SENSORY, MOTOR & PSYCHIC NEURONS 403 



blood change going on within the elements of that fluid 

 as it passes through the peculiar chemico-physical changes 

 characteristic of the pia mater metabolism, consisting 

 mainly of the separation from the blood stream of 

 cerebro-spinal lymph, to maintain the circulation of that 

 fluid throughout its whole extent, and the deposition of 

 glial substance within the neuroglial matrix, for the suste- 

 nance of the nerve units, or neurons, with the necessary, 

 and the consequent, intensification of the colouring of 

 the remainder, or residuum, of the blood substance, and 

 hence the conferring on that remainder of the peculiar 

 shade so markedly belonging to the " grey matter." That 

 colour is neither in the overlying, or inter-penetrating, 

 cerebro-spinal fluid, the proper neuronal structures, nor 

 the glial substance on which the neurons are supported ; 

 it must, therefore, be intimately connected with the 

 capillary blood vasculature, and its contents, as it passes 

 through it the residual blood material of the pial 

 circulation. 



This colour is only observed where nerve cell growth 

 is in progress, or where the neurons arise as cells from 

 the neuroglial matrix, or feltage, and pass out into the 

 white substance as axonal processes, to be connected with 

 other nerve cells, or to be distributed as nerve terminals 

 to skin or muscle, according as they are sensory or 

 motor. Those cells, therefore, which are not terminal, 

 sensory, or motor, nerve cells, must end individually by 

 passing into, or through, other nerve cells, which must 

 also necessarily be surrounded by a matrix of grey sub- 

 stance, and thereafter end in sensory or motor terminals, 

 or in turn pass through further grey matter extensions 

 until they finally reach their terminal stage of distribution 

 in, or to, sensory or motor textures ; from which it 

 becomes obvious that a large proportion of the cerebral 

 neurons must begin and end histologically and func- 

 tionally there, i.e. within the proper structural matrix of 

 the cerebrum, and, therefore, that they do not necessarily 

 directly pass out into the peripheral regions of the systemic 

 nervous system. The widely distributed areas of grey 

 substance to be seen overlying and inter-penetrating that 

 organ, therefore, thus assume the character of great 



