VEINS AND LYMPHATICS IN TRAGULUS 209 



doubtedly denote the connections of the sac with lymphatic 

 trunk lines of the head. The main interest with reference to 

 these processes centers about those which are derived from the 

 caudal extremity or what may be termed the cervix of the sac 

 Upon reaching the level marking the entrance of the cephalic 

 vein into the jugular, the cervix of the sac breaks up into four 

 processes, namely: 



1. The dorsal process which extends dorsad accompanying 

 the cephalic vein and receives the dorsal somatic tributary of 

 the sac, the subcutaneous duct. 



2. The mesial process, the now much reduced primary veno- 

 lymphatic connection which extends mesad dorsal to the jugular 

 vein but does not tap into it. 



3. The dorsal descending process which in the 20 mm. embryo 

 was designated the thoracic duct approach and which has now 

 acquired its full connection with the preazygos segment of the 

 duct, especially on the left side. This process descends along 

 the dorso-lateral surface of the thyreo-cervical artery. 



4. The ventral descending process which affords communi- 

 cation with the broncho-mediastinal channels. It appears in 

 a position ventral to the thyreo-cervical artery. As this process 

 enters the sac it forms an acute angle with the dorsal descending 

 process which lodges the cephalic vein as it is opening into the 

 jugulo-subclavian junction. In addition to the broncho-medias- 

 tinal approach, the ventral descending process has developed 

 still a third, the jugulo-subclavian approach. This is a slender 

 prolongation from the mesial side of the process which, upon 

 reaching the jugulo-subclavian angle, forms a direct communi- 

 cation with the venous system. This is the so-called secondary 

 venous tap of the lymphatic into the venous system and by its 

 establishment determines the transition from the prelymphatic 

 to the lymphatic stage. The secondary tap is made in a charac- 

 teristic manner. From the wall of the vein at its jugulo-sub- 

 clavian junction a process which has the appearance of a tubular 

 redundancy pushes its way cephalad between the lateral vein 

 wall and the mesial wall of the sac. After a short distance it 

 meets and opens into the jugulo-subclavian approach of the 

 lymph sac thus producing a channel between the latter and the 



