Florence E. Sabin 365 



ular vein, and thirdly in the depth along the internal jugular vein. 

 In the surface the capillaries have grown from the apex of the l3-mph 

 heart to the shoulder and back of the head and from the long plexus to 

 the face, neck, thorax, and fore leg. The capillaries of all these sets of 

 ducts anastomose freel}^ in the skin and there are no valves to check the 

 spreading of an injection mass. The lymphatics of the axilla belong to 

 the deep set which grow along the arteries rather than the veins. 



The spreading of the lymphatics for the lower part of the body can 

 be constructed from Fig. 5, Vol. Ill, p. 188. The position of the pos- 

 terior lymph heart is just caudal to the kidney, and at this point a 

 h'mpli node develops. The superficial lymphatics for the lower part 

 of the body grow in two directions, one set following the vein to a point 

 over the crest of the ileum, where a node is formed which drains the 

 skin of the back and hip; the other set coming to the surface in the 

 inguinal region where a long node is formed which drains the abdominal 

 wall and the hind legs. These three nodes with the abundant chain of 

 nodes along the aorta represent the distribution of the glands which 

 drain the skin of the lower part of the body. In following the histo- 

 genesis of the lymph nodes all of these different nodes have been studied, 

 but most of the figures presented are from different stages of one node, 

 namely, the first one to develop in the body. 



Histogenesis of the primary lymph node. — We turn now to the histo- 

 genesis of the lymph node, which will involve determining the structural 

 unit, and tracing the two elements, the vascular element with the adenoid 

 tissue, and the lymphatic element or sinus. 



The first lymph node in the body develops from the apex of the lymph 

 heart and will be referred to as the primary lymph node. This node 

 will be traced in its development until its condition, is practically adult. 



The first evidence of the formation of lymphatic nodes occurs when 

 the embryo is 3 cm. long. At this stage the lymph heart, which has 

 been a smooth walled sac, as shown in Figs. 3 and 5, lined with a single 

 layer of endothelial cells, begins to show a slight modification at the apex 

 in that bands of connective tissue begin to push into the lumen Avithout 

 destroying the lining. The apex of the lymph sac is pictured in Fig. 8 

 from a pig 3.6 cm. long. The section is taken from the same series as 

 Fig. 5, by which the transverse plane of the section can be noted. The 

 figure shows the character of the surrounding tissue consisting of a syn- 

 cytium of protoplasm with nuclei in the nodes. The wall of the sac 

 consists of a single layer of endothelial cells, and in the left hand side 

 there is no perceptible modification of the connective tissue. On the right 



