THE LYMPH NODES 235 



The lymphocytes are loosely packed in the center of the nodule, and 

 in this portion cell division by mitosis is most active. This central por- 

 tion is the germinal center of Flemming. The germinal center Ts sur- 

 rounded by a denser circumferential layer of lymphoid tissue in which 

 cell division is less active. Between this denser portion and the sur- 

 rounding connective tissue the lymphocytes are again more loosely 

 packed, and over a greater portion of the nodule are separated from the 

 trabecula? by a lacuna-like space, the peripheral lymph sinus. 



The nodule is usually supplied with a thin-walled artery, occasionally 

 two, which penetrates to the middle of the nodule to form a wide 

 meshed capillary plexus. The capillaries, at the periphery of the nodule, 

 unite to form two or more veins, which are contained in the adjacent 

 connective tissue. 



The lymph cells are mostly of the mononuclear type of leukocyte, the 

 small mononuclear or lymphocyte type being the most abundant. Poly- 

 morphonuclear and eosinophil leukocytes are also found in the lymph 

 nodules, though in much smaller numbers. Mitosis is most frequently 

 observed in the large mononuclear type. Because of the nomadic tend- 

 encies of the leukocytes the boundaries of the lobule are not always 

 sharp, the lymph cells frequently infiltrating the surrounding connective 

 tissue so as to render it most difficult to distinguish the latter from the 

 true lymphoid tissue of the nodule. 



THE LYMPH NODES 



(Lymph Glands) 



These structures occur in the course of the lymph circulation in vari- 

 ous parts of the body. They are found in the neighborhood of the 

 large joints, as in the axilla, the groin, the popliteal space, in the pre- 

 vertebral and mediastinal connective tissue of the abdominal and thoracic 

 cavities, and in the mesentery. They are frequently in relation with 

 the large arteries, e.g., the renal, internal and external carotids, etc. 



Each lymph node consists of a mass of nodular lymphoid tissue in- 

 closed within a fibro-elastic connective tissue capsule. The capsule also 

 contains a little smooth muscle tissue, but this is never so abundant as to 

 form any considerable portion of the fibrous membrane ; in fact, as com- 

 pared with the somewhat similar capsule of the spleen, that of the lymph 

 node is notably deficient in smooth muscle. 



