. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 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
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 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 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 FIG. 279.—Diagrammatic cross section of heart showing atrio- ventricular valves; a, atrium; ct, chorda tendinea; m, muscula pap- illosa; v, ventricle; vl, atrio-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, the
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