854 HUMAN ANATOMY. 



the bronchial and pulmonary veins in the region of the smaller bronchi are abundant, 

 and, in addition, the main stems of the pulmonary veins receive at the hilum of the 

 lung one or more branches from the larger bronchial veins. They also receive com- 

 munications from the venous plexus which surrounds the thoracic aorta in the pos- 

 terior mediastinum, and occasionally also a vein from the pericardium. There is 

 thus a certain commingling of venous blood with the arterialized blood which forms 

 the principal contents of the pulmonary veins. 



Variations. — At one stage in the development of the embryo the veins from each lung 

 converge to a single short tf unk before opening into the portion of the atrium which corresponds 

 to the left auricle. As the development of the heart proceeds, this trunk is graduall>' taken up 

 into the auricle, until the two stems which unite to form it open independently into that 

 structure. An inhibition of this process occasionally obtains, so that but a single vein, repre- 

 senting the original terminal trunk, opens into the auricle from one lung or from both. On the 

 other hand, the taking up of the pulmonary vein into the wall of the auricle may proceed further 

 than usual, or, to state it perhaps more correctly, the union of the various stems emerging from 

 the hilum of the lung may be partly delayed until they have reached the original terminal trunk, 

 so that when this is taken up into the auricle an additional vein will open independently into the 

 latter. This e.xtra vein is most frequently that from the middle lobe of the right lung, but three 

 distinct veins have also been observed upon the left side. 



THE CARDINAL SYSTEM. 



The cardinal system of veins is so named because its main trunks are the repre- 

 sentatives of the cardinal veins of the embryo. These veins are four in number, 

 disposed symmetrically in pairs, two returning the blood from the head, neck, and 

 upper extremities, while the other two return that from the thoracic and abdominal 

 walls, from the thoracic viscera, and from the lower extremities. Just before they 

 reach the heart, the superior and infe7'ior or posterior cardinal veiiis of each side 

 unite (Fig. 776) to form trunks known as the dncts of Ciivier, the two ducts opening 

 independently into the primitive right auricle. By a series of changes, which are 

 described more fully in the section on the development of the veins (page 927), the 

 left superior cardinal becomes connected with the right at the base of the neck, the stem 

 so formed constituting what is termed the superior vena cava. The portion of the 

 left superior cardinal between the connecting vessel and the heart becomes greatly 

 reduced in size, indeed, almost completely degenerates ; the left duct of Cuvier, 

 however, persisting as the coronary sinus, which receives the coronary veins returning 

 the blood from the heart's walls. On the development of the vena cava inferior the 

 veins of the lower extremity make connection with it, separating from the inferior 

 cardinals; these latter become considerably reduced in size, especially in the abdominal 

 region, a cross-connection develops between the left and right veins, and the former 

 severs its connection with the left ductus Cuvieri, the final result being the formation 

 of the venae azygos and hemi-azygos of the adult. 



There are, then, developed from the cardinal veins of the embryo three sub- 

 systems of veins : (i) that of the cardiac veins ; (2) that of the superior vena cava, 

 which includes the jugular and subclavian groups of veins, the original superior 

 cardinals being represented by the internal jugular veins ; and (3) the azygos sub- 

 system. These will be considered in the order in which they have been named. 



THE CARDIAC VEINS. 



The Coronary Sinus. 



The coronary sinus (sinus coronarius ) (Fig. 750) is a short venous trunk about 

 3 cm. (a little over an inch) in length, which occupies the right half of that portion of the 

 posterior auriculo-\^entricular groove which lies between the left auricle and ventricle. 

 At its right end it opens into the right auricle, its orifice (Fig. 657) being situated 

 upon the posterior surface of the auricle, below that of the inferior vena cava, and being 

 guarded by the ThebesiaJi valve (valvula sinus coronarii). At its left end it receives the 

 great coronary vein, from whose proximal portion it is not always clearly distinguish- 

 able upon superficial examination. A close inspection usually reveals, however, 

 either a constriction or a slight dilatation at the union of the two vessels, and on 



