204 HISTOLOGY 



trophih'c. Some are coarsely granular and eosinophilic; others contain 

 the basophilic mast cell granules, but these are not well preserved in ordi- 

 nary specimens. In certain diseases myelocytes enter the circulating 

 blood, and they appear in smears as shown in Fig. 187, p. 198. 



Erythroblasts are generally found in clusters, some being young with 

 vesicular nuclei, others being normoblasts with dense irregular nuclei, such 

 as have already been described. Rarely a nucleus may be found which 

 apparently is partly extruded. Cup-shaped corpuscles are seen in the 

 tissue meshes. 



Lymphocytes are not a conspicuous element of the marrow, yet they 

 are present and sometimes in disease become abundant. 



The relations of the blood vessels to the reticular tissue are of great 

 interest. It has been thought that the endothelium blends with the retic- 

 ulum so that no sharp distinction can be made between the two. It 

 seems more probable that the endothelium is merely more permeable 

 than usual, by a freer separation of its cells. The same problem is pre- 

 sented by the blood vessels and reticular tissue of the lymph glands and 

 spleen. 



The functions of the marrow are the production and dissolution of 

 bone, the storing of fat, the formation of granular leucocytes (neutrophiles, 

 eosinophiles, and mast cells), of red corpuscles, and to a less extent of 

 lymphocytes; to these some would add the destruction of red corpuscles, 

 as indicated by ingested fragments and intercellular granules. 



LYMPH NODULES AND LYMPH GLANDS. 



The lymph glands arise as nodules of dense tissue in close relation 

 with an artery, a vein and a lymphatic vessel, as seen in the photographs, 

 Figs. 192 and 193. The first distinct lymph glands in the body are a pair 

 in the axillary region, a pair in the iliac region, and a pair or two in the 

 maxillary region. They are found in rabbit embryos of about 30 mm., 

 and in human embryos of about 40 mm. These first glands are soon fol- 

 lowed by others in their vicinity, producing axillary, inguinal and cervical 

 groups, respectively; and scattered glands more peripherally situated 

 along the vessels develop later. At the same time, the tissue around the 

 jugular and mesenteric lymph sacs becomes transformed into dense lym- 

 phoid tissue, which is resolved into the chains of deep lymphatic glands. 

 These acquire a structure similar to that of the superficial glands. There 

 is no satisfactory evidence that the dense lymphoid tissue of which the 

 glands are composed is produced by the emigration of cells from either the 

 arteries, veins or lymphatics associated with them. 



In further development the lymph glands become organized as shown 

 in the diagrams, Figs. 194 and 195. The left half of each diagram repre- 



