THE CENTRAL NERVOUS SYSTEM. 251 



form the great bulk of the white substance, where they are inex- 

 tricably interwoven with the other fibres there present. 



Besides the centripetal neurites of the association and commissural 

 neurons, their collaterals and those of the projection-fibres, the gray 

 matter of the cortex receives terminal neurites from larger fibres 

 that are probably derived from the cerebellum and cord (Fig. 230, 

 E). These give off numerous collaterals and teleneurites, which are 

 distributed to the small pyramidal cells of the second layer, and 

 probably also penetrate into the molecular layer, where they end in 

 numerous teleneurites among the cells of that layer. 



In the diagrammatic figure 230 the probable course of nervous 

 stimuli to and from the cerebral cortex is indicated. The possi- 

 bilities of transmission within a structure of such marvellous com- 

 plexity are incalculable. 



The above structural details of the central nervous system are 

 chiefly taken from the publications of Ramon y Cajal. They are 

 the result of researches carried on by the application of the methods 

 devised by Golgi to the nervous structures of the lower vertebrates 

 and embryos. Such details cannot be observed when specimens 

 have been hardened and stained by methods used for the study of 

 other structures. In such specimens the nuclei of the nerve-cells 

 and those of the neuroglia are stained and become prominent. But 

 the multitude of nervous filaments lying between the cells and the 

 processes of the neuroglia-cells are not differentiated, but appear 

 as an indefinite, finely granular material, in which the cell-bodies 

 apparently lie. Where the cells are sparse or small, as in the first 

 layer of the cerebral gray matter, the tissue appears finely molecu- 

 lar. Where the cells are numerous but small, their stained nuclei 

 give the tissue a granular appearance, as, for example, in the second 

 layer of the cerebellar cortex. 



The brain and spinal cord are invested by a membrane of areolar 

 tissue, called the "pia mater." Extensions of this areolar tissue 

 penetrate the substance of the cord and brain, giving support to 

 bloodvessels and their accompanying lymphatics. This areolar 

 tissue also extends into the ventricles of the brain, where it receives 

 an external covering of epithelium continuous with that lining the 

 ventricles, which is ciliated. Externally, the areolar tissue is con- 

 densed to form a thin superficial layer. 



