VASCULAR SYSTEM. 



455 



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, z>, 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 arching vessels diffusing the 

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

 blood into a dorsal aorta which runs along under the backbone and 

 supplies the body. 



So 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 life it sends important vitelline arteries to the yolk, and {in 

 Reptiles, Birds, and ^Iammals) equally important allantoic arteries to 

 the allantois. 



Returning to the arterial system of a fish, we must consider the arches 

 more carefully, and compare them with those of Sauropsida and ]\Iani- 

 mals, where they are no longer connected with functional gill clefts, and 

 also with those of Amphibians, where the complications due to lungs, 

 &c., bee;in. 



Fishes. 



(ci) Mandibular aortic arch 

 usually aborts ; has 

 a persistent trace in 

 Elasmobranchs (spir- 

 acular artery). 



{h) Hyoid aortic arch 

 aborts, or is rudi- 

 mentary, persists in 

 Elasmobranchs and 

 some Ganoids. 



((■) ist branchial. 



((/) 2nd branchial. 



{c) 3rd branchial. 



{/) 4th branchial 

 off artei-y to ' ' 

 of Dipnoi). 



(gives 

 lung" 



Amphibians. 



Aborts, or is not de- 

 veloped. 



Aborts. 



Carotid. 



Systemic arches, 

 unite to form 

 dorsal aorta. 



Rudimentary or dis- 

 appears. 



Pulmonary. 



Sauroi'siua and Mammals. 



At most merely em- 

 bryonic. 



At most merely em- 

 bryonic. 



Carotid. 



Systemic. Only the right 

 persists in Birds ; only 

 the left in Manmials. 



Possibly the pulmonary 

 (unless that be/). 



The pulmonary (unless 

 that be c). 



