640 CIRCULATION Ol- 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 (PI. 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). 



DES< KI 1-1 KIN or l'i A 1 1 XLV. 



The anterior part of the dorsal vessel and pericardium of the Ulow-lly. 



The pericardium lus lieen removed on the ri^ht side to show the three anterior 

 chaml ITS nf tin- dorsal vessel. <7, aorta; a /;/, alar muscle; //, fat bodies; 

 ;_;ant;lion cells ; //, pericardia] membrane ; fr, trachcal vessels. 



