56 PSYCHOBIOLOGY 



The spinal cord is composed mostly of ' white ' matter surrounding a 

 core of ' gray ' matter, the latter having in cross-section roughly the shape 

 of the letter H [Fig. 41]. Outside the 'white' matter are three pro- 

 tective membranes [Fig. 40]: (1) the pia mater, a fibrous connective 

 tissue coat, closely applied, continuous with the inner surface of which 

 are fine septa, penetrating the ' white ' matter; (2) the arachnoid, a thin 

 membrane loosely wrapped around outside the pia mater, leaving an inter- 

 val (the ' subarachnoid space ') filled with ' cerebrospinal fluid ' ; and (3) 

 the dura mater, forming a dense tubular sheath, considerably larger than 

 the cord, and extending downwards below the limit of the cord into the 

 sacral part of the canal. The cord is attached to the inner surface of the 

 sheath of dura mater by two lateral wing-like ligamenta denticulata. 



Between the dura mater and the wall of the spinal canal is a small space 

 filled by fatty tissues and blood vessels. 



On the anterior and posterior sides of the cord there are fissures, into 

 each of which a fold of the pia mater extends, nearly dividing the cord 

 into two halves. A fold of the arachnoid (the septum) extends to the 

 dorsal side of the cord, across and dividing the subarachnoid space. 



GRAY MATTER AND WHITE MATTER. 



The ' white matter ' of the brain, cord and nerves is made up chiefly of 

 medullated nerve fibers, running through the framework of neuroglia, 

 The 'gray matter' (found only in brain, cord and ganglia) is composed 

 of cell-bodies, and non-medullated fibers, in a neuroglia framework. 



Neuroglia is composed of branched or fibrillated cells (neuroglia cells; 

 derived from the same embryonic layer as the nerve cells) whose branches 

 anastomose, forming a reticular tissue or network [Fig. 42]. 



There appear to be two kinds of neuroglia cells, in the one of which the 

 processes branch repeatedly, in the other of which (the ' spider cells') the 

 processes are entirely unbranched. 



THE ' CEREBROSPINAL ' AND ' SYMPATHETIC ' SYSTEMS. 



It is customary to class together the neurons whose cell bodies lie in the 

 brain, cord, spinal ganglia, ganglia of the cranial nerve roots, and ' sense 

 organs ', as the cerebrospinal system. The neurons whose bodies lie 

 in the other ganglia ('sympathetic' ganglia) are classed as the sympa- 

 thetic or autonomic system. This classification is useful if it is under- 

 stood that there are not two separate systems, but two intimately associated 

 parts of one system. Classification and terminology in respect to these 

 divisions of the total nervous system are not well agreed upon. We shall 

 refer to this point later. 



