BLOOD CIRCULATION, 
57 
worker there are five ventricles, and each is provided 
with an opening on either side (Fig. 28, d)^ which 
acts, by muscular contraction, as a valve. 
Each ventricle com- 
municates with the one 
before it by means of a 
similar valve (Fig. 28, ^), 
50 that the blood flows 
Fig. 27. — Dorsal Vessel. 
Ventricles and 
Valves. 
in one direction (indicated by the arrows in the 
figure), when the dorsal vessel is contracted by means 
of the central muscular layer, and is forced out at the 
head, returning thence and permeating every part of 
the body. When the ventricles are expanded the 
blood enters by the side openings, and the valves 
between them close, but again on contraction the 
reverse takes place, so that there is a constant stream 
of blood flowing to the head. 
The dorsal vessel is kept in its place by muscles 
