THE VASCULAR SYSTEM 187 



arise normally four arterial arches, although occasionally only three 

 may be present. These arteries do not^ as in the Frog, leave the 

 truncus as a single vessel, divided internally, but are separate from 

 the beginning, and although a certain amount of connective tissue 

 envelops their bases, it is clearly distinguishable from the arterial 

 walls. In this connexion Ziillich is a little misleading, since he refers 

 to a 'Truncus arteriosus dexter' and 'sinister' (cf. his Abb. i with 

 Fig. 60 of this work). 



The auricles (or atrid)^ as already stated, are displaced to the left. . 

 They are separated from the ventricle by a deep groove — the sulcus 

 coronarius — which is especially apparent from the ventral side. Dor- 

 sally it is covered by the sinus venosus. It is not usually possible to 

 distinguish accurately, from the outside, the partition between the 

 auricles — the septum atriorum — but the left auricle is considerably 

 smaller than the right. The sinus venosus (s.v.) is a large triangular 

 sac which may normally be seen from the ventral side, lying dorsally 

 and to the left of the heart (right as viewed), although obviously the 

 exact size and shape will depend mainly on the amount of blood it 

 happens to contain. It is much larger than in the Frog. The apex 

 of the sinus venosus is directed posteriorly and receives the post-caval 

 vein, while the two ducts of Cuvier enter the basal angles. On 

 removing the heart, or by turning the apex of the ventricle forwards 

 so that the dorsal side may be seen, there are exposed to view the 

 right ductus Cuvieri lying across the anterior rim of the ventricle, 

 and the pulmonary vein passing forwards along the right margin of 

 the sinus venosus to enter the left auricle. 



Internal Details. 



In the detailed description of the internal structure and relations 

 of the several parts of the heart, the order followed will correspond 

 with the course of the blood through that organ, namely, from the 

 sinus venosus to the truncus arteriosus. 



Sinus venosus (s.v.). The cavity is undivided and the walls are 

 very thin and devoid of striped (cardiac) muscle elements. It joins 

 the right auricle at the anterior, right-hand, basal angle, quite close 

 to the point of entry of the right duct of Cuvier, and the cavities of 

 the sinus venosus and the right auricle are there confluent. The 

 opening — the ostium venosum sinus (Ziillich) — is situated near the 

 septum atriorum, and is guarded by a large single flap valve which 

 forms an antero-dorsal outgrowth from the septum atriorum, and 

 prevents regurgitation of the blood from the auricle to the sinus 

 venosus. It lies to the left of the ostium. 



