Lymph then passes into much larger lymph vessels known as lymph ducts. Hilar fat and lymphatic tissue with lymph nodes surround these structures. Tumors on the superolateral bladder wall spread to external iliac lymph nodes. These selective nodes act as sentinel lymph nodes. Furthermore, the tonsils filter tissue fluid whereas lymph nodes filter lymph. A chain of superficial inguinal lymph nodes drain to the deep nodes. Diagnosis is by examination of a bone marrow or lymph node biopsy. The intervening lymph nodes can trap the cancer cells. The inguinal depots enclose the inguinal group of lymph nodes. This may include some of the surrounding organs or tissues or lymph nodes. The neck may swell in part due to enlarged lymph nodes. The majority of these vessels drain into the external iliac lymph nodes. It can also spread to the liver, bones, abdomen or distant lymph nodes. The disease usually begins in the lungs, skin, or lymph nodes. These circulate through the bloodstream and enter and reside in lymph nodes.