745 



dominai one, since the latter would be inclined in an opposite direction 

 to them, vlx. downwards, in the curve of the ventral Ijody-wall. 



; However, this factor is not of much importance in animals like 

 Keptilia in which the iliac veins lie ajiproximately in a horizontal plane. 

 For this reason then, viz., because the dorsal limit of the ])ody-cavity 

 is more in line with the caudal and iliac veins than the ventral, the 

 l)osterior cardinals (post-caval) lie dorsally and therefore in close appo- 

 sition to the vertebral colunni. Confirmatory of this explanation is the 

 fact that when the dorsal limit of the coelom is not so much in line with 

 the caudal and iliac veins as the ventral (as e. g. in Chcloiiia., where the 

 extremely convex curvature of the vertebral column and the Hat plastron 

 conduce to this result) then the veins pursue the ventral course instead 

 of the dorsal; likewise, because they are as much in line Avith the epi- 

 gastrics as with the jDost-caval and nearer, the horizontal iliacs of Cro- 

 codilia (not the caudal, which is very large and quite in line with the 

 dorsal post-posterior cardinals) are at liberty to open into the former 

 instead of the latter, which they do; and it is also j^ossible that the 

 blood contained in the coccygeo-mesenteric of birds i^ursues a ventral 

 course merely because such is, owing to the form and position of the 

 trunk, as easy of passage as the dorsal, the reduced "renal portal 

 system" not offering so great an obstruction to the flow of the blood in 

 the avian post-caval as in that of lower vertebrata. 



But now, granted that the post-caval has been formed midw'ay 

 down the dorsal side of the trunk, there has yet to be effected a direct 

 communication with the heart in order to entirely replace the posterior 

 cardinals in the anterior region. How easily this communication may 

 be made is shown by the fact, stated by Hochs tetter, that even in 

 Elasmobranchs an anastomosis is formed between the hepatic sinus and 

 the posterior cardinal veins, this anastomosis being, and very rightly, in 

 Hochstetter's opinion, tantamount to the formation of a post-caval ■*. 

 The Dipnoi also only differ from other fishes in that one of the jjosterior 

 cardinals opens into the heart instead of into the Cuvierian duct. A 

 similar anastomosis is formed during the ontogeny of Clieiropterjigia. 

 the post-caval Ijecoming connected with the right hepatic vein, which, 

 losing its connection with the liver, conducts the blood returned from 

 the posterior jiortions of the body directly to the heart. Hence at the 

 time of formation of tliis anastomosis the blood in the post-caval can 

 return by three routes to the heart — directly by the median anasto- 

 mosis, indirectly by the lateral persistent anterior portions of the poste- 

 rior cardinals. 



* See footnote on previous iiage. 



