ANATOMY OF A 17.8 MM. HUMAN EMBRYO 91 



immediately lateral to the internal jugular vein. Through the 

 sacs pass branches of the cervical plexus. 



Saccus jugularis sinister. The cephalic end of the left sac 

 communicates with the internal jugular vein through a small 

 channel (a). There is also a large caudal connection between 

 the sac and vein which cannot be seen in the reconstruction. 

 The latter opening is in the caudo-medial wall of the sac near 

 the temporary opening of the external jugular vein (V.jug.ex.) 

 into the internal jugular. Incomplete valves, dorsally and ven- 

 trally placed, guard the opening. 



The branches of the cervical plexus which pass through this sac 

 are seen in plate 5. Through the cephalic extremity extends the 

 great auricular (N.aur.m.). A little more caudally there issues a 

 branch of the third cervical which is joined lateral to the sac by 

 a branch of the fourth cervical nerve. The trunk thus formed, 

 immediately gives off the small occipital, and then bends ventrally 

 across the sac as the N. cutaneous colli. 



The vasa lymphatica superficialia (Vas.lym.sup.), arising from 

 the cephalo-lateral portion of the left lymph sac, are seen plainly 

 in plate 6. They extend laterally and dorsally into the subcu- 

 taneous tissue. A prolongation from the caudal extremity of 

 the left sac overlies the lateral aspect of the terminal part of 

 the subclavian vein. 



Saccus jugularis dexter. The cephalic end of the right lymph 

 sac does not communicate with the internal jugular vein as 

 does the left. Caudally at a level corresponding approximately 

 with the large communication on the left, the right sac opens 

 into the internal jugular. The opening in this case is a very 

 small slit-like aperture between two valves, a lateral and a medial. 

 The lateral valve is adjacent to the permanent temiination of the 

 external jugular vein, while the medial projects into the cavity 

 of the internal jugular. 



A prolongation of the sac overlies the lateral surface of the 

 proximal portion of the subclavian vein as occurred on the left. 



Saccus mesentericus. There is a plexus of vessels, situated 

 immediately ventral to the aorta, which extends for the most part 

 between the proximal parts of the superior and inferior mesen- 



