Karel Frederik Wenckebach "second degree AV block" and later named the "Wenckebach phenomenon" and reclassified as Mobitz type I block in Mobitz's 1924 paper. A similar phenomenon can also occur in the Sinoatrial node where it gives rise to Type I second degree SA block, and this is also known as a Wenckebach block; the two have distinct features on an ECG however. Wenckebach is credited for describing the median bundle of the heart's conductive system that connects the sinoatrial node to the atrioventricular node. This bundle was named Wenckebach's bundle, and is also known as the middle internodal tract. Wenckebach's bundle is
Lymph node stromal cell the "T cell zone". B lymphocytes (cells which help to fight certain bacteria) are located in the "B cell follicle". The primary B cell follicle matures in germinal centers. In the medulla are hematopoietic cells (which contribute to the formation of the blood) and stromal cells. Near the medulla is the hilum of lymph node. This is the place where blood vessels enter and leave the lymph node and lymphatic vessels leave the lymph node. Lymph vessels entering the node do so along the perimeter (outer surface). The lymph nodes, the spleen and Peyer's patches, are known as secondary lymphoid