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, chordz tendinez, 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 cesophagus. 
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 [811] does not deny such an anterior 
termination, but says he was unable to observe it. In 1870 
[62] I described independently a similar branching of the 
vessel in the Blow-fly, but the observation of its anterior 
termination is far from easy. 
