ioo VERTEBRATE ZOOLOGY 



At the forward end of the ventricle is the auriculo-ventricular 

 opening, which leads into the ventricle from both auricles. The 

 septum between the auricles divides this opening into two pas- 

 sages, one of which leads from each auricle. This opening is 

 guarded by a semilunar valve extending into the ventricle, to the 

 dorsal wall of which it is attached by tendinous cords. Look for 

 them with the aid of the blowpipe. 



At the forward end of the ventricle, also, to the right, is the 

 opening into the truncus arteriosus. It is guarded by three semi- 

 lunar valves. Look for them with the aid of the blowpipe. 



The space in the truncus is incompletely divided into two 

 compartments by a large longitudinal ridge, springing from its 

 dorsal wall, called the spiral valve. Of these compartments the 

 right-hand one may be called the arterial compartment, and the 

 left-hand one the pulmocutaneous compartment. Note carefully 

 the shape of the spiral valve and its relation to the opening 

 into the ventricle. 



At the forward end of the spiral valve are three additional 

 pouch-shaped valves. The one of these on the right side is the 

 anterior end of the spiral valve; it is the largest of the three. 

 The other two are on the left and on the dorsal side, respectively, 

 of the truncus and are much smaller. These things are difficult to 

 see except in a large frog. 



Make a cross section of one of the two branches of the truncus 

 and note that it is divided by longitudinal partitions into three 

 chambers which communicate with the three aortic arches. Probe 

 these chambers. The innermost, or anterior, chamber takes blood 

 to the carotid artery, the middle chamber takes it to the systemic 

 artery, and the outer, or posterior, chamber takes it to the pul- 

 mocutaneous artery. Note that the pulmocutaneous chambers on 

 the two sides meet and open posteriorly through a single aperture 

 at the anterior end of the pulmocutaneous compartment of the 

 truncus, which has received venous blood from the ventricle. The 

 arterial compartment, which has received arterial blood, delivers 

 it to the carotid and systemic arches. 



It will be seen that the purpose of this arrangement is to effect 

 such a division of the blood that the pulmocutaneous arch will 



