﻿168 Proceedings of the Royal Physical Society. 



cavity the two spiral ridges, their position at this level 

 being practically dorsal and ventral, so that they divide 

 the lumen into two moieties, a right and a left. Sections 

 farther forwards (h, c, d) show how these two chambers alter 

 their, relative position, until eventually the one which was 

 on the right side posteriorly comes to be dorsal, while that 

 which was on the left side comes to be ventral. The two 

 valves have now become fused together, so as to form a 

 complete horizontal septum, which forms the floor of the 

 dorsal cavity. This floor, when traced forward, is seen to 

 slope upwards towards the dorsal wall of the ventral aorta, 

 with which it completely blends (e). The dorsal chamber is 

 thus closed anteriorly. The point at which its floor meets 

 and fuses with its roof is situated behind the point of 

 origin of the fourth aortic arch, and in front of the point 

 of origin of the fifth. Both fifth and sixth aortic arches 

 arise from the dorsal side of the ventral aorta, i.e., they 

 receive their blood from the dorsal chamber. 



It is the fact of the extension forwards of this dorsal 

 chamber, so as to include the origin of the fifth aortic arch, 

 that appears to be answerable for the reduction of this arch 

 in the higher lung-breathing forms, for it is quite clear 

 that increased development of the lung, as the respiratory 

 organ, would lead to increased size of the pulmonary 

 arteries, and it would obviously be advantageous that the 

 pulmonary arteries should drain away a larger and larger 

 proportion of the venous blood (from the right side of the 

 heart) in the dorsal chamber. The fifth arch, which is 

 dependent on the same source of supply, would thus get 

 less and less blood, and we should therefore have every 

 expectation that it would diminish in size and finally 

 disappear, as has been the case in the higher vertebrates. 



