48 COMPARATIVE ANATOMY 



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 a pair of 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 tendines, 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 it 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. 



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. The middle valve is the smallest, and just above it 

 a horizontal partition springing from the spiral valve separates 

 a dorsal from a ventral chamber. The dorsal chamber, which 

 is much the smaller, twists round to the right and passing for- 

 wards branches right and left to form the carotid arches. The 

 ventral chamber is divided by a vertical partition into two wide 

 right and left passages, and each of these is again subdivided 

 by a partition into a systemic aortic arch, above and a pulmo- 

 cutaneous arch below (fig. 7, B). We have already noticed 

 that the truncus arteriosus bifurcates at its anterior end, and that 

 each bifurcation very shortly divides into three vessels. The 

 bifurcations themselves are triple in constitution, as may be 

 seen by cutting them across, when each is observed to be made 

 up of three vessels — an inner and upper, the carotid ; a median, 

 the systemic ; and a lower and outer, the pulmo-cutaneous. 

 It is by means of these vessels that blood is conveyed from 



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