DEVELOPMENT OF THE VEINS IN THE EMMKVO 1'Ki. l!> 



of the azygos system (1X30, p. (17) seems to have been based on theoretical grounds; he 

 said that in sheep the hemiazygos vein replaced the left inferior vena cava and that in the 

 sheep and the pig the azygos vein did not develop. Subsequently (1X3X) he stated the 

 theory that the azygos and hemiazygos veins were transformed posterior cardinal veins 

 down to the sixth dorsal vertebra, and that below this point they were from longitudinal 

 anastomoses between the intercostals. The work of Hochstetter forms the basis of the 

 modern work on the venous system. His work on the azygos veins was done especially on 

 guinea-pigs and his conclusions are that down to the tenth dorsal segment ( 1X93, p. 574), 

 or, stated in general terms for mammals (1906, p. 145), through the thoracic cavity, the 

 azygos and hemiazygos are transformed posterior cardinal veins, while below that level 

 they are new veins. This new vein he described as formed in the following manner (1894, 

 p. 488): the segmental veins give up their connection with the posterior cardinal vein and 

 become collected into a longitudinal trunk which lies dorsal to the aorta. That is to say, the 

 lower part of the azygos is a longitudinal anastomosis between the roots of the spinal veins. 

 In this connection he noted that the segmental veins were originally ventral to the sympa- 

 thetic system and subsequently became dorsal. This point will be thoroughly explained 

 in the following pages, as well as the true relation of the spinal veins to the azygos system. 



Zumstein was the first to give an entirely correct statement of the origin of the azygos 

 veins (1897, p. 175). In studying the veins of an embryo guinea-pig of about 23 days, he 

 says that near the cephalic end of the posterior cardinal veins two new veins appear, dor- 

 sal to the cardinal near the bodies of the vertebrae. They join with the cardinal near the 

 duct of Cuvier and become the azygos and hemiazygos veins. In embryos a little larger 

 he noted that both the cardinals and the more dorsal azygos veins were present, and that 

 subsequently the cardinals became rudimentary. In his figure 9 he shows both the pos- 

 terior cardinals and the azygos veins in the same specimen. 



In the previous year Zumstein had published a study of the relations of the vena cava 

 to the azygos and hemiazygos veins in human embryos and in the adult, in which he took 

 the view that the azygos and hemiazygos veins were transformed posterior cardinal veins. 



The following year Parker and Tozier made the next step in advance. In studying 

 the development of the posterior cardinal veins in the pig, they noted that the cardinal 

 veins, after becoming incorporated in the Wolffian bodies, become interrupted within the 

 Wolffian bodies at the stage of 12 to 13 mm., and that the anterior portion of each posterior 

 cardinal vein received blood not only from the Wolffian bodies but from the tissue between. 

 This they found was due to a new outgrowth of vessels from the posterior cardinal veins 

 near the anterior end of the mesonephros. These new accessory veins were the azygos and 

 hemiazygos veins, which they described as having extremely delicate walls (capillaries) 

 without a complete lumen, and as gradually progressing caudalward. The point of not 

 having a complete lumen is, I think, due to a collapsing of the walls of the capillaries, since 

 injections of a complete capillary network are readily made at the same stage. But the 

 discovery of Parker and Tozier that the azygos and hemiazygos are new veins from a new 

 subvertebral capillary plexus is the correct view, and is the main point, They found that 

 these new vessels start from the posterior cardinal veins near the cephalic end of the meso- 

 nephros, but the permanent place of union of these new vessels with the cardinal system 

 they thought to be at the level of the tenth rib, so that they finally agreed with Hochstetter 

 that the thoracic part of the vessels came from the posterior cardinal veins and the rest from 

 a new subvertebral plexus. This work of Parker and Tozier corrects the view of Rathke 

 that the azygos veins are longitudinal anastomoses of the intercostals, and the view of 



