504 THE ABSOEBEXT VESSELS. 



Yalved 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 sulj- 

 stance of both ventricles. 



The small or anterior cardiac veins {vena3 cordis parvas) 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 provided with more or less complece 

 valves at their terminations. 



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

 considered with reference to their early foetal condition and certain malfonna- 

 tlons 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 wiU be found in the description of the development of these 

 veins. 



2.— THE ABSOEBENT VESSELS. 



The absorbent vessels are divisible physiologically into two sets ; — 

 the lacteals , which convey the chyle from the alimentary canal to the 

 thoracic duct ; and the lympliatics, which take up the lymph from all 

 the other parts of the body, and return it into the venous system. 

 Anatomically considered, however, the lacteals are not different from 

 the lymphatics, and may be regarded as the absorbents of the mucous 

 membrane of the intestine. The larger lacteals and lymphatics arc 

 provided with numerous valves, which give them, when distended, a 

 somewhat moniliform appearance ; and both are connected in their 

 course with lacteal or lymphatic glands. 



The general anatomy of the absorbents having been elsewhere de- 

 tailed, only their course and position remain to be here described. 

 They are gathered into a right and a left trunk, which open into the 

 angles of union of the subclavian and internal jugular veins. The 

 large vessel of the left side traversing the thorax is named the thoracic 

 duct : it receives not only the lymphatics of its own side of the head 

 and arm, and the most of those of the trunk, but likewise the lymphatics 

 of both lower limbs, and the whole of the lacteals. The vessel of the 

 right side is named the riyht lympliatic duct, and receives the Ij-mphatics 

 only of that side of the head and neck and upper part of the trunk. 



THE THORACIC DUCT. 



The thoracic duct is the common trunk which receives the absorbents 

 from both the lower limbs, from the abdominal viscera (except part of 



