EXTRACT XXVI. 



ON THE NEUROGLIA, AND HOW, AND WHEN, THE 

 ELEMENTS OF THE INGESTA BECOME ALIVE. 



IN dealing, here, with the more solid parts of the nervous 

 system, let us begin with the substance known as the 

 neuroglia, the most largely developed structure entering 

 into the composition of the brain, spinal cord, and 

 ganglia. 



The neuroglia may be looked upon, as the scene of the 

 great problem of what may be described as a secondary 

 digestion , and assimilation , i.e. the cerebro-spinal, or central, 

 as well as the peripheral, nervous system, may be 

 regarded, so to speak, as a product of, or, at any rate, as 

 having been developed within, and from, that structure 

 in other words, the blood arterio-capillary circulation 

 terminates, so far as its nutritive function is concerned at 

 least, within that structure, and leaves the brain portion of 

 it, by the vessels known as the sinuses (Fig. 117), i.e. it 

 conveys hither the materials, from which the nerve cell 

 structures, or neurons, are developed (Figs. 117, 1 18), and 

 by which they are nutritionally, as well as mechanically, 

 sustained, and, therefore, the neuroglia, thus, becomes the 

 .storehouse, to which the raw materials are conveyed, and 

 from which the nerve structures select what they require 

 for their, development, maintenance, and repair. Hence 

 it may be regarded as the great terminus, to which the 

 circulation proper, or a section of it, is constantly engaged 

 conveying nutritive material, whence it can be taken up, 

 and distributed, to meet the requirements of the great 

 systemic nervous system and this latter, the systemic 



