vi HEAET AND AETEEIES 393 



about a substitution of automatically acting pocket- or flap-valves 

 for the original endocardiac ridge or cushions which for their 

 functioning were dependent upon a complex and fallible neuro- 

 muscular mechanism. 



Arterial System. — The conus arteriosus is primitively prolonged 

 forwards into the ventral aorta which gives off on each side the 

 series of aortic arches. The most important evolutionary change 

 which the originally simple tubular ventral aorta undergoes is a 

 process of splitting whereby the stream of blood to the lungs is 

 separated from that to the tissues generally. This splitting is seen 

 for the first time in the Lung-fishes. Here a dorsal portion of the 

 cavity is separated off, continuous behind with the pulmonary cavity 

 of the conus and ending 1 ilindly in front by its floor meeting its roof. 

 The anterior termination of this cavity is just in front of the point of 

 origin of aortic arch V, and this arch along with arch VI, owing to 

 the fact that they branch off from the ventral aorta somewhat dorsally, 

 open from the cavity in question. The horizontal partition which 

 forms the floor of this dorsal pulmonary cavity begins in ontogeny 

 as a rudimentary ingrowth on each side between the origins of 

 arches IV and V. The two rudiments grow back, fuse, and form a 

 horizontal partition continuous at its hinder end with the Eight and 

 Left ridges of the conus which, as already explained, form the floor 

 of its pulmonary cavity. 



A similar splitting off of a dorsal, pulmonary, part of the ventral 

 aorta occurs in air-breathing Vertebrates in general. 



As regards the remaining, or systemic, portion of the ventral 

 aorta, the main point to notice is its tendency to split into two 

 separate halves in its anterior portion, a process correlated probably 

 with economy of material, allowing as it does the origins of the aortic 

 arches to be displaced outwards so as to shorten these arches. This 

 splitting or bifurcation of the ventral aorta spreads backwards for a 

 variable distance, commonly to about the level of aortic arch III 

 or IV. 



In the Eeptiles there comes about an independent splitting 

 into two lateral halves of the aortic cavity posterior to the region 

 of the bifurcation just alluded to. A vertical septum grows 

 backwards from the anterior lip of the opening into aortic arch IV 

 of the left side and becomes continuous with the septum separating 

 the two systemic cavities of the conus. 



When this aortic septum is formed the ventral aorta in its 

 hinder portion contains three cavities — a dorsal, pulmonary, leading 

 to aortic arches V and VI of both sides, a left ventral leading to 

 aortic arch IV of the left side, and a right ventral leading to aortic 

 arch IV of the right side together with the paired anterior part of the 

 ventral aorta and the aortic arches springing from them. Each of the 

 three cavities is continuous behind with the corresponding cavity of 

 the conus. In the majority of Eeptiles (not in Lace-rta) the separation 

 of these cavities is followed by splitting of the septa between them 



