CIRCULATORY ORGANS. 



2 73 



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 

 passes through the heart at short inter- 

 vals, this is not sufficient for the nourish- 

 ment of that organ. Therefore its mus- 

 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. 



FIG. 279. Diagrammatic cross 

 section of heart showing atrio- 

 ventricular valves; a, atrium; ct, 

 chorda tendinea; m, muscula pap- 

 illosa; v, ventricle; vl, a trio- ven- 

 tricular valves. 



THE ARTERIES. 



Aorta and Aortic Arches. The ventral aorta is the trunk in 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 aortae. 



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 



