig. 39). 

 The heart has no opening at its hinder end, but from 

 the foremost chamber a slender tube — the aorta — 

 passes forward to the head (l, 30). 



Aorta. — In most insects the blood-tube is only 

 chambered in the abdomen so that the aorta is said to 

 extend throughout the thorax (fig. 47 d. %'.). In the 

 neighbourhood of the 

 brain the aorta ends in 

 a trumpet - shaped 

 opening. A special 

 contractile sac is 

 situated at the base of 

 each feeler, into which 

 it propels blood re- 

 ceived from the head- 

 cavity (32). 



Pericardium. — 

 The heart lies in an 

 irregular chamber — 

 the pericardium — 

 which is bounded 

 above by the upper 

 body-wall and its 

 muscles, and below by a delicate perforated membrane 

 {pericardial diaphragrti). The cavity is mostly filled with 

 fatty tissue, and is traversed by air-tubes. Below, and 

 supporting the perforated membrane, are a series of 

 muscles {alary muscles) (fig. 37), a pair to each seg- 

 ment of the body. Each alary muscle is attached by 

 a tendon to a point at the outer region of the front 

 edge of each tergite. Its fibres spread in a fan-like 

 manner, joining those of its fellow of the pair in the 



Fig. 3Q. — Diagram of junction between two 

 chambers of Cockroach's heart. ML. 

 median lobe; V. valve ; /. lateral inlet 

 Magnified. From Miall & Denny. 



