CIRCULATORY ORGANS. 273 
In many fishes the conus arteriosus is followed by a strongly muscu- 
lar region, the bulbus arteriosus (fig. 287, B) which has muscles like 
those of the heart (p. 125), while the truncus in front of this has smooth 
muscles, like the rest of the blood-vessels. Hence conus and bulbus 
are to be regarded as a part of the heart, while the region in front is a 
part of ventral aorta to be described below. 
When first formed, the heart lies 
close behind the mandibular artery (first 
aortic arch to be described below), but 
as other vessels are formed it is forced 
farther back into a position, in the lower 
vertebrates, ventral to and a little behind 
the pharynx, but in the adult tetrapoda 
it is carried back, as a result of unequal 
growth even into the thorax, the extreme 
of migration being seen in the giraffe 
and the long-necked birds. ; 
Although all of the blood of the body Fic. 279.—Diagrammatic cross 
passes through the heart at short inter- section of heart showing atrio- 
on acs ‘ ventricular valves; a@, atrium; ct, 
vals, this is not sufficient for the nourish-  chorda tendinea; m, muscula pap- 
ment of that organ. Therefore its mus- rare a a hy Hone 
cles are usually supplied with blood 
through coronary arteries which arise from the aortic arches and 
run back along the truncus arteriosus to reach the atrium and ventricle. 
THe ARTERIES. 
Aorta and Aortic Arches.—The ventral aorta is the trunkin front 
of the pericardium, extending from the truncus arteriosus to the mandib- 
ular artery (first aortic arch). It runs, not through a cavity, but be- 
tween muscles and through connective tissue. The mandibular arter- 
ies continue dorsally on either side of the pharynx until they reach its 
dorsal surface. With development, the ventral aorta elongates and at 
the same time other aortic arches arise between the mandibular arteries 
and the pericardium, these extending dorsally until they meet the back- 
ward prolongations of the first, thus forming a pair of longitudinal 
tubes, dorsal to the alimentary tract, the radices aorte. 
The number of pairs of aortic arches varies with the number of gill 
clefts, the vessels coursing between the clefts. The number of arches 
18 
