114 ANATOMY OF THE RABBIT 



in the accompanying diagram (Fig. 64), the arched condition is re- 

 tained by the aorta and by the puhnonary artery. It is interesting 

 to note also that the primary connection of these vessels is repre- 

 sented in the foetus by an open canal, the ductus arteriosus (Bo- 

 talli), which closes shortly after birth but is indicated in the adult 

 by a short fibrous cord between the left pulmonary artery and the 

 aorta, the arterial ligament (Fig. 57, 1). It will be evident from an 

 examination of Fig. 64 that this is a vestige of the dorsal part of 

 the sixth aortic arch on the left side. The adult aortic arch 

 represents the left one of a pair (the fourth) while that of the 

 right side is represented only imperfectly by the base of the right 

 subclavian artery (the innominate artery and the common carotids 

 being derived from parts of the ventral aorta, as is apparent in 

 Fig. 64). Hence a condition of asymmetry results, which is ex- 

 pressed mainly in the sinistral position of the aortic arch with 

 reference to the oesophagus (Plate VII). By comparison with the 

 embryonic plan, it is seen that the primitive features of the heart 

 and the arterial vessels include the ventral position of the heart 

 itself, the equivalence of the two atria and of the two ventricles — 

 these structures being partitioned internally but imperfectly 

 divided externally — the forward position of the first portion of the 

 aorta (derived from the ventral aorta of the embryo), and the 

 dorsal position of the descending part of the aorta (beyond its arch) 

 as a median vertebral trunk. 



The vascular system is noteworthy for several departures from 

 the condition of symmetry, one of which has already been men- 

 tioned. In addition, it is seen that in a mammal, as in terrestrial 

 vertebrates generally, the base of the pulmonary artery (Figs. 57, 

 62) is rotated in a spiral fashion about the base of the aorta, so that 

 from its beginning on the right ventricle it passes across the ventral 

 surface of the base of the aorta to divide on the dorsal side of 

 the latter into its two main branches. Moreover, the separation 

 of the ventricular portion of the heart into two chambers is as- 

 sociated with an enormous increase in the muscularity of the wall 

 in the left ventricle, or, in other words, in that portion which is 

 concerned with the larger, systemic circulation. The inferior caval 

 vein (Plate VIII), a highly specialized vessel, is asymmetrical. 



