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STRUCTURE OF VERTEBRATA. 



In Crocodilia there are two auricles and two ventricles. But the 

 dorsal aorta, which supplies the posterior parts of the body, is formed 

 from the union of two aortic arches, one from each ventricle. Therefore 

 it contains mixed blood. 



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

 and one aortic arch supplies 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. 



Summary as to Aortic Arches. 



Fishes. 



Ami'hibians. 



Sauropsida and 

 Mammals. 



(a) Mandibular aortic 

 arch usually aborts ; 

 there is a persistent 

 trace in Elasmo- 

 branchs (spiracular 

 artery). 



{6) Hyoid aortic arch 

 aborts, or is rudi- 

 mentary, persists in 

 Elasmobranchs and 

 some Ganoids. 



(c) 1st 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. 



Pulmonary. 



At most merely em- 

 bryonic. 



At most merely em- 

 bryonic. 



Carotid. 



Systemic. Only the right 

 persists in Birds ; only 

 the left in Mammals. 



Possibly the pulmonary 

 (unless that be/.). 



The pulmonary (unless 

 that be e. ). 



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

 forwards from the heart, of a number of arching 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, 



