Lymphatics in Small Intestine of the Pig. 91 



terior the sac itself persists. In tracing the series caudalward, the 

 central mass of lymph nodes corresponding with the cistema chyli, 

 can be traced to the pelvis, where the mass turns a little to the side 

 and joins the jiosterior lymph sacs. The posterior lymph sacs are 

 really enormous in size, measuring 2.8 x 2 x 3.5 mm. (dorso-ventral). 

 These measurements include the glandular masses in the edge of the 

 sac. 



The sacs are well shown in Fig. 19, which illustrates that the 

 posterior sacs are being transformed into lymph nodes. In some 

 sections of the pelvis these masses of lymphatic tissue seem to 

 take up almost two thirds of the area of the cross section. From 

 the posterior sac two sets of vessels extend, one along the sciatic 

 vein and the other along the femoral. There is one lymph node along 

 the sciatic vessels and a chain of nodes along the femoral. In Fig. 

 19 is a tiny lymph node, labeled Lg., which illustrates well the 

 simplest form of a lymph node, a central mass of lymphocytes 

 with a plexus of lymph ducts around. This plexus of ducts is so 

 close that it may already be termed a sinus, so the node consists 

 of a single follicle with its peripheral sinus. It is the structural 

 unit of the lymph node. 



From the description of this specimen it will be seen that the 

 foundations of the lymphatic system as it is found in the adult have 

 been laid down in an embryo of 80 mm. 



The primitive system is complete, and the sacs are forming the 

 primary nodes. The peripheral vessels have extended to the skin 

 and to the viscera, and secondary nodes are forming along these 

 vessels. I think that we have the key for working out the peripheral 

 spread of the lymphatics and carrying them to their capillary bed. 

 Injections of the retroperitoneal sac give us the material for tracing 

 this development. 



