116 CARNEGIE INSTITUTION OF WASHINGTON. 



An important study has been made by Dr. C. R. Essick on the 

 structure and behavior of the arachnoidal tissue surrounding the cen- 

 tral nervous system. He has shown that when particles of foreign 

 matter are injected into the subarachnoid cavity of a living animal 

 a complete transformation occurs on the part of the cells lining the 

 space. They undergo hypertrophy, and, changing from fixed elements 

 to free-living cells in the form of macrophages, undertake the work 

 of removing the debris. In other words, this is an instance where cells 

 with a specialized function revert to the more primitive role of uni- 

 cellular organisms and engage in phagocytosis and amoeboid wander- 

 ing. This reaction on the part of the arachnoid cells may be brought 

 about by the injection of inert particles, such as carbon or cinnabar, 

 as well as by active matter, such as fragmented red blood-corpuscles 

 or dead leucocytes. The most striking results were obtained by the 

 injection of blood that had been partially laked with distilled water. 

 Using cats for the experiments, a hollow needle was introduced through 

 the occipito-atlantoid ligament and another into the lumbar sub- 

 arachnoid space. In this way the cerebro-spinal fluid surrounding the 

 spinal cord was displaced by laked blood. Within 6 hours this induced 

 a sterile meningitis characterized by a marked increase in the leucocyte 

 content and evidence of active phagocytosis of the fragments of the 

 red blood-cells. At the end of 48 hours the increase of leucocytes in the 

 cerebro-spinal fluid had subsided and microscopic examination of the 

 meninges revealed little evidence of the injection of laked blood. The 

 behavior of the cells lining the arachnoid space following the injection 

 of laked red blood-cells is best studied by dissecting out tha arachnoid 

 and studying it in aqueous solutions, with or without stain. In such 

 preparations the protoplasm of the arachnoid cells, which is normally 

 thin and hardly demonstrable, increases in amount, so that the cell 

 projects sharply from the trabecula. The nucleus becomes circular in 

 outline and assumes an eccentric position. Some cells show small inclu- 

 sions of fragmented erythrocytes, while others are gorged with them. 



As the cells further enlarge, their attachment to the trabeculse 

 becomes more and more restricted, until they bud off as free amoeboid 

 macrophages, in which state they tend to become still further dis- 

 tended with erythrocytes and pigment. Histologically these cells, 

 even before their detachment is completed, are identical with the large 

 mononuclear elements which are found in the cerebro-spinal fluid in 

 large numbers 48 hours after the onset of the reaction. Under normal 

 conditions there occur a few of these large mononuclear cells in the 

 cerebro-spinal fl.uid, and it is probable that they result in a similar way 

 from the stimulus provided by the small quantity of tissue debris 

 which normally finds its way into the subarachnoid space. While this 

 reaction of the arachnoid cells is best followed on the trabeculse, the 



