384 MACROPHAGES FROM ARACHNOID CELLS. 



so as to be hardly demonstrable, it begins to increase in thickness and the nucleus 

 no longer stands out sharply as viewed in profile (fig. 2a and fig. 3). Other cells 

 containing small inclusions of cellular fragments (fig. 4a) are found. Their cyto- 

 plasm now forms a respectable accumulation around the nucleus and the whole 

 cell projects sharply from the trabecula. The nucleus is more distinctly circular 

 in outline and is seen to occupy an eccentric position in the cell (figs. 2c and 46). 

 Still other cells are literally gorged with fragments of erythrocytes, some of which 

 may be almost whole. In this condition (fig. 46) they are about ready to leave the 

 sessile position always occupied and become amoeboid. 



Histologically, these fixed cells do not differ from the free, round phagocytes 

 which appeared in the fluid tappings and were identified as macrophages. They 

 are physiologically still a portion of the membranous lining of the fluid cavity, 

 although their attachment becomes more and more restricted. After budding off 

 they tend to become still further distended with erythrocytes and pigment, often 

 reaching a diameter of 16 microns (figs. 6 and 7). Other cells (fig. 8) occur with 

 relatively little vacuolization of their protoplasm and few fragments of erythrocytes. 

 They are smaller (9 to 1 1 microns) and represent cells losing their attachment while 

 still in the stage represented in figure 3. Their protoplasm is finely granular and 

 dense, showing a paler zone around the eccentric nucleus. The free-moving macro- 

 phages gather in clumps (fig. 12) and are most numerous where the debris is greatest. 

 Unless the quantity of matter is very large they quickly store it in their bodies. 

 Occasionally a polymorphonuclear leucocyte, probably representing a dead cell, 

 suffers the same fate as the red blood-cell (fig. 6). The cycle of development may 

 be followed more easily on the trabeculse, but the cells covering the membranous 

 portion of the arachnoid, as well as those normally identified with the outer surface 

 of the pia mater, undergo the same physiological reactions to the stimulus of the 

 blood (lower portion of fig. 11). Intracellular inclusions are seen clearly enough 

 when looking down upon a cell, but better evidences of the membrane's participa- 

 tion are obtained in cross-section. 



The cells of the arachnoid facing the dura mater show similar changes, but 

 in only one instance was a subdural extravasation of blood produced in the region 

 of the spinal cord without the hemorrhage involving the subarachnoid space. Over 

 the cerebral cortex it is rather common to find a hemorrhage separating the dura 

 and the arachnoid membrane. It is then possible to obtain fixed preparations of 

 macrophages in a confined space, with their pseudopodia thrust out for a consider- 

 able distance. Such a cell is illustrated in figure 10, showing the characteristic 

 vacuolated appearance of the protoplasm. 



This brings up the question of the specificity of certain cells to produce macro- 

 phages, and their response to a stimulus applied at a distance. No portion of the 

 arachnoid membrane shows any differences in its behavior towards particulate 

 matter; to produce a response, actual physical contact alone seems necessary. Thus, 

 where the collections of debris are thick almost every cell shows signs of swelling 

 up, while adjoining regions look relatively quiet. This is illustrated more clearly 

 by the different reactions of the cells situated on the two sides of the arachnoid 



