204 FREDERICK TILNEY 



maximum transverse diameter of the right sac is .54 mm., that 

 of the left sac is .48 mm. It will be convenient to describe the 

 structure as having a body with caudal and cephalic processes. 



The body of the lymph sac {jig. 7, 25). This portion of the 

 sac appears as a prominent feature of cross sections in the neck 

 region. The walls of the .sacs are thinner than those of the veins. 

 The mesial and ventral walls are smooth and regular, the lateral 

 wall presents many irregular projections. The broad ventral 

 portion of the sac lies directly back of the jugular vein (fig. 7, 35). 

 Mesially it is in relation with the carotid artery, the vagus and 

 sympathetic nerves, but separated from them by an extensive 

 plexus of venous channels draining into the jugular system. 

 In its subsequent development, this plexus allies itself with the 

 lymphatic system. 



Processes of the sac. The contour of the sac becomes irregu- 

 lar in several regions by prolongations from its walls. These 

 prolongations serve as the processes by which the sac acquires 

 its ultimate connections with the venous system and the systemic 

 lymphatics. From the cephalic pole a large number of processes 

 reach upward into the head region and end blindly. Some of 

 these prolongations serve to connect with the systemic lymphatic 

 trunks from above. The caudal processes are larger and show 

 a more definite arrangement. Their number and disposition 

 differ on the two sides. This difference depends upon the rela- 

 tions which each sac bears to the adjacent veins. The left sac 

 is still freely connected with the jugular vein; the right sac is 

 entirely cut off all venous connection. The left sac is typical 

 of the conditions in the late veno-lymphatic period. The right 

 sac affords a good example of the pre-lymphatic stage. Due 

 to its free communication with the left jugular vein, the left 

 sac is filled with blood. Caudally, it is divided into two main 

 processes, one of which turns mesad and ventrad in the direction 

 of the jugular vein, the ventro-mesial process; the other continues 

 caudad in line with the general axis of the sac, the dorso-lateral 

 process. The ventro-mesial process enters directly into the vein, 

 On the right side the sac has already been emptied of blood and 

 has lost connection with the jugular vein. The dorso-lateral 

 process (fig. 7, 26) is present on each side. It does not connect 



