NEURONS AND NEURON-CHAINS 



47 



to the myelinated fibers a whitish color. This sheath is interrupted at regular 

 intervals by constrictions in the nerve-fiber known as the nodes of Ranvier. 

 The constrictions are produced by a dipping in of the neurilemma sheath toward 

 the axon, which runs without interruption through the node. The part of a 

 fiber between each node is an internodal segment, and each such segment pos- 

 sesses a nucleus which is surrounded by a small amount of cytoplasm and lies 

 just beneath the neurilemma. The latter is a thin membranous outer covering 

 for the fiber. Each segment of the neurilemma sheath, together with the cell 

 which lies beneath, is the product of a single sheath cell of ectodermal origin. 

 Fibers such as have just been described are found in the cerebrospinal nerves, 

 and give these their white glistening appearance. 



The myelinated filers of the brain and spinal cord are of somewhat different 

 structure. There is no evidence of segmentation in the myelin sheath and 

 neither the neurilemma nor its cells are present. This fact is of much im- 

 portance in the phenomena of regeneration, as will be explained later. These 

 are the fibers which give the characteristic color to the white matter of the 

 brain and spinal cord. 



Unmyelinated fibers are of two kinds, namely, Remak's fibers and naked , 

 axons. The former possess nuclei which may be regarded as belonging to a 

 thin neurilemma. They are found in great numbers in the sympathetic nervous 

 system, and many of the fine afferent fibers of the cerebrospinal nerves also 

 belong to this class (Ranson. 1911). Naked axons are especially numerous in 

 the gray matter of the brain and spinal cord, and it may be added that every 

 axon at its beginning from the nerve-cell, as well as at its terminal arborization, 

 is devoid of covering. 



By way of summary we may enumerate four kinds of nerve-fibers: (1) myelin- 

 ated fibers with a neurilemma, found in the peripheral nervous system, especially 

 in the cerebrospinal nerves; (2) myelinated fibers without a neurilemma, found 

 in the central nervous system; (3) unmyelinated fibers with nuclei (Remak's 

 fibers), especially numerous in the sympathetic system, and (4) naked axons, 

 abundant in the gray matter of the brain and spinal cord. 



Neuroglia cells and fibers will be considered in connection with the structure 

 of the spinal cord. 



Structure of Neurons. Like other cells, a neuron consists of a nucleus sur- 

 rounded by cytoplasm, and these possess the fundamental characteristics which 

 belong to nucleus and cytoplasm everywhere, but each presents certain features 

 more or less characteristic of the nerve-cell. The nucleus is large and spheric; 



