RECORDS OF MEETINGS 369 



intravertebral or spinous^ juxta vertebral or azygos^ agree in drainage 

 area, in the rationale of their development and in having connection with 

 the districts both of precava and of postcava, between which they so 

 extend as to be able to serve as an equilibratory anastomosis tending to 

 equalize conditions of flow in the two great drainage areas of the systemic 

 veins. Both have numerous connections across the vertebrae with their 

 antimeres. In the case of the azygos the enlargement of some of these 

 connections permits of the development of asymmetry and the ultimate 

 jDreponderance of the vein of one side. This, as might be expected from 

 the normal dextral position of single pre- and postcavse, is usually the 

 right and presumably the factor common to all three cases in the early 

 shift of the venous end of the heart to the right. While appearing first 

 in embryo sauropsids in many and perhaps the majority of adult forms 

 the azygos is reduced and this seems to depend in general upon the selec- 

 tion of the spinous channels as the favored line for segmental drainage ;- 

 a more variable factor exists in the establishment of connections between 

 the right azygos and portal tributaries first in the caval lobe of the liver, 

 later very generally in the extent of the foregut. 



The locus dassicus for the history of the azygos is Eathke's study of 

 the development of Tropidonotus natrix. He also investigated the con- 

 ditions in birds. Later Hochstetter examined the subject in several 

 lacertilians. In all it appears that the cervical portion of the system 

 becomes included in the costo-transverse space giving rise to the anterior 

 vertebral vein. The same position may be occupied by the posterior ver- 

 tebral vein where the costocentral articulations are present as in birds 

 and for three segments in saurians (Kathke). This position is secondary 

 and depends upon the interruption of the plexus about the vertebrae by 

 the developing capitela with the persistence of the portion of the plexus 

 included between rib and transverse process. Chelonians stand apart 

 from other sauropsids in that they develop a homodynamous vessel dorsal 

 to the transverse processes. 



In mammals essentially similar conditions obtain, as Eathke pointed 

 out in 1837. A series of anastomoses extends on each side of the spine 

 from occiput to tail connected across the vertebrae by numerous anas- 

 tomoses. The whole system has been termed the supracardinai v.eins 

 by Huntington and McClure, to whom we owe the recognition of its par- 

 ticipation in the formation of the postrenal segment of the post- 

 cava in placentals. It was later termed prevertebral plexus by Hunting- 

 ton. From this plexus the azygos developes in a manner analogous to its 

 formation in reptiles. This was recognized by Eathke, who interpreted 



