THE VASCULAR SYSTEM 219 



smaller factors without the aid of a good dissecting microscope such 

 as is not usually available to the ordinary student, the detailed de- 

 scription of the venous system is prefaced by the following short 

 account of the sinus venosus and the main venous trunks entering it. 



The sinus venosus forms a triangular swelling on the anterior end 

 of the post-caval vein, almost directly after the latter emerges from 

 the substance of the liver, on the left side of the heart. The base of 

 the triangle is anterior. It may be seen from the ventral side, after 

 opening the pericardium, by gently pressing back the left auricle. 

 The ducts of Cuvier enter the basal angles, while the post-caval vein 

 enters the apex. 



The left ductus Cuvieri is either very short or entirely wanting, since 

 the union of the post-cardinals with the anterior veins — the internal 

 and external jugulars — occurs very close indeed to the sinus. 



The right ductus Cuvieri^ on the other hand, is quite long, and ex- 

 tends across the dorsal side of the heart — at about the level of the 

 anterior end of the ventricle — since the union of the venous trunks 

 on this side is symmetrical with that on the left, and occurs outside 

 the pericardium. Thus a long ductus Cuvieri is necessary to reach 

 the sinus venosus, which is situated well to the left of the middle line. 

 It may easily be seen by turning the tip of the ventricle forwards. 



The venous trunks uniting to form the ducts of Cuvier on either 

 side are four in number, viz. the external jugular, the internal jugular, 

 the subclavian, and the post-cardinal. The exact order in which these 

 veins join up varies a little, but they may be easily identified as 

 follows. The external jugular (v.j.e.) is the most anterior, and com- 

 prises (i) the common facial vein (v.f.c.) arriving laterally from the 

 paratoid region, and (ii) the lingual and thyroid veins arriving from 

 a mesial direction, the thyroid vein lying ventral ^.nd the lingual vein 

 dorsal to the arterial arches. They unite to form a common vessel 

 immediately posterior to the arches, and the vessel thus formed 

 usually unites with the common facial vein to form a short trunk — 

 the external jugular proper — which, in turn, joins the subclavian 

 vein. It may, however, occasionally enter the subclavian separately 

 from the facial vein. 



The subclavian vein (v.scl.) is the largest and most prominent of 

 the vessels entering the ductus, and also, with the exception of the 

 thyroid vein, the most ventral, while its direction is almost exactly 

 transverse, so that it is easily found and forms a useful landmark for 

 identifying the other vessels. 



The internal jugular (v.j.i.) arrives from an antero-dorsal direction 

 and joins the subclavian vein close to the external jugular. It is most 



