48 COMPARATIVE ANATOMY 



The right-hand interior corner of the ventricle is prolonged 

 on its ventral side, into a stout vessel, which passes forward 

 and to the left across the auricles, and divides at the upper 

 limits of the latter into two vessels, right and left, each of 

 which almost immediately divides again into three. The 

 large single vessel is called the truncus arteriosus. Though 

 it appears single up to its bifurcation, the distal part of the 

 truncus really consists of vessels closely united together, hence 

 the truncus is conveniently subdivided into a proximal single 

 portion, the pylangium, and a distal multiple portion, the 

 synangium. The opening of the pylangium into the ventricle 

 is large, and is guarded by three membranous valves, 

 shaped like watch-pockets with their mouths turned towards 

 the cavity of the pylangium. The free edges of each of these 

 valves known as the semi-lunar valves are tied to the inner 

 walls of the pylangium by cordse tendineae, similar to those 

 of the auriculo-ventricular valve. Further, the cavity of the 

 pylangium is incompletely divided into two by a membranous 

 fold which commences on the ventral side close above the 

 opening into the ventricle, runs forward with a spiral course, 

 becoming deeper as its goes, and it ends on the dorsal and 

 right side of the pylangium, where it is fused to one of the 

 three semi-lunar valves, similar to those of the ventricular 

 aperture, which guard the opening from the pylangium into 

 the synangium. The ventral border of this fold hangs free 

 in the cavity of the pylangium, and extends across two-thirds 

 of its diameter when fully extended, dividing it into a chamber 

 beginning on the ventral side and curving round to the right, 

 the cavum aorticum, and another beginning on the dorsal 

 side and curving round to the left, the cavum pulmo-cutaneum. 

 The synangium is the wide but very short part of the truncus 

 which lies in front of the three valves separating it from the 

 pylangium. It is completely divided by a horizontal partition, 

 called the principal septum, into a dorsal and a ventral chamber. 

 The dorsal chamber communicates with the cavum pulmo- 

 cutaneum by an aperture situated just above the middle and 

 smallest of the three valves, and anteriorly it is continued 

 into the pulmo-cutaneous arteries. The ventral chamber is 

 divided by a vertical partition, called the septum medium, into 

 two wide right and left passages. The left passage leads 

 into the systemic aorta of that side. The right passage is 



