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STRUCTURE OF VERTEBRATA 



In Birds and Mammals the heart has two auricles and two ventricles, 

 and one aortic arch suppUes the body with wholly pure blood. This 

 aortic arch always arises from the left ventricle, but in Birds it curves 

 over the right bronchus, i.e. is a right aortic arch, and in Mammals 

 over the left, i.e. is a left aortic arch. Impure blood from the body 

 enters the right auricle, passes into the right ventricle, is driven to the 

 lungs, returns purified to the left auricle, enters the left ventricle, and 

 is driven to the body. 



The arterial system of a fish consists of a ventral aorta continued 

 forwards from the heart, of a number of afferent vessels diffusing the 

 impure blood on the gills, and of efferent vessels collecting the purified 

 blood into a dorsal aorta. 



In the embryo of higher Vertebrates the same arrangement persists, 

 though there are no gills beyond Amphibians. From a ventral arterial 

 stem arches arise, which are connected so as to form the roots of the 

 dorsal aorta. This aorta gives off vessels to the body, while in embry- 

 onic hfe it sends important vitelline arteries to the yolk, and (in 

 Reptiles, Birds, and Mammals) equally important allantoic arteries to the 

 allantois. 



Summary as to Aortic Arches 



Fishes. 



Amphibians. 



(a) Mandibular aortic 



arch usually aborts ; 

 there is a persistent 

 trace in Elasmo- 

 branchs (spiracular 

 artery). 



(b) Hyoid aortic arch 



aborts, or is rudi- 

 mentary. 



(c) I St branchial. 



(d) 2nd branchial. 



{e) 3rd branchial. 



(/) 4th branchial (gives 

 off artery to " lung " 

 of Dipnoi). 



Aborts, or is not 

 developed. 



Aborts. 



Carotid. 



Systemic arches, 

 unite to form 

 dorsal aorta. 



Rudimentary or 

 disappears in 

 most forms. 



Pulmonary. 



Sauropsida and 

 Mammals. 



At most merely em- 

 bryonic. 



At most merely em- 

 bryonic. 



Carotid. 



Systemic. Only the right 

 persists in Birds ; only 

 the left in Mammals. 



Disappears. 



The pulmonary. 



