THE VESSELS. 347 



600. The Veins of the vertebral column, 



to which also vv. azygos and hemiazygos belong, connect together 

 the veins of all parts of the trunk, so that, if even one of the two 

 cavse is obstructed in any place, the circulation of the blood is, in 

 consequence, uninterrupted. They are partly external, upon the 

 arches of the vertebrae, partly internal, in the canalis spinalis, 

 and they form rete in the whole length of the vertebral column. 



1. Internal: 



a. Plexus" 1 spinales interni anteriores; they lie close upon the bones, on the 

 posterior surface of the bodies of the vertebrae ; they appear as a broad 

 longitudinal canal, consisting however of transverse canals lying close 

 together, with thin walls, connected at the foram. magnum with sinus 

 occipital.; they receive the blood from the vertebras and the dura mater. 



b. Plexus spinales interni posteriores; between dura mater and vertebral 

 arches, the meshes are wider ; they form a ring with the anterior 

 plexus between two vertebrae, besides two canals passing close to the 

 middle line ; they anastomose with the : 



2. External: 



a. PI. spinales externi posteriores; they are placed with wide meshes upon 

 the vertebral arches between the transverse and spinous processes ; 

 more narrow in the neck (pi. colli poster.) where there is an anasto- 

 mosis with sinus transversus, (through foram. condyloid. post, and 

 mastoid.) 



b. PI. spinales externi anteriores; do not exist throughout the spine ; pi. 

 colli anter., pi. sacralis. 



The Lymphatic vessels, Vasa lymphatica s. resorbentia. 



I. The Trunks. 



601. Ductus thoracicus, the thoracic duct, 



the common trunk of the lymphatics of the entire inferior and 

 the left superior half of the body, measures transversely in width 

 two lines, has a few valves, and frequently divides into two or 

 several trunks, which again unite. 



Origin : before the first to the second lumbar vertebra, to the 

 right of the Aorta, to the left of the right internal crus of the 

 diaphragm, close under the hiatus aorticus ; by the confluence of 

 three (to six) short thick ramuli, the central one of which com- 

 prises the lymphatics of the organs of digestion (chyliferous ves- 

 sels), whilst the lateral consist of the united lymphatics of the 

 inferior extremities and the pelvis. One or two of the united 

 ramuli is enlarged, one to two inches, before the commencement 



