The lymphatic organs are classified as primary (thymus and bursa of Fabricius) or secondary tissue (spleen; Harderian gland; pineal gland; bone marrow; scattered aggregates of lymphoid tissue). The anatomy and function of these organs and tissues arc described in Chapter 1, Clinical Anatomy and Physiology pp. 42–43. An important feature to note is the lack of lymph nodes in birds; rather, lymphoid tissue is diffusely disseminated in many tissues including the liver, spleen, kidney, and gastrointestinal tract.