DEVELOPMENT OF THE VEIXS. 483 



auricle and ventricle, ends in the coronary sinus : a valve of two segments 

 closes its aperture in the sinus. In this course it receives branches from 

 the ventricles, especially from the left, and also from the left auricle : and 

 as it passes round the thick margin of the left ventricle, it receives a vein of 

 some size, which ascends to join it. 



The posterior cardiac veins ascend on the back of the ventricles, espe- 

 cially on the left, and open into the coronary sinus by four or more valved 

 orifices. One of these, larger than the rest ('middle or posterior cardiac 

 vein), ascends along the groove between the ventricles upon the posterior 

 surface of the heart. It commences by small branches at the apex of the 

 heart, which communicate with those of the preceding vein, and then 

 ascends to the base, receiving branches from the substance of both ventricles. 



The small or anterior cardiac veins (vense cordis parvre) are several small 

 branches, which commence upon the anterior surface of the right ventricle, 

 and passing upwards and outwards, open separately into the right auricle, 

 after having crossed over the groove between it and the ventricle. 



The coronary sinus is about an inch in length, and is placed at the back 

 of the heart in the transverse groove between the left auricle and ventricle, 

 where it is covered by the muscular fibres of the auricle. At one end it is 

 joined by a small vein from the right side, and opens into the right auricle 

 beneath the Thebesian valve ; at the other, it receives the large coronary 

 vein, and a small straight vein directed obliquely along the back of the left 

 auricle ; whilst between those points other veins enter it from the back of 

 the heart. All the veins joining it, except the small oblique vein, are pro- 

 vided with more or less complete valves at their terminations. 



The coronary sinus, together with the small oblique vein above referred to, con- 

 sidered with reference to their early foetal condition and certain malformations to 

 which they are subject along with other neighbouring veins, may be looked upon 

 rather as the persistent terminal parts of a typically distinct left superior vena cava, 

 than as simply the main stem of the cardiac veins. The explanation of this will 

 be found in what follows on the development of these veins. 



DEVELOPMENT OF THE GREAT VEINS. 



In the young foetus before the development of the allantois, a right and a left 

 omphalo-niesenteric vein bring back the blood from the walls of the umbilical vesicle, 

 and unite to form a short trunk, which is continued into the auricular extremity 

 of the rudimentary heart. 



In the first commencement of the placental circulation, or in the third week of fcetal 

 life (Coste), two umbilical veins have been seen coming from the placenta, and unit- 

 ing to form a short trunk, which opens into the common omphalo-mesenteric vein. 

 Very soon the right omphalo-mesenteric vein and right umbilical vein disappear. 

 In connection with the common trunk of the umbilical and omphalo-mesenteric veins 

 two sets of vessels make their appearance in the young liver. Those furthest from 

 the heart, named venae hepaticce, advehentes, become the right and left divisions of 

 the portal vein ; the others are the hepatic veins, vence hepaticce revehentes. The 

 portion of vessel intervening between those two sets of veins forms the ductus 

 venosus (p. 329), and the part above the hepatic vein, being subsequently joined by 

 the ascending vena cava, forms the upper extremity of that vein. Into the remaining 

 or left omphalo-mesenteric vein open the mesenteric and splenic veins. The part 

 above the latter forms the trunk of the portal vein ; and the portion of vessel be- 

 tween the union of this with the umbilical vein and the origin of the venae hepaticse 

 advehentes is so altered that the portal trunk opens into the commencement of the 

 right vena advehens. 



At the time of the commencement of the placental circulation, two short trans- 

 verse venous trunks, the ducts of Cuvier, open, one on each side, into the auricle of 



