Lymphatic System
Avian lymph vessels are less numerous and less widely distributed than the analogous structures in mammals. They run satellite to blood vessels and most ultimately converge on paired thoracic ducts that, in turn, empty into the paired cranial venae cavae.
Only Anseriformes possess true encapsulated lymph nodes; in these species, the cervicothoracic lymph nodes are found within the thoracic inlet and the lumbar lymph nodes ventral to the vertebral column in the region of the kidneys. Lymph nodules are found abundantly associated with most organs and in the walls of the gastrointestinal tract. These are especially prominent in the ceca of chickens, where they may be referred to as cecal tonsils.The cloacal bursa has already been mentioned as a diverticulum of the proctodeum. It is well developed in young birds and disappears following sexual maturation. During its short existence, it is an important lymphatic organ, being the maturation site for B-lymphocytes.
The avian spleen lies in contact with the right lobe of the liver adjacent to the gizzard. In addition to its important role in erythrocyte physiology, the spleen produces lymphocytes that are clustered in sheaths around splenic arterioles (the so-called white pulp of the spleen).