208 



HISTOLOGY 



Lymph 

 sinuses. 



of the preponderance of nuclear material, is very dark, and its appearance 

 even under low magnification is quite characteristic; it is shown in the 

 medullary cords in Fig. 197, which illustrates also its relation to the 

 lymph sinuses. 



The lymph sinuses are not well-defined endothelial tubes, but appear 

 rather as washed-out portions of the reticular tissue. If the endothelial 

 tubes which line the lymphatic vessels enter the lymph gland to form the 

 sinuses, it must be considered that their cells separate and that strands 

 of reticular tissue pass across them. Some authorities consider that the 

 endothelial tissue blends freely with the reticular tissue, so that any 

 distinction is here arbitrary. The reticular tissue cells, or endothelial 

 cells, lining the sinuses are highly phagocytic, and ingested fragments 

 may be seen within them in sections. Certain of these cells become de- 

 tached, and there is reason to 

 believe that they are the source 

 of the large mononuclear leuco- 

 cytes. Lymphocytes from the 

 adjacent cords and nodules 

 also enter the lymph as it 

 passes through the sinuses, 

 and thus they are added to 

 the circulation. Within the 

 cords and nodules they are 

 enclosed in a closer meshed 

 reticulum than that of the 

 sinuses, which may prevent 

 them from escaping too freely. 

 The germinal centers con- 

 tain cells with larger and paler 

 nuclei than those of lympho- 

 cytes. These central cells 

 resemble premyelocytes, and 

 they are supposed to give rise to lymphocytes. Mitotic figures are abun- 

 dant. The germinal centers, however, are not found in certain nodules, 

 and they are absent from the medullary cords. This has been explained 

 as due to the slower and more scattered multiplication of cells in those 

 places, but the germinal centers are absent also from the early stages of 

 embryonic glands. Presumably they are not adequately explained by 

 stating that they are centers for lymphocyte production. 



The capsules of the lymph glands consist of fibrous connective tissue, 

 containing elastic elements which increase in abundance with age. 

 Smooth muscle fibers are present as scattered cells or as slender bundles. 

 The trabeculae, which are extensions of the capsule, are composed of the 



FIG. 197. FROM THE MEDULLA OF A LYMPH GLAND OF AN 

 Ox. X 240. 



