390 THE PINEAL ORGAN 



lining the central canal of the spinal cord and brain stem in animals and 

 the human subject. The existence of nerve cells in the ependyma which 

 are connected by their peripheral processes with nerve cells in the adjacent 

 grey matter suggests that impulses originating in the ependyma may be 

 transmitted to ganglion cells in the grey matter of the spinal cord or 

 brain stem. Whether the impulses are transferred to the cortex of the 

 brain and give rise to a conscious sensation or not, it seems quite possible 

 and even probable that they may originate reflex actions. 



The function of the subcommissural organ, which is developed as a 

 thickening of the ependyma below the posterior commissure and inferior 

 peduncle of the pineal body (Fig. 185, B, p. 261), is not known, nor is that 

 of Reissner's fibre, which is developed in relation with the subcommissural 

 organ (Fig. 134, p. 188). 



Microglia 



Any investigation into the structure and pathology of an organ such 

 as the pineal body, which is derived as an outgrowth from the central 

 nervous system would be incomplete without a reference to the nature 

 and origin of microglia as it occurs in the cerebrospinal system generally. 

 The microglia or mesoglia is sometimes alluded to as the " third element " 

 in the composition of nervous tissue. This name was originally given 

 by Cajal (1913) to a group of small non-nervous elements which were 

 afterwards differentiated by the special staining methods of Del Rio- 

 Hortega into oligodendrocytes and microglia, the former neuroglial in 

 nature, the latter mesodermal. Hortega accordingly proposed to restrict 

 the application of the term " third element " to the microglia, and allocate 

 the oligodendrocytes to the neuroglial constituents or " second element." 

 Thus, of the three constituents of nerve tissue excluding the connective 

 tissue and vessels, the " first element " comprises the nerve cells ; the 

 " second element " the neuroglia, including both astrocytes and oligo- 

 dendrocytes ; and the " third element " is represented by the microglia. 

 The microglia cells are small branched elements with minute nuclei of 

 irregular form which stain deeply by Nissl's method ; the ordinary nuclear 

 dyes such as hematoxylin ; and the silver carbonate method of Del 

 Rio-Hortega which also brings out clearly the cell-body and processes. 

 The latter are irregular in form and size and are characterized by small 

 thorn-like spines. The cells have phagocytic properties and are under 

 certain conditions capable of amceboid movements. Thus, according 

 to the descriptions of Hortega, during the migratory phase in the develop- 

 ment of the microglia, throughout the nervous tissue, it consists of 

 roundish cells with pseudopodia, the various shapes of which indicate the 

 motility of the cells. " After this initial phase the cells become branched, 



