282 ZOOLOGY SECT. 



(obd.) which passes forwards in the ventral body-wall, between the 

 linea alba and the peritoneum, to the level of the sternum, where 

 it turns inwards and divides into two branches, both breaking up 

 into capillaries in the liver. Just as it enters the liver it is joined 

 by the hepatic portal vein (hp. |tf.), bringing the blood from the 

 stomach, intestine, spleen, and pancreas. The abdominal vein also 

 receives vesical veins (ves.) from the urinary bladder, and a small 

 cardiac vein from the heart (cd). It represents the lateral veins 

 of Elasmobranchs united in the middle ventral line : the pelvic 

 veins are their posterior free portions. 



The blood is collected from the kidneys by the renal veins (rn.\ 

 which unite to form the large unpaired postcaval vein (pt. ?.). 

 This passes forwards through a notch in the liver, receives the 

 hepatic veins (h.p.) from that organ and finally opens into the sinus 

 venosus. Thus the blood from the hind-limbs has to pass through 

 one of the two portal systems on its way back to the heart : part 

 of it goes by the renal portal veins to the kidneys, and thence by 

 the renal veins to the postcaval, part by the pelvic and abdominal 

 veins to the liver, and thence by the hepatic veins to the postcaval. 

 Lastly, the blood which has been purified in the lungs is returned 

 by the pulmonary veins (pul.*) directly to the left auricle. 



It will be seen that there is no trace of cardinal veins in the 

 Frog. But in the larva both anterior and posterior cardinal veins 

 are present: during the metamorphosis the ductus Cuvieri in 

 which, as in Fishes, they unite, become converted into the 

 pre-cavals, while the posterior portions of the posterior cardinals 

 contribute to the formation of the postcaval, and the anterior 

 portions disappear. 



It will be perceived that the blood poured into the right auricle 

 is mostly impure or venous, that poured into the left fully aerated 

 or arterial. When the auricles contract, which they do simultane- 

 ously, each passes its blood into the corresponding part of the 

 ventricle, which then instantly contracts, before the venous and 

 arterial bloods, kept separate as they are to some extent by the 

 muscular trabeculae acting as incomplete partitions, have time 

 to mix. Since the conus arteriosus springs from the right side 

 of the ventricle, it will at first receive only venous blood, which, 

 on the contraction of the conus, might pass either into the bulbus 

 aortse or into the aperture of the pulmo-cutaneous trunks. But 

 the carotid and systemic trunks are connected with a much more 

 extensive capillary system than the pulmo-cutaneous, and the 

 pressure in them is proportionally great, so that it is easier 

 for the blood to enter the pulmo-cutaneous trunks than to 

 force aside the valves between the conus and the bulbus A 

 fraction of a second is, however, enough to get up the pressure 

 in the pulmonary and cutaneous arteries, and in the meantime 

 the pressure in the arteries of the head, trunk, &c., is constantly 



