2?4 ZOOLOGY SECT. 



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 simulta- 

 neously, 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 trabeculse 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 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 labyrinth, 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 arterial. Finally, as the 

 pressure rises in the systemic trunks, the last portion of blood from 

 the ventricle, which, coming from the left side, is arterial, will pass 

 into the carotids and so supply the head. 



The red blood-corpuscles are, like those of Fishes, oval, nucleated 

 discs. The lymphatic system (Fig. 945) is very well developed, 

 and is remarkable for the dilatation of many of its vessels into 

 immense lymph-sinuses. Between the skin and muscle are large 

 subcutaneous sinuses (Fig. 940, v. ly. s.), separated from one another 

 by fibrous partitions, and the dorsal aorta is surrounded by a 

 spacious subvertebral sinus. The lymph is pumped into the veins 



