Development of the Jugular Lymph Sacs. 299 
of the internal jugular vein, between the sympathetic nerve 
and the thyro-cervical artery. It extends caudad as far as 
the base of the first of the (promontorial) dorsal somatic tributar- 
ies (4S in figs. 34, 45 and 60), where it ends blindly. The connec- 
tion of this process with the jugular sac lies dorsal to that of the 
jugular approach, the position of the latter, on the ventral surface 
of the vein, being represented by a plus sign (+) in fig. 60. 
The thyro-cervical artery, a branch of the subclavian, runs 
cephalad along the dorsal-lateral surface of the jugular vein and 
on reaching the lymph sac turns abruptly laterad, ventral to the 
subclavian approach, and then divides into three branches one of 
which runs dorsad along the lateral surface of the jugular sac. 
The relations of the thyro-cervical artery to the jugular sac are 
represented in fig. 60. The branch which follows the lateral sur- 
face of the lymph sac and its accompanying vein are not repre- 
sented, however. It will be observed that the relations of the 
thyro-cervical artery to the jugular lymph sac are exactly the 
same as in all of the preceding stages (figs. 49, 51, 52 and 57). 
The external jugular vein is a vein of large size in this embryo 
(fig. 61), having practically assumed its adult condition. It usually 
functions as the main drainage canal of the head and neck in 
the adult although cases are sometimes met with in which the 
external and internal (precardinal) jugulars are subequal in 
size. 
Series 15, 16” Embryo 
Reconstruction of left side, 
Dorso-lateral aspect, fig. 62 
The jugular lymph sac now appears relatively reduced and 
shortened in its cephalo-caudal dimension. It endsblindly in front 
in a bluntly pointed extremity, and receives the subcutaneous 
duct on its lateral surface. 
A number of features characterize this embryo, which show 
an advance in lymphatic development over that of the preceding 
stage (14 mm.). 
1. The jugular approach, not shown in the dorso-lateral view, 
communicates with the venous system at the angle of confluence 
of the internal jugular and the combined trunk of the external 
