<P> The medulla contains large blood vessels, sinuses and medullary cords that contain antibody - secreting plasma cells . </P> <P> The medullary cords are cords of lymphatic tissue, and include plasma cells, macrophages, and B cells . The medullary sinuses (or sinusoids) are vessel - like spaces separating the medullary cords . Lymph flows into the medullary sinuses from cortical sinuses, and into the efferent lymphatic vessel . There is usually only one efferent vessel though sometimes there may be two . Medullary sinuses contain histiocytes (immobile macrophages) and reticular cells . </P> <P> Lymph nodes are present throughout the body, are more concentrated near and within the trunk, and are divided in the study of anatomy into groups . Some lymph nodes can be felt when enlarged (and occasionally when not), such as the axillary lymph nodes under the arm, the cervical lymph nodes of the head and neck and the inguinal lymph nodes near the groin crease . Some lymph nodes can be seen, such as the tonsils . Most lymph nodes however lie within the trunk adjacent to other major structures in the body - such as the paraaortic lymph nodes and the tracheobronchial lymph nodes . </P> <P> There are no lymph nodes in the central nervous system, which is separated from the body by the blood - brain barrier . </P>

Where are lymph nodes concentrated in the body