DORSAL VESSEL OF THE IMAGO OF THE BLOW-FLY. 643 



other and extend to the edges of the mesophragma, where they 

 communicate with the lateral venous sinuses of the thorax. 

 These sinuses have a wall which is indubitably formed by a 

 prolongation of the pericardial septum, so that they resemble 

 veins, and can be dissected out and removed from the ab- 

 dominal wall with the pericardium. But the smaller sinuses, 

 which extend over the air-sacs, cannot be recognised after the 

 pericardium has been removed from the body of the insect. 



In preparations of the fresh pericardium treated with silver 

 nitrate, I have observed indications of an endothelium, consist- 

 ing of thin cells of irregular form on the dorsal surface of the 

 pericardial septum. These form the endothelial lining of the 

 pericardial sinus. 



7\ 



FIG. 87. A transverse section through the pericardial sinus and the parts in imme- 

 diate relation with it. a, a, great air-sacs ; dv, dorsal vessel ; g, ganglion cells ; 

 /, intestine; /, pericardial septum ; ps, pericardial sinus; s, s, blood sinuses 

 on the surface of the air-sacs opening into the pericardial sinus. 



The Relation of the Dorsal Vessel and Pericardial Septum 



(Fig. 87). The dorsal vessel lies loosely in the pericardial 

 cavity, but in transverse sections, slender muscle fibres and 

 fine adenoid tissue are seen crossing the pericardial space, 

 which unite the wall of the dorsal vessel with the walls of the 

 pericardial sinus. The union is more intimate in the posterior 

 part of the pericardial space than in front ; the anterior 

 portion has only the most delicate reticulum of stellate cells, 



