184 THE I'KOG. 



vein, at the level of the hinder border of the pericardial cavity, 

 to form the Cuvierian vein (Fig. 71). The two Cnvierian veins, 

 right and left, run almost vertically downwards to open into the 

 outer ends of the sinus venosus (Fig. 71). The Cuvierian veins 

 persist as the anterior venae cava3 of the frog. 



e. The inferior jugular veins collect the blood from the floor 

 of the mouth, including the large sinuses above the sucker into 

 which the mandibular and hyoidean veins open (Fig. 78) : they 

 then run back in the side walls of the pericardial cavity, a ad open 

 into the right and left Cuvierian veins respectively, just before 

 these reach the sinus venosus. 



f. The posterior vena cava is a large median vein (Fig. 76, 

 vi), which develops shortly after the mouth opening is esta- 

 blished ; it is continuous posteriorly with the hinder portions of 

 the two posterior cardinal veins, which unite together ; and, 

 further forwards, it runs in a deep groove along the left side of 

 the liver (Fig. 71, vh), joins with the hepatic veins, and then 

 opens into the sinus venosus. The blood from the hinder part 

 of the body can now return to the heart either along the posterior 

 cardinal and Cuvierian veins, or else by the posterior vena cava. 

 In the later stages of tadpole life more and more of the blood 

 follows the latter course, and the anterior ends of the posterior 

 cardinal veins gradually diminish in size, and during the meta- 

 morphosis disappear completely. 



g. The renal portal veins are formed by longitudinal anasto- 

 motic communications between the transverse, or vertebral, veins 

 of the hinder part of the body ; they are joined posteriorly by the 

 iliac veins, and with these form the afferent renal system of veins. 



h. The anterior abdominal vein is at first paired, and is in 

 connection , not with the liver, but the heart. The pair of vessels 

 appear first in the ventral body wall, extending backwards a 

 short distance from the sinus venosus ; they soon extend further 

 backwards, and acquire communications with the veins of the 

 hind legs and of the bladder. At a later stage, the two anterior 

 abdominal veins unite at their hinder ends, in front of the 

 bladder, while further forwards the vein of the right side disap- 

 pears, the left one alone persisting. Later still, the anterior abdo- 

 minal vein loses its direct communication with the sinus venosus, 

 and acquires a secondary one with the hepatic portal veins, or 

 afferent veins of the liver. 



