92 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



venous trunks of the right and left sides, involving the distal part 

 of the internal jugular, the common jugular and the innomi- 

 nate veins. 



Out of this general antecedent sinistro-dextral capillary reticu- 

 lum the definite retro- or suprathymic innominate anastomosis 

 develops subsequently in normal individuals, but in the earlier 

 stages this plexus extends both ventral and dorsal to the thymus 

 anlage. 



The venous network develops, in addition to the prevertebral 

 and dorsal somatic components, chiefly on the lateral aspect of the 

 oesophagus, in the peri-tracheal area, and around the thymus and 

 the common carotid artery. It trends cephalo-laterad towards 

 the main venous lino (jugulo-innominate), into whose mesal sur- 

 face the terminals of the plexus discharge. 



In so doing they encounter, and partially surround, the sym- 

 pathetic and vagus nerves, and the thymus body. Consequently 

 the terminal branches of this ventral mediastinal plexus enter 

 the main vein both ventral and dorsal to these three structures 

 and through the intervals between them. Thus the composite 

 plan of this venous area appears as shown in the schematic fig. 

 101, which represents the combination of a number of transverse 

 sections of the anterior thoracic region. 



The terminal connections of the ventral mediastinal plexus 

 may be grouped in reference to their derivation and their relation 

 to adjacent fields of venous drainage into the following five sets, 

 (fig. 101): 



1. Dorso-medial somatic tributaries of internal and common 

 jugular and of innominate veins (fig. 101, 16} entering the main 

 venous channel dorsal to the sympathetic nerve, and frequently 

 combined with 



2. Terminal of prevertebral venous plexus anterior to azygos 

 veins (fig. 101, 17). 



3. Terminal of peritracheal plexus passing laterad between 

 sympathetic (1) and vagus (22) nerves (fig. 101, 18). 



4. Ventral mediastinal branch, draining, as " suprathymic 

 plexus," the area between the thymus (21), carotid (23) and 

 vagus (22). This is the line of the future typical permanent left 

 innominate anastomosis dorsal to the thymus (fig. 101, 19). 



