LYMPHATIC VESSELS 



183 



large branch is given off, which has a semicircular valve at its origin, and 

 moreover is white and full of aqueous humor." 



" Not far from its source, it splits into two parts which come together a little further 

 on. Giving off no branches, and lying against the left side of the vertebrae, having 

 penetrated the diaphragm, it is borne along to the middle of the loins. There, having 

 become larger and folded around the great artery, it has an obscure ending, not clearly 

 made out by me up to the present time." 



The vessel so well described by Eustachius is now known as the 

 thoracic duct. It has the structure of a vein, and empties its contents into 

 the blood at the junction of the left internal jugular and left subclavian 

 veins. It receives branches from the left side of 

 the head and the left arm, as well as from the 

 trunk of the body. There is a corresponding vessel 

 on the right side, known as the right lymphatic duct. 

 It drains the right side of the head, the right arm, 

 and adjacent territory, emptying at the junction of 

 the right internal jugular and subclavian veins. 

 Having no connection with the abdominal lymphatic 

 vessels, however, it is much smaller than the thoracic 

 duct on the left. 



The connection between the lacteal vessels in 

 the mesentery, seen by Aselli, and the thoracic duct 

 observed by Eustachius, was demonstrated physio- 

 logically by Pecquet (Experimenta nova anatomica, 

 1651). He found a whitish fluid coming from the 

 vena cava superior of a dog from which the heart 

 had been excised, and observed that its flow was 

 increased by pressure on the mesenteries. Moreover 

 he described the receptaculum chyli, or enlargement 

 of the thoracic duct dorsal to the aorta, which receives the chylous fluid. 

 This is now called the cisterna chyli. The distribution, of the lymphatic 

 vessels, which are ramifications of these main trunks, was followed out 

 by skillful injections, and the results of such studies were presented in 

 great folios by Mascagni (1787) and Sappey (1874). Considered as a 

 whole the lymphatic system may be compared with a venous system 

 which has no corresponding arteries; it is composed entirely of afferent 

 vessels. 



Recent anatomical studies of these vessels have been concerned with 

 their origin, and their relation in the adult to the surrounding connective 

 tissue. The vessels have long been known as absorbents, and it was 

 thought that they opened freely at their distal ends into the connective 

 tissue spaces; through these openings they were supposed to suck in the 

 tissue fluid which had escaped from the vessels, and the chylous fluid, 



FIG. 174. THE AZYGOS 

 VEIN (a) AND THO- 

 RACIC DUCT (b) AS 

 FIGURED BY EUSTA- 

 CHIUS. 



