CHAPTER XXXV. 489 



By means of CajaPs method two categories of neuroglia elements 

 become stained a dark purple on a much lighter purplish back- 

 ground. The first category consists of neuroglia cells provided with 

 a changing number of variously ramified protoplasmic processes, 

 which inter-cross with those of other cells, and thus give origin to 

 CajaFs pleurigenic plexus. These neuroglia cells prevail in the grey 

 layers of the human cerebral cortex, and form the bulk of the 

 protoplasmic neuroglia ( 909). In Cajal's preparations they appear 

 beset with vacuoles, situated both within their cytoplasm and along 

 their processes. The vacuoles or spaces are occupied by granules 

 (gliosomes), which may be stained either by Cajal's uranium nitrate 

 method ( 847) (superficial sections) or by methods generally 

 used for the demonstration of mitochondrial formations as well as 

 by the methods of Eisath and Fieandt. The other category of 

 neuroglia elements shown by the gold chloride and sublimate 

 method consists of astrocytes, viz., of neuroglia cells, also provided 

 with a changing number of processes, but chiefly characterised by 

 the absence of gliosomes and the presence of fibres which, though a 

 product of differentiation of the protoplasmic portions of the 

 astrocytes, never become entirely independent of the latter. These 

 fibres appear to correspond to those stainable by the methods 

 described in 910 to 916. The astrocytes prevail in the white 

 matter of the central nervous system, and form the bulk of the 

 fibrous neuroglia ( 909). Neuroglia cells, in part protoplasmic and 

 in part fibrous, occur chiefly at the points of transition between the 

 grey and the white substances of central nervous organs. 



The gold chloride and sublimate method leaves unstained a 

 third category of elements, the existence of which was at first recog- 

 nised by Cajal by means of this negative character, but they were 

 subsequently studied by him in superficial sections of pieces stained 

 by his uranium nitrate method and other cytological methods. 

 The cells belonging to the category now considered appear in 

 uranium nitrate preparations as roundish elements, but, as a matter 

 of fact, they also are provided with a changing number of variously- 

 ramified protoplasmic processes (see 919). As Cajal was not able 

 to come to any definite conclusion in regard to their nature, he 

 proposed to term them the " third element," i.e., a category of cells 

 which, though non-nervous in character, do not plainly form part 

 either of the connective tissue (blood-vessels, pial septa) or of the 

 neuroglia, this term being, in Cajal's opinion, reserved for those 

 elements which are genetically derived from an evolution of the 

 ependymal epithelium. 



