THE CIRCULATORY SYSTEM. 125 



of lymphatics are blackened as the result of an accumulation of 

 particles of carbon that have been inhaled and then absorbed into 

 the lymphatics. 



The lymph-nodes may, therefore, be considered as filters which 

 remove suspended foreign particles from the lymph ; but it is 

 probable that the dissolved substances in the lymph are also 

 affected in its passage through the nodes, and that a purification 

 of that fluid is thereby occasioned. A fresh access of leucocytes 

 further alters the character of the lymph during its transit through 

 the lymph-nodes. 



Bone-marrow (Fig. 109). — In early life the medullary cavities of 

 the long bones, as well as the cancellae of the spongy bones, are all 

 occupied by that form of marrow known as " red " bone-marrow. 

 This is functionally the most important variety. In after-life the 

 marrow in the medullary cavities of the long bones becomes fatty 

 through infiltration of its cells with fat, which converts them 

 into cells quite similar to those of adipose tissue. Marrow so modi- 

 fied is called "yellow" marrow. It may subsequently undergo a 

 species of atrophy, during which the fat is absorbed from the cells 

 and the marrow becomes serous, fluid taking the place of the mate- 

 rials that have been removed. This process results in the produc- 

 tion of a " mucoid " marrow. 



The marrow of bones possesses a supporting network of reticular 

 tissue not unlike that of the lymph-nodes. In the meshes of this 

 tissue are five different varieties of cell (Fig. 110) : First, myelo- 

 cytes, cells resembling the leucocytes of the blood, but somewhat 

 larger in size and possessing distinctly vesicular nuclei. They are 

 capable of amoeboid movements, and not infrequently contain gran- 

 ules of pigment which they have taken into their cytoplasm. 

 Second, erythroblasts, or nucleated red blood-corpuscles, which 

 divide by karyokinesis and eventually lose their nuclei, becoming 

 converted into the red corpuscles of the circulating blood. Third, 

 acidophilic cells, containing relatively coarse granules having an 

 affinity for " acid " anilin-dyes, such as eosin. These cells are 

 larger than the majority of the leucocytes circulating in the blood. 

 Their nuclei are spherical or polymorphic and vesicular. Fourth, 

 giant-cells with unusually large bodies and generally several nuclei, 

 though occasionally only one nucleus is present. They possess the 

 power of executing amoeboid movements and appear to act as phago- 

 cytes. Where absorption of bone is taking place they are found 



