XIII.] THE FEOG. 1G9 



which are the openings, posteriorly, of the -pulmonary ar- 

 teries, anteriorly of the carotid trunks; while, at the sides, 

 the cavity of the synangium opens into those of the right and 

 left aortic arches. The apparently simple branches into 

 which the truncus arteriosus divides, are, in fact, each made 

 up of three separate trunks, the pulmo-cutaneous trunk behind, 

 the aortic arch in the middle and the carotid triink in front. 



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

 atrium, the ventricle and the truncus arteriosus contract in 

 the order in which they have just been named. Each 

 contracts as a whole, so that the two auricles are emptied 

 simultaneously. The blood from each is forced into the 

 corresponding 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 

 arteriosus (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 

 and less resistance is offered elsewhere, the next portion of 

 blood, consisting of the venous and arterial blood which have 

 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, th& longi- 

 tudinal 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. 



