4 86 



STRUCTURE OF VERTEBRATA. 



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. 



(a) Mandibular aortic 

 arch usually aborts; 

 there is a persistent 

 trace in Elasmo- 

 branchs (spiracular 

 artery). 



(/>) Ilyoid aortic arch 

 aborts, or is rudi- 

 mentary, persists in 

 Elasmobranchs and 

 some Ganoids. 



(c) 1st branchial. 



(d) 2nd branchial. 



(e) 3rd branchial. 



AMPHIBIANS. 



SAUROPSIDA AND 



MAMMALS. 



Aborts, or is not At most merely em- 



developed. 



Aborts. 



(/) 4th branchial (gives Pulmonary, 

 off artery to "lung" 

 of Dipnoi). 



bryonic. 



At most merely em- 

 bryonic. 



Carotid. 



Systemic arches, 

 unite to form 

 dorsal aorta. 



Rudimentary or 

 disappears. 



Carotid. 



Systemic. Only the right 

 persists in Birds ; only 

 the left in Mammals. 



Possibly the pulmonary 

 (unless that be/.). 



The pulmonary (unless 

 that be <?.). 



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, 



