642 CIRCULATION OF THE BLOOD AND 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 alz, 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 ale ; 
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 ‘4 mm. wide between 
the subcutaneous fat bodies. 
The pericardial septum may be described as a thin sheet of 
fibro-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 endothelial 
wall of the sinus. The anterior extremity of the sinus com- 
municates with two large sinuses, which diverge from each 
