THE CORONARY SINUS AND THE VEINS OF THE HEAET. 959 



side, is in the lowest part of the root, and it is in a plane posterior to that in which 

 the upper vein lies. 



On the right side the upper pulmonary vein passes behind to the superior vena cava, 

 and the lower passes behind the right atrium. They both terminate in the upper and 

 posterior part of the left atrium close to the interatrial septum. 



On the left side both upper and lower pulmonary veins cross anterior to the descend- 

 ing aorta, and they terminate in the upper and posterior part of the left atrium near its 

 left border. 



All four pulmonary veins perforate the fibrous layer of the pericardium, and receive 

 partial coverings of the serous layer before they enter the atrium. 



SYSTEMIC VEINS. 



The systemic veins return blood to the right atrium of the heart through the 

 superior vena cava, the inferior vena cava, and the coronary sinus. The two first- 

 named receive blood from the veins of the body and limbs and from most of the 

 abdominal and pelvic viscera. The coronary sinus receives blood from the veins of 

 the walls of the heart alone. 



General Arrangement. The veins of the body wall and limbs form two groups 

 (1) the superficial veins ; (2) the deep veins. 



The superficial veins lie in the superficial fascia; they commence in the 

 capillaries of the skin and subcutaneous tissues, and are very numerous. They 

 frequently anastomose with one another, and they also communicate with the deep 

 veins, in which, after piercing the deep fascia, they terminate. They may or may 

 not accompany superficial arteries. 



The deep veins accompany arteries, and are known as vence comites. The large 

 arteries have only one accompanying vein, but with the medium-sized and small 

 arteries there are usually two venae comites, which anastomose freely with each 

 other by short transverse branches of communication. 



Visceral veins usually accompany the arteries which supply viscera in the 

 head, neck, thorax, and abdomen. As a rule there is only one vein with each 

 visceral artery, and, with the exception of those which enter into the formation of 

 the portal system, they terminate in the deep systemic veins. 



SINUS CORONARIUS ET VEN^ CORDIS. 



The coronary sinus (Fig. 750) is a short, but relatively wide, venous trunk 

 which receives the majority of the veins of the heart. It lies in the inferior 

 portion of the coronary sulcus, between the left atrium and the left ventricle, and 

 it is covered superficially by some of the muscular fibres of the atrium. 



It terminates in the lower and posterior part of the right atrium, between the 

 orifice of the inferior vena cava on the right, and the right atrio- ventricular 

 orifice anteriorly ; an imperfect valve, consisting of one cusp, called the valve of the 

 coronary sinus (Thebesius), is situated at the right margin of the opening of the 

 sinus into the atrium. 



The apertures of the tributaries of the coronary sinus, except those of the great 

 and small cardiac veins, are not provided with valves, and the valves of the two 

 veins mentioned are often incompetent. 



Tributaries. (1) The great cardiac vein (Fig. 754) commences at the apex of the 

 heart. It ascends, in the anterior interventricular sulcus, to the coronary sulcus ; it then 

 turns to the left, and, passing round the left margin of the heart, into the inferior part of 

 the coronary sulcus, terminates in the left extremity of the coronary sinus. It receives 

 tributaries from the walls of both ventricles and from the wall of the left atrium. It 

 receives also the left marginal vein ; that vein commences at the lower extremity of the 

 left margin of the heart, along which it ascends to its termination. 



(2) The small cardiac vein is very variable ; as a rule it commences at the inferior 

 margin of the heart and passes to the right to the coronary sulcus in which it turns to 

 the left, on the inferior surface of the heart, and terminates in the right extremity of 

 the coronary sinus. It receives tributaries from the walls of the right atrium and the 

 right ventricle. 



