158 EDWARD MCCRADY, JR. 



The jugular lymph sacs. During stage 33 the many, small, 

 separate, lymphatic spaces formed in the paracardinal region 

 fuse together to form a continuous, though somewhat compli- 

 cated, jugular lymph sac; and this sac acquires a connection 

 with the venous system by means of a bicuspid valve located 

 on the ventral side of the junction of the internal jugular 

 (anterior cardinal) and external jugular (jugulocephalic) 

 veins. 



The anatomy of the jugular sac will be treated in detail in 

 Doctor Zimmermann's monograph, so only a few words are 

 necessary here. It is composed of two principal divisions: 

 1) a cephalic division (fig. 52), which is penetrated by the 

 levator scapulae ventralis muscle (also called the m. atlanto- 

 scapularis anticus) and branches from the third and fourth 

 cervical nerves; and 2) an axillary portion lying mediad of 

 the scapula, which is derived from the dorsolateral lymphatics 

 previously described as paralleling the dorsal ulnar vein. 

 Two small extensions or diverticuli from the cephalic portion 

 should be mentioned on account of their prospective signifi- 

 .cance. One of these, the medial jugular extension, runs 

 mediad from the slender, lower segment of the cephalic por- 

 tion. It represents part of the jugular arch which will later 

 connect the jugular sac with the thoracic duct. The other, the 

 intermediate caudal extension, runs laterad from the same 

 slender neck of the cephalic portion of the jugular sac, and will 

 later become the cervical approach of the lymphatic channel 

 which parallels the external mammary artery. 



The thoracic supracardinal (azygos} veins. The thoracic 

 intersegmental veins from the postcardinal at first (i.e., up 

 until stage 32) lie consistently lateral to the sympathetic 

 trunks. During stage 32 supplementary, minor channels pass 

 around the nerves on the medial side and there is thus a double 

 drainage into the postcardinals. In stage 33 longitudinal an- 

 astomoses form between the medial radices of the interseg- 

 mentals, the lateral radices drop out, and the postcardinal 

 trunk ahead of the mesonephros degenerates. The longi- 

 tudinal vessels thus left on the medial side of the sympa- 

 thetic trunks and draining into the common cardinal, are the 

 supracardinal or azygos veins. 



