II. THE NORMAL AND THE ATYPICAL VENA CAVA POSTERIOB 



(INFERIOR) 



The explanation and interpretation of the atypical condi- 

 tions of the vena cava posterior encountered by us in an 

 extensive series of adult examples of Felis domestica are 

 fully to be found in the comprehensive system of embryonic 

 venous channels normally functioning in different onto^-ciu-tic 

 stages. The acquisition of what is regarded as the normal 

 type of vena cava in the cat is contingent on the selection and 

 the continued development in the majority of individuals of 

 certain venous pathways into the adult stage. The average 

 individual thus assumes the venous type considered normal 

 for the species. Any deviation from this plan is to be re- 

 garded as an atypical condition or variant. It is brought 

 about by the selection and continued development of other 

 existing embryonic channels which normally are not carried 

 into the adult stage. 



The vena cava posterior in any one of its atypical forms 

 has usually been described by anatomists as an 'abnor- 

 mality.' In the sense that an atypical condition is not nor- 

 mally encountered in the adult, such a designation is correct. 

 On the other hand, all atypical cavae mentioned in the follow- 

 ing pages are normal rather than pathological, in the sense 

 that they are derived from vessels which during the course of 

 ontogenetic development have normally functioned in the 

 embryo. 



III. THE SUPRACARDINAL SYSTEM OF VEINS 



In 1905, when we first began a comprehensive study of the 

 development of veins in the cat, it was generally supposed 

 that the posterior cardinal vein of the embryo entered directly 

 into the formation of the lumbar portion of the vena cava 

 posterior. The acceptance of this view was due to the bril- 

 liant pioneer work of Hochstetter ('93), whose figures illus- 

 trating the development of the veins are well known to all 

 anatomists. While we recognize the importance of Hoch- 



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