xii PHYLUM CHORDATA 423 



veins to the post-caval, part by the abdominal vein to the 

 liver, and thence by the hepatic veins to the post-caval. 

 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 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, etc., is constantly 

 diminishing, owing to the continual flow of blood towards 

 the capillaries. Very soon, therefore, the blood forces the 

 valves aside and makes its way into the bulbus aortse. Here 

 again the course taken is that of least resistance ; owing to 

 the presence of the carotid gland the passage of blood into 

 the carotid trunks is less free than into the wide, elastic, 

 systemic trunks. These will, therefore, receive the next 

 portion of blood, which, the venous blood having been 

 mostly driven to the lungs, will be a mixture of venous and 



