638 CIRCULATION OF THE BLOOD AND DORSAL VESSEL. 



enlarging the pericardial cavity at the expense of the abdominal 

 cavity and causing the blood to flow into it. 



The Ventricles of the Dorsal Vessel are fusiform enlargements 

 of the abdominal portion of a muscular tube, which extends 

 from near the posterior extremity of the abdomen into the head. 

 Each ventricle communicates with the pericardial sinus by a 

 pair of ostia, or openings guarded by valves opening inwards, 

 so that the blood can pass freely from the pericardial sinus 

 into the dorsal vessel, but cannot return from the vessel into 

 the sinus. 



Each valve consists of a pair of valve flaps, the arrangement 

 of which is similar to that of the auriculo-ventricular valves of 

 the vertebrate heart. The free edges of the valve-flaps are 

 held in their places by fine cords, chordae tendineae, which are 

 inserted into the wall of the ventricle in front of the ostia in 

 the middle line, both on the dorsal and ventral wall of the 

 vessel. 



There are also inter-ventricular valves, which are very 

 variable in structure. In some insects they consist of 

 sphincters of muscle-fibres, and in others there are membranous 

 valves which permit only of a forward movement of the circu- 

 lating fluid in the narrow inter-ventricular parts of the vessel 

 and at the root of the aorta. 



The Aorta. The cylindrical anterior portion of the dorsal 

 vessel, which is not surrounded by a pericardial sinus, conveys 

 the blood to the head. Like the ventricular portion, it is 

 a muscular tube. It is deeply placed in the thorax, and lies 

 immediately over the chyle stomach and oesophagus. 



Anterior Termination of the Aorta. Authorities are not agreed 

 as to the exact manner in which the dorsal vessel terminates in 

 the head. Newport [9] described its bifurcation and numerous 

 branches given off from the two trunks into which it divides in 

 sphinx ligustri ; Verloren [311] does not deny such an anterior 

 {niiination, but says he was unable to observe it. In 1870 

 [62J I described independently a similar branching of the 

 vessel in the Blow-fly, but the observation of its anterior 

 termination is far from easy. 



