Development of the Jugular Lymph Sacs. 301 
approximately coincides with, or follows very shortly upon, the 
establishment of the subclavian vein as the chief channel of venous 
return from the anterior limb and the resulting abolishment of the 
primitive ulnar vein. 
The further fate of the primitive ulnar lymphatic is involved 
in the establishment of the definite systemic lymphatic channels of 
the anterior extremity, and is hence not considered at this time. 
5. The relations of the thyro-cervical artery to the subclavian 
approach and to the common trunk formed by the external jugu- 
lar and cephalic veins, is well illustrated by the series of photomic- 
rographs taken through this region, and should be compared with 
the reconstruction (fig. 62). Beginning caudad, figs. 7, 6, and 5 
represent the thyro-cervical artery situated dorsal to the com- 
mon trunk of the external jugular and cephalic veins and medial 
to the subclavian approach. Fig. 4, on the other hand, represents 
a transverse section taken through the thyro-cervical artery as it 
bends laterad, ventral to the subclavian approach and dorsal to 
the jugulo-cephalic trunk. This last section lies shghtly caudal 
to the point where the thyro-cervical artery divides into three 
branches; one ventrally directed branch; one which follows the 
cephalic vein, and another which runs dorsally along the lateral 
surface of the jugular sac and is accompanied by a branch of the 
cephalic vein (fig. 62). The latter branch as well as the vein, as 
mentioned above, are at times embedded in the lymph sac (see fig. 
64, series 88). 
6. The first, second, third and fourth (fig. 62, SP.N. DIG EE 
and JV) spinal nerves no longer pass through a foramen in the 
lymph sac as in fig. 57, but now pass between the sac and the in- 
ternal jugular (precardinal) vein. The fifth (SP.N .V) spinal 
nerve still penetrates the jugular sac while the sixth (SP.N. VI) 
lies upon the lateral surface of the subclavian approach, and, as 
stated above, is now in no way related to the primitive ulnar 
lymphatic. . 
The present relations of the first four spinal nerves to the jugu- 
lar sac are explained by comparing fig. 17 with the preceding 
fig. 16. In course of further development, as illustrated in the 
present instance, the large lymphatic foramen through which 
