MACROPHAGES FROM ARACHNOID CELLS. 383 



mononuclear element is found in the cerebro-spinal fluid in increasing numbers. 

 It, too, is actively phagocytic and when studied on the warm stage presents the 

 well-known morphological characteristics of the amoeboid macro phage. The 

 nucleus is large, measuring 7 to 9 microns, and usually has a well-developed nucle- 

 olus. The cytoplasm shows a large number of inclusions and vacuoles which take 

 up the main body of the cell and flatten out the nucleus against the limiting mem- 

 brane. Such inclusions consist mainly of fragmented erythrocytes or pigment 

 granules of reduced hemoglobin. The number of these large cells may reach the 

 neighborhood of 50 per cubic millimeter of cerebro-spinal fluid, withdrawn at the 

 end of 48 hours. 



Macroscopic examination of the spinal cord at the end of 48 hours reveals little 

 evidence of the injection of laked blood. A diffuse, pale salmon-pink may be seen 

 through the dura, but unless one were looking for blood the cord might be easily 

 regarded as perfectly normal in color. After five injections of blood, made at 

 48-hour intervals, a brownish tinge was evident macroscopically, but here again it 

 was not pronounced, except in the immediate site of the injection. 



Our chief interest lies in the microscopical appearance of the cells lining the 

 arachnoid space when filled with laked red blood-corpuscles. The normal arachnoid 

 membrane has been beautifully pictured by Key and Retzius, whose technique of 

 preparing specimens for microscopic study was in most respects similar to that 

 employed in these experiments. All of their illustrations show a finely granular 

 protoplasm, becoming coarser around the poles of the oval nuclei. Their figure 1, 

 plate x, corresponds very nearly to the resting normal trabecula which is pictured 

 in figure 3. Their interpretation of this appearance is expressed in the following 

 quotation (p. 127) : 



"Um die Kerne, besonders aber an ihren beiden Polen liegt ein Haufen von Kornchen, 

 welche theils feiner, mehr protoplasmatisch sind, theils aber grossere gliinzendere Kugeln 

 ausmachen. Diese Kornchen kommen fast an jedem Kerne vor, sind aber zuweilen nur 

 sehr sparsam vorhanden, zuweilen aber auch sehr zahlreich, die Enden der Kerne fast 

 verdeckend. Diese Kornchenzone, welche bei jiingeren Individuen im Allgemeinen reich- 

 licher erscheint und als mehr oder weniger veranderter Ueberrest des ursprunglichen Pro- 

 toplasma zu betrachten ist, streckt sich in verschiedener Entfernung vom Kern auf die 

 Oberflache der Scheide, sich allmiihlig verdiinnend und verschmalernd, hinaus, bald 

 hat sie eine bestimmtere Begrenzung, bald erstreckt sie sich in verschiedener, zuweilen 

 phantastischer Form, als Seesternarme u. s. w. nach verschiedenen Richtungen, am 

 gewohnlichsten aber bipolar vom Kern hinaus." 



These granules could be stained with rosanilin. Other granulation, similar to 

 fat but not staining so deeply with osmic acid, were observed by these authors in the 

 normal arachnoidal cells. Such appearances are shown in their Taf. x, fig. 3, and 

 Taf. xi, fig. 1. 



Specimens dissected from the arachnoid can be best studied in aqueous solu- 

 tions. A faint stain with toluidin blue will help to differentiate the cell-structures, 

 but the natural differences of refraction and normal color of the fragmented erythro- 

 cytes give the most impressive preparations. The first effect of a change in cerebro- 

 spinal fluid is reflected in the protoplasm of the cell. Normally very thin (fig. 1) 



