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 some evidence to show that the number is really six, an arch dropping out 

 between the fourth and fifth of those recognized here. 



