In this paper the characteristics of vascular system of the rabbit popliteal lymph nodes, especially the function of post-capillary venules was investigated. The presence of direct passage from lymph flow to blood flow within lymph nodes was proved on various substances of macromolecules. The results are as follows:
1. The most characteristic feature of the microcirculatory architecture of the rabbit popliteal lymph nodes is the presence of the post-capillary venule which was named after Schulze.
2. The post-capillary venules form complicated venous plexuses in the cortical lymphatic matrix.
3. The post-capillary venules does not appear in the embryonic stage and is able to be observed on the second day after birth.
4. The secondary nodules are formed in the area of the peripheral lymphatic matrix where capillary networks face the lymphatic sinuses.
5. The capillary networks in secondary nodules and the venous plexuses of post-capillary venules in the lymphatic matrix form a functional unit of the vascular architecture of lymph nodes, and these two subunits, i.e. the secondary nodules and the venous plexuses react antagonistically to various stimmulations given to lymph nodes.
6. Pontamin sky blue in Ringer's solution, human whole serum, human gamma globulin introduced through the catheter into the afferent lymphatics of the rabbit popliteal lymph nodes were detected immunoelectrophoretically in the blood plasma obtained from vein draining from the lymph node. Molecular weight of substances introduced is the important factor for determining the rate of the passage.
7. The Fesin particles introduced lymphogenously into lymph nodes were found in the lumen or the wall of the post-capillary venules.
8. From these facts, it is concluded that various substances would be able to pass from lymphatics into veins in lymph nodes in physiologic conditions and that the site of passage from lymph flow into blood flow i.e. the site of absorption of the fluid is the post-capillary venule.
9. The movement of reticular frame-work has a great influence upon the absorption of the fluid in lymph nodes.
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