9© ANAtOMY OF THE RaBBIT. 



Indicated in the adult condition by a short. fibrous cord, the arterial 

 ligament. The adult aortic arch represents only the left one of a 

 pair, and since that of the right is only represented imperfectly 

 by the innominate artery and the base of the subclavianof that side, 

 a condition of asymmetry results, which is mainly expressed by 

 the sinistral position of the 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, and the position 

 of the aorta as a median vertebral trunk. 



The vascular system is noteworthy for several departures from 

 the condition of symmetry, one of these having already been 

 mentioned. In addition, it is seen that in a mammal, as in terres- 

 trial vertebrates generally, the base of the pulmonary artery 

 (Fig. 44) is rotated in a spiral fashion about the base of the aorta, 

 so that from its beginning on the right ven tricle it passes across the 

 ventral surface of the base of the aorta, and divides on the dorsal 

 side of the latter into its two main branches. Moreover, the 

 separation of the ventricular portion of the heart into two cham- 

 bers is associated 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, since from its beginning in the pelvic cavity to its 

 termination on the right atrium it lies wholly to the right of the 

 median plane. The azygos vein of the thorax (Plate VII), a 

 vessel uniting the majority of the paired intercostal veins, and 

 interesting as a remnant of the primitive circulation, is also asym- 

 metrical, since the trunk lies to the right of the bodies of the verte- 

 brae, and is connected at its base with the right superior caval vein. 



THE LYMPHATIC SYSTEM. 



The lymphatic system, both in its fuhctidnal relation and in 

 Brigitf, 'ife'aii appendage of the venous portidn of the vascular system. 

 The%?stfem is an iriiportaftt bne, of which, unfortunately, little 



