X LUNG-FISH 385 



passes thence through left ventricle, systemic cavity of the conus, and 

 aortic arches III and IV to the aortic roots and so on to the tissues of 

 the body generally. The blood from the sinus venosus on the other 

 hand passes by way of right auricle, right ventricle, pulmonary division 

 of the conus, and arches V and VI, into the common vessel formed by 

 the junction of these. From this it drains away into the pulmonary 

 artery and so to the lung, only an insignificant proportion passing on 

 into the aortic root. 



There is thus in Lepidosiren a perfectly recognizable double circula- 

 tion, but the two circuits, pulmonary and systemic, have not as yet 

 become completely shut off from one another. A considerable admixture 

 of the two blood-streams must still take place owing to the splitting 

 of atrium, ventricle and conus into two halves being not yet complete. 



The venous system of the Lung-fishes is also of great interest for we 

 find in it for the first time an arrangement of the main venous trunks 

 in the hinder part of the bpdy that occurs in all the higher vertebrates 

 and also a clear indication of the manner in which that arrangement has 

 come about in the course of evolution. In the typical fishes, as in 

 Scyllium, the blood from the kidneys and posterior region of the body 

 is carried forwards towards the heart by a pair of posterior cardinal 

 veins which open anteriorly into the outer ends of the ducts of Cuvier. 

 In the higher vertebrates on the other hand this holds only for early 

 stages of development : in the adult the posterior cardinals are reduced 

 or absent in the region in front of the kidneys and the blood passes 

 directly to the sinus venosus by a large new unpaired vein known as the 

 posterior (or in Man " inferior ") vena cava, which arises in the embryo 

 by sprouting out from the veins of the liver. It is in regard to the 

 evolutionary origin of this important vein of the higher vertebrates that 

 the Lung-fishes give us a clue. 



There are present in Lepidosiren and Protopterus the same main 

 venous trunks, as in Scyllium. The caudal vein divides into the two 

 renal portals which run forwards along the outer edge of the kidneys. 

 After traversing the kidneys the blood is collected by the two posterior 

 cardinals which pass forwards, communicating with one another here 

 and there by cross channels, to the outer ends of the ducts of Cuvier. 

 Anterior cardinal and subclavian veins also open into the duct of Cuvier. 

 Into the sinus venosus opens the hepatic vein from the liver. 



The important new feature of the venous system is correlated with the 

 fact that the kidneys are much elongated and that the tip of the hver 

 has come into intimate contact with the front end of the right kidney. 

 Complete fusion has taken place between the tissues of the two organs 



2 C 



