THE VASCULAR SYSTEM, 



395 



In Crocodilia, there are two auricles and two ventricles. 

 But the dorsal aorta, which supplies most 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 supplying the body with 

 wholly pure blood. 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, 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 em- 

 bryonic life it sends important vitelline arteries to the yolk, and (in 

 Reptiles, Birds, and Mammals) 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 Mam- 

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

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

 etc., begin. 



Fishes. 



[a) Mandibular aortic arch 

 usually aborts ; has a 

 persistent trace in Elas- 

 mobranchs (spiracular 

 artery). 



(J>) Hyoid aortic arch aborts, 

 or is rudimentary, per- 

 sists in Elasmobranchs 

 and some Ganoids. 



(c) ist branchial. 



{d) 2nd branchial 



{e) 3rd branchial. 



C/) 4th branchial (gives off 

 artery to " lung " of 

 Dipnoi). 



Amphibians. 



Aborts, or is not de- 

 veloped. 



Ahorts. 



Carotid. 



Systemic arches, unite 

 to form dorsal 

 aorta. 



Rudimentary or dis- 

 appears. 



Pulmonary. 



Sauropsida and Mammals. 



At most merely embryonic. 



At most merely embryonic. 



Carotid. 



Systemic. Only the right 

 persists in Birds ; only 

 the left in Mammals. 



Pulmonary (perhaps this is 

 the 6th). 



(Perhaps it is the 5th which 

 disappears.) 



