640 CIRCULATION OF THE BLOOD AND DORSAL VESSEL. 
chambers of the dorsal vessel will be seen as a fine white line 
between the two lateral groups of subcutaneous fat cells. The 
pericardium may then be removed with the dorsal vessel 
attached to it, spread out carefully on a slide, and examined 
under the microscope. To make this dissection, the abdomen 
of a large Blow-fly should be opened on its ventral surface, 
and the insect placed in a 0°5 per cent. solution of potassium 
bichromate, or in dilute Flemming’s mixture for an hour or 
two; the dissection can then be proceeded with in the same 
fluid. 
It is by no means easy even with practice to remove the 
whole of the dorsal vessel uninjured; but parts are easily 
obtained in this way, and their relations to the pericardial 
septum are readily made out, or the pericardium may be 
removed from the vessel itself with needles. 
The abdominal or ventricular portion of the dorsal vessel 
consists of five chambers; the most anterior of these is 
nearly spherical, and not half the diameter of the second 
chamber, which is the largest of all. I term the anterior 
chamber the aortic bulb, and the second chamber the anterior 
ventricle. 
The Aortic Bulb (Pl. XLV., Fig. 86) lies close to the 
margin of the mesophragma ; it gives off the aorta (a), which 
dips under the margin of the mesophragma, and enters the 
thorax. 
In Eristalis, in which these parts are far larger and more 
readily studied than in the Blow-fly, the aortic bulb appears 
to give off three vessels, the thoracic aorta and a pair of smaller 
lateral vessels, which diverge from each other and enter the 
thorax, one on each side of the mesophragma. They lie in 
the walls of the two great sinuses, which enter the pericardial 
sinus in front (see p. 648). 
DESCRIPTION OF PLATE XLV. 
The anterior part of the dorsal vessel and pericardium of the Blow-fly. 
The pericardium has been removed on the right side to show the three anterior 
chambers of the dorsal vessel. a@, aorta; am, alar muscle; //, fat bodies; 
£& ganglion cells; Af, pericardial membrane; /, tracheal vessels. 
