94 THE FROG CHAP. 



in these vessels rapidly diminishes, and the blood soon 

 forces aside the valves between the conus and the 

 bulbus and fills the latter. Here again the question of 

 pressure comes in. It is easier for the blood to make its 

 way into the wide systemic trunks (syst. tr) uniting 

 immediately into the long dorsal aorta (d. ao) than into 

 the comparatively narrow carotid trunks (car. tr}, 

 obstructed by the carotid labyrinths. Hence, the non- 

 aerated blood having been mostly driven into the pulmo- 

 cutaneous trunk, the mixed blood, from the middle part 

 of the ventricle, goes into the systemic trunk, and 

 thence to the various arteries supplying the limbs (scl. a, 

 il. a) and the viscera (sp. a, etc.). Finally, when the 

 pressure is sufficiently raised in the systemic trunks the 

 remaining blood, which, coming from the left side of the 

 heart, is aerated, is pumped into the carotid trunks 

 (car. tr), and thence to the head. 



Thus, owing to the arrangement of the valves, and to 

 the varying pressures in different parts of the vascular 

 system, the non-aerated blood returned from the various 

 parts of the body to the heart is mostly sent to the lungs 

 and skin to be aerated. Mixed blood is sent to the 

 trunk, limbs, and viscera, while for the head with its 

 contained brain the directing and controlling organ of 

 the whole animal a special supply of pure, aerated 

 blood is reserved. 



We see then that the course of the circulation may be 

 proved, as a simple matter of induction, from the 

 structure of the heart and its valves, the direct observa- 

 tion of its beat, and the manner in which the flow 

 from cut vessels takes place. It was by observation 

 and experiments of this kind that the circulation of the 

 blood in the higher animals was demonstrated by 

 William Harvey in the seventeenth century. But the 

 final and most conclusive proof of the circulation from 



