Pith, or medulla, is a tissue in the stems of vascular plants. Pith is composed of soft, spongy parenchyma cells, which store and transport nutrients throughout the plant. In eudicots, pith is located in the center of the stem. In monocots, it extends also into flowering stems and roots. The pith is encircled by a ring of xylem; the xylem, in turn, is encircled by a ring of phloem.
Throughout the lymph node's interior are a series of street-like channels called sinuses that help guide the flow of lymph from the afferent lymphatic vessels, through the cortex and medulla, and finally to the efferent lymphatic vessels at the hilum.ach lymph node chain is responsible for draining a particular body region or structure. For example, the lymph node chains in your armpits drain your arms and upper chest, while the chains in your groin are responsible for filtering lymph from your feet, legs, genitals, and lower abdominal wall.
One main lymph node area (the armpit, or axilla) and two secondary lymph node areas (the internal mammary and supraclavicular regions) filter the lymph fluid draining away from the breast area. Since the job of the lymph nodes is to filter out bad guys like cancer cells, this is a logical place to look for breast cancer cells that have escaped the original tumor and are trying to go elsewhere in the body. Cancer cells may also leave the breast through the bloodstream and bypass the lymph nodes.