94 THE FROG chap. 



{d.ao) than into the comparatively narrow carotid trunks 

 {car. tr), obstructed by the carotid plexuses. 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 tjie 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 observation of its 

 beat, and the manner in which the flow from cut vessels 

 takes place. It was by obserX^ation and experiments of this 

 kind that thee irculation 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 directly observing the flow — ^became 

 possible only after the invention of the microscope. This 

 instrument, by furnishing a sufficiently high magnifying 

 power, allows us to see for ourselvest he actual movement 

 of the blood in an animal or organ of sufficient transparency ; 

 and, at the same time, clears up the question, previously 



