380 MACROPHAGES FROM ARACHNOID CELLS. 



pull in their protoplasmic processes and become globular and phagocytic (Alzheimer, 

 1910). The endothelium of blood vessels has been recognized for a long time as 

 furnishing large phagocytic cells in areas of inflammation. Evans (1915), by offering 

 trypan-blue to the endothelium of the liver, lymph glands, and spleen, has observed 

 the formation of a new circulating mononuclear blood element (macrophage) which 

 buds off the lining of the vessel lumen, but only after prolonged irritation. 



To Metschnikoff (1892) we owe the physiological term macrophage. This term 

 was adopted because it did not imply a fixed ancestry. An excellent presentation 

 of the biological activities of the phagocytic cells in inflammation of the brain is 

 given by Mertzbacher (1909). His term Abraumzell is much more suggestive in 

 that it calls to mind the attempts at repair which go hand in hand with efforts to 

 nullify the destruction of tissue continuity. This study is concerned with the 

 efforts of the arachnoid membrane towards such removal of foreign material. To 

 include the polyblast of Maximow, the pyrrol-zell of Goldmann, the clasmatocyte 

 of Ranvier, the endothelioid cells, and Kornschenzellen in the class of macrophages 

 is to simply express a histological similarity combined with a common physiological 

 behavior. The common histological properties are the single nucleus eccentrically 

 placed and a cytoplasm usually reticulated because of the intracellular inclusions. 

 The cells may contain fat-globules, inert bodies, blood elements (either red or 

 white), and albuminous granules, as well as vacuoles. Among their physiological 

 functions are independent freedom of movement and ingestion of porticulate 

 matter. In the very first weeks of embryonic life one comes upon such cells fully 

 developed, before any others, to their adult form, first in the placenta and later in 

 the body of the embryo (Essick, 1915). Certainly we are dealing with a universal 

 expression of the need, in the physiological economy of the organism, for cells whose 

 function is to remove the products of tissue destruction. A very good resume of 

 the literature up to 1909 is given by Schott, who discusses the relation of fixed tissue 

 and macrophages. 



For a better understanding of the results of experimental introduction of 

 minute particles into the subarachnoid space with the subsequent formation of 

 macrophages, it seems necessary to give a brief description of the anatomy of the 

 leptomeninges and the mode of preparing specimens for study. 



The pia mater and arachnoid may be aptly compared to a living sponge 

 accurately filling the irregularities between the brain and the dura, the exterior 

 surface of which is formed by a closed semi-permeable membrane, while the surface 

 approximating the nervous tissue is perforated by the entrance of the perivascular 

 spaces. Vessels, nerves, and ligaments pass through this layer of tissue without 

 lying free in the cavity. Just as the spaces in a sponge are in free intercommu- 

 nication, so the cerebro-spinal fluid is normally continuous everywhere. The 

 connective-tissue framework is largely made up of white fibrous tissue, covered by 

 a continuous layer of flattened cells externally, where they form a comparatively 

 simple uninterrupted surface looking toward the dura. The underlying space, 

 known as the subarachnoid cavity, is broken up by anastomosing strands, but the 

 lining presents a continuous unbroken surface of cells except where the perivascular 



