I.] THE FROG. 19 



branches into which the truncus arteriosus divides, are, in 

 fact, each made up of three separate trunks, the pul mo- 

 cutaneous trunk behind, the aortic arch in the middle and 

 the carotid trunk in front. 



When the heart is in action, the sinus venosus, the atrium, 

 the ventricle and the truncus arteriosus contract in the 

 ordeF in which they have just been named. Each contracts 

 as a whole, so that the two auricles are emptied simulta- 

 neously. The blood from each is forced into the correspond- 

 ing half of the spongy cavity of the ventricle, so that the 

 right half of the ventricle contains venous blood and the 

 left arterial blood. When the systole of the ventricle takes 

 place, the blood which is first driven into the truncus arte- 

 riosus (the opening of which is, as has been seen, at the 

 right end of the cavity) is therefore venous. It fills the 

 conus arteriosus and, finding least resistance in the short 

 and wide pulmonary vessels, passes along the left side of the 

 median valve into them. But as they become distended 

 the next portion of blood, consisting of the venous and 

 arterial blood which have become mixed in the middle of 

 the ventricle, passes on the right side of the longitudinal 

 valve into the aortic arches. And, as the truncus becomes 

 more and more distended, the longitudinal valve, flapping 

 over, tends more and more completely to shut off the 

 openings of the pulmonary arteries and to prevent any 

 blood from flowing into them. 



Finally, the last portion of blood from the ventricle, 

 representing the completely arterialized blood of the left 

 auricle which is the last to arrive at the opening of the 

 truncus, passes into the carotid trunks and is distributed to 

 the head. 



The principal vessels of the Frog are disposed as fol- 

 lows : — 



