THE HEART. 33 



4. The Truncus Arteriosus consists of two parts -, a proximal 

 part or pylangium, which is a single vessel arising from the 

 ventricle ; and a distal part or synanginm, which consists of the 

 basal parts of the aortic arches closely united together. 



a. The pylangium (Fig. 6) is a short tube arising from 



the right hand ventral corner of the anterior end of 

 the ventricle : it has thick muscular walls and is 

 widest about the middle of its length. 



The opening from the ventricle to the pylangiimi 

 (Fig. 6 B) is guarded by three semilunar pocket valves. 



The opening from the pylangium to the synangium 

 is also guarded by three semilunar valves which are 

 of very unequal size, a large right one, a small left 

 one, and a still smaller dorsal valve. 



The spiral valve is a longitudinal ridge, projecting 

 into the cavity of the pylangium : it commences at 

 the left side of the ventricular aperture and runs 

 forwards somewhat spirally along the dorsal wall of 

 the pylangium to its anterior end, where it fuses 

 with the large right valve of the three between the 

 pylangium and the synangium. The ventral edge of 

 the spiral valve is free and rounded, and the valve is 

 much wider at its anterior than at its posterior end. 



b. The synangium is the distal part of the truncus 



arteriosus. In its dorsal wall, immediately beyond 

 the valves separating it from the pylangium, is an 

 aperture (Fig. 6 P') leading to the right and left 

 pulmo-cutaneous arches (Fig. 6 P,P'). Beyond this 

 the synangium contains a wide cavity continued right 

 and left into the two systemic arches — S,S'. The 

 cavity is partially divided by a vertical tongue-like 

 projection from its dorsal wall : on the ventral 

 surface of this tongue are two small openings, very 

 close together, which lead into the right and left 

 carotid arches, C,C'. 

 Cut across the aortic arches, just heyond the division of the 

 truncus into right and left branches, and note that though each 

 branch is apparently a single vessel its cavity is really divided 

 into three vessels corresponding to the three aortic arches. Pass 

 bristles down these aortic arches, and note the points at which they 

 severally open into the truncus arteriosus. 

 D 



