420 MANUAL OF ZOOLOGY SECT. 



the hind limb is carried by the renal portal vein (rn. pt.) 

 to the corresponding kidney. 



The blood is collected from the kidneys by the renal 

 veins (rn.\ which unite to form the large unpaired postcaval 

 vein (pt. cv.). This passes forwards through a notch in the 

 liver, receives the hepatic veins (hp.) 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 abdominal vein 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 (put.) directly to the left 

 auricle. 



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 simultaneously, each passes its blood into 

 the corresponding part of the ventricle, which then instantly 

 contracts, before the venous and arterial bloods 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 aortae 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 



