126 ENTOMOLOGY 
within the chambers occur other valvular folds that allow the 
blood to move forward only. With few exceptions (Ephe- 
meridz) the dorsal vessel is blind behind and the blood can 
enter it only through the lateral ostia. 
Aorta.—The posterior, or 
pulsating portion (heart) of 
TG. 162 
the dorsal vessel is confined 
for the most part to the abdo- 
men; the anterior portion, or 
aorta, extends as a_ simple 
attenuated tube through the 
thorax and into the head, 
where it passes under the 
brain and usually divides into 
two branches (Fig. 162), 
each of which may again 
branch, “lm the (head) iie 
blood leaves the aorta ab- 
ruptly and enters the general 
body cavity. 
Mir Alary Muscles.—Fxtend- 
Lr~yw ; ; a 9 
Y ing outward from the “heart, 
Diagram’ to indicate the ‘course pt the: or propulsatory portion), aild 
blood in the nymph of a dragon fly, . aM 
Epitheca. a, aorta; h, heart; the arrows Making with the dorsal wall 
how directions tak "4 rents of C : E 
show irections taken by currents of of the body 24 pericardial 
blood.—After Ko se. 
chamber, isa loose diaphragm, 
formed largely by paired fan-like muscles—the alary muscles 
(Figs. 158, 160). These are thought to assist the heart in its 
propulsatory action. 
Structure of the Heart.—The dorsal vessel has a delicate 
lining-membrane, or intima, and a thin enveloping membrane; 
between these, in the heart, is a layer of fine muscle fibers, cir- 
cular or spiral in direction, which effect the contractions of the 
organ. 
Ventral Sinus.—In many if not most insects a pulsatory 
septum (Fig. 177, v) extends across the floor of the body cav- 
