THE NERVOUS TISSUES. 



997 



contains the nerve-cells of the sensory ganglia and is principally composed of the 

 nerve-fibres that pass to and from the end -organs. Intimately associated with and 

 in fact a part of the peripheral nervous system, but at the same time possessing a 

 certain degree of independence, stands the sympathetic system, which provides for 

 the innervation of the involuntary muscle and glandular tissue throughout the body 

 and the muscle of the heart. 



When sectioned, the fresh brain and spinal cord do not present a uniform appear- 

 ance, but are seen to be made up of a darker and a lighter substance. The former, 

 the gray matter, owes its reddish brown color not only to the numerous nerve-cells 

 that it contains, but also to its greater vascularity ; the hue of the lighter substance, 

 the white matter, is due to its chief constituents, the medullated nerve-fibres, in 

 conjunction with its relatively meagre blood supply. 



THE NERVOUS TISSUES. 



The Neurones. The neurones, the essential morphological units of the 

 nervous system, consist of the cell-body and the processes. The latter, as seen in 

 the case of a typical motor neurone (Fig. 835), are of two kinds : (a) the branched 

 protoplasmic extensions, the dendrites, which may be multiple and form elaborate 

 arborescent ramifications that establish relations with other neurones, and (^) the 

 single unbranched axone (neuraxis, neurite) that ordinarily is prolonged to form the 

 axis-cylinder of a nerve-fibre, and, hence, is often termed the axis-cylinder process. 

 The dendrites are usually uneven in contour and relatively robust as they leave 

 the cell-body, but rapidly become thinner, due to their repeated branching, until 



they are reduced to delicate threads that con- 

 stitute the terminal arborizations, the telodendria, 

 formed by the end-branches. The latter are 

 beset with minute varicosities and finally end in 

 terminal bead-like thickenings. The axones, 

 slender and smooth and of uniform thickness, 

 are much less conspicuous than the dendrites. 

 They may be short and only extend to nearby 

 cells ; or they may be of great length and con- 

 nect distant parts that lie either wholly within the 



FIG. 836. 



Dendrites 



FIG. 835. 





Dendrites 





Aiborization 



of axone 



Telodendrion 

 Diagram of typical neurone. 



/ / 



Diagram of nerve-cell of type 

 II, in which axone is not prolonged 

 as nerve-fibre. 



cerebro-spinal axis (as from the brain-cortex to the lower part of the spinal cord) or 

 extend beyond (as from the lower part of the cord to the plantar muscles of the foot). 



