XV THE CIRCULATORY SYSTEM 267 



divided by the interauricular septum. This opening is 

 guarded by four valves, two large ones on the dorsal and 

 ventral edges, and a small valve at either end ; small fibers 

 extend from the ventricular wall to be inserted into the free 

 edges of the valves ; they prevent the edges of the valves 

 from being turned back into the auricles when the ventricle 

 contracts, and thus keep the blood from flowing from the 

 ventricle into the auricles. The ventricle possesses in 

 addition to the central cavity at its base a number of fissure- 

 like chambers in the thick muscular wall ; these chambers 

 are separated from each other by muscular partitions and 

 extend outwardly nearly to the periphery of the ventricle ; 

 they receive a large part of the blood that passes through 

 the heart, and have the important function, as will be seen 

 later, of preventing the mixing of the blood that comes in 

 from the two auricles. 



The opening from the ventricle into the bulbus cordis is 

 guarded by three semilunar valves. They are in the form 

 of pockets, open anteriorly, whose walls may be pressed 

 down when blood is passing out of the ventricle, but when 

 blood tends to pass the other way, they fill and prevent its 

 return. In front of these valves is a pecuHar and important 

 structure known as the spiral valve. It consists of a longi- 

 tudinal fold attached along the dorsal wall of the bulbus, its 

 ventral edge lying free. At its posterior end it is attached 

 to the left side of the bulbus, near the opening into the ven- 

 tricle. It passes obliquely across the bulbus, and widens 

 out at its anterior end into a cup-like valve. Two smaller 

 valves occur at the same level. In front of these valves is 

 the unpaired portion of the truncus, which is partly divided 

 by a ridge extending from the point of union of the two 

 branches to the middle of the anterior enlargement of the 

 spiral valve. Each of the two branches of the truncus is 



