MACROPHAGES FROM ARACHNOID CELLS. 381 



channels open into it. The spinal cord has been chosen as a place for studying the 

 arachnoid tissue, because of the ease in approaching it experimentally and pre- 

 paring specimens for microscopic study. 



In these observations cats of various ages were used, but the results were 

 uniform. After sacrificing the animals with ether the thorax was opened and 10 

 per cent formalin was injected immediately into the aorta. A few hours later the 

 bony covering of the central nervous system was removed en bloc and the dorsal 

 region of the brain and cord was exposed, care being taken not to rupture the dura. 

 These specimens, partially exposed, were immersed for 5 days in 10 per cent for- 

 malin to insure good fixation, after which all of the remaining bone was removed. 

 The dural covering of a section of the cord was carefully removed and the arachnoid 

 separated from the pia as an intact membrane. This was best accomplished by the 

 aid of a binocular microscope, as great care had to be exercised to avoid undue 

 tension on the delicate strands uniting the pia and arachnoid. Eye-scissors were 

 used to cut the trabeculse. The membranes thus obtained were best studied by 

 immersion in a weak aqueous solution of toluidin blue. Permanent preparations 

 were made by the technical methods usually applied to celloidin sections. The 

 conventional microscopic section of the pia arachnoid, in addition to its shrunken 

 and distorted picture, gives one a very limited field to study, as only a small frag- 

 ment of the rich ttabecular system appears in any one specimen. To this limitation 

 is added the fact that the strands are usually cut in such an oblique direction as to 

 render them almost unintelligible. When examined in a dissected specimen the 

 cells clothing the smaller trabeculae are almost completely isolated from their neigh- 

 bors and furnish a brilliant opportunity for studying them in profile or for noting 

 their cellular contents without confusion. This clearness of picture makes the study 

 of cell hypertrophy and proliferation more convincing because of its comparative 

 isolation, and it approaches more nearly the conditions seen in tissue cultures where 

 cells become amoeboid and separate themselves from their normal environment. 



The cells covering the membranous expansion of the arachnoid have large, 

 pale, oval nuclei with very indistinct chromatin network (fig. 11). With the 

 ordinary cytological stains the cell-boundaries can not be made out, yet their irreg- 

 ular arrangement may be demonstrated by silver precipitate. Distributed through- 

 out the brain and cord are clusters of closely placed nuclei within the arachnoid 

 membrane. These are well shown in the upper left corner of figure 11. Such areas 

 are irregular in shape, size, and distribution. Histologically they correspond to the 

 arachnoid cell clusters found by Weed (1914, p. 64) in the dura. They represent 

 normal structures and, like the arachnoid trabeculae, become the seat of calcium 

 deposits with the advancing age of the animal. In no sense should they be mis- 

 taken for a cellular proliferation in response to a degenerative process. One does 

 not choose by preference the membranous expansion of the arachnoid in studying 

 the formation of macrophages. One meets here the same difficulties that are 

 encountered in the flat serous surfaces, such as the peritoneum. It is hard to elimi- 

 nate doubt concerning the exact relations of a single cell to the membrane spread 

 out as a flat preparation. Analogous processes can be made out, but not with the 



