642 CIRCULATION OF THE BLOOD AXD DORSAL VESSEL. 



with those of Graber. The narrow parts of the dorsal vessel 

 also appear to be inter-segmental. 



The Pericardial Septum is 75 mm. wide in front, and gradually 

 narrows as far back as the posterior margin of the second 

 abdominal segment, where it is about '4 mm. in breadth. 



It consists of a delicate fenestrated elastic membrane, and 

 of two great bundles of striated muscle, the alas, which arise, 

 one on each side, from the posterior surface of the meso- 

 phragma (PI. XLV., am}. 



Straus Durckheim [40] described the alar muscles of the 

 heart in Melolontha as ' paired broad fan-shaped muscles, 

 which arise from the anterior margins of the eight superior 

 arches of the abdomen. These muscles spread out into a 

 web of minute bands, of which an inferior set unite with a 

 membrane adhering to the inferior surface of the heart, and 

 with those of the opposite side ; whilst a superior set pass 

 directly into the sides of the heart itself, and those directed 

 towards the auriculo-ventricular openings (ostia) form little 

 arcades around them.' 



In most insects the alar muscles arise as Straus Durckheim 

 describes them in the Cockchafer, but in the Blow-fly I have 

 been quite unable to discover any such lateral muscular alae ; 

 only the single pair of muscles above described enter into the 

 formation of the septum. The posterior part of the pericardial 

 septum maintains the same width throughout its entire length; 

 it is seen as a transparent streak about '^ mm. wide between 

 the subcutaneous fat bodies. 



The pericardial septum may be described as a thin sheet of 

 tibro-muscular tissue which separates the pericardial sinus 

 from the general cavity of the abdomen. It is attached to the 

 fat bodies and hypodermis on each side. Numerous lateral 

 sinuses, especially on the surface of the great abdominal air- 

 sacs, communicate with the edges of the pericardial sinus, and 

 at these spots it is evident that the septum is not attached 

 to the abdominal wall, but is continuous with the endothulial 

 wall of the sinus. The anterior extremity of the sinus com- 

 municates with two large sinuses, which diverge from each 



