. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. ORGANS OF CIRCULATION. I8S plete. Hence in all amniotes we can recognize an arterial (left) and a venous (right) side to the heart, each side consisting of practically an auricle and a ventricle. In the mammals and birds the divis- ion also extends to the truncus as far forward as the first aortic arch, so that these vessels are connected with the right auricle, the other arches being connected with the arterial half of the heart. In the reptiles the division is carried farther ; for the fourth arch of the left side has its
. Text book of vertebrate zoology. Vertebrates; Anatomy, Comparative. ORGANS OF CIRCULATION. I8S plete. Hence in all amniotes we can recognize an arterial (left) and a venous (right) side to the heart, each side consisting of practically an auricle and a ventricle. In the mammals and birds the divis- ion also extends to the truncus as far forward as the first aortic arch, so that these vessels are connected with the right auricle, the other arches being connected with the arterial half of the heart. In the reptiles the division is carried farther ; for the fourth arch of the left side has its own trunk, and this is connected with the right side of the heart. The effect of this will be apparent after we consider the aortic arches. Aortic Arches.— In all vertebrates except the cyclostomes and lower sharks the aortic arches are typically five in number; ^ but in all except the elasmobranchs the number is re- duced by the disappearance of the second normal arch. In the follow- ing the arches are numbered one to five, although that number may not be actually present. In the ichthy- opsida these arches really consist of two parts, one arising from the ven- tral aorta, the others connecting with the dorsal aorta. In the gill arches these two vessels run parallel to each other, the con- nection between them being effected by capillary loops which run through the external or internal gill filaments. In passing through these gills the blood loses its carbon dioxide and takes up oxygen, and thus enters the dorsal aorta as arterial blood. In dipnoi, amphibia, and higher groups, in which lungs appear, the posterior (fifth) arch of either side sends a branch, the pulmo-. FiG. 198. Diagram of the heart and aortic arches of the alligator, after Hertwig. la, left auricle; lao, left aortic arch; lav, left auriculo-ventric- ular aperture; Ic, left carotid; Ip, left pulmonary artery; h, left subclavian; Iv, left ven- tricle. The right side with cor- responding letters. 1 There is som
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