64 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



IV. NERVOUS TISSUE. 



Nervous tissue has usually been described as being composed of two 

 distinct substances, nerve-fibers and nerve-cells. The modern view of the 

 nature of nerve tissue is, however, that the nerve-cell and the nerve fibers 

 are to be considered together as one unit, called the neurone. The neurone 

 is embedded in and supported by a substance called neuroglia. This neurone 

 consists of a cell body, a number of branching processes termed dendrites, 

 and a long process running out from it, the neuraxone, or axone, which be- 

 comes eventually a nerve fiber. The nerve-cell and the nerve fiber are parts 

 of the same anatomical unit, and the nervous centers are made up of those 

 units, arranged in different ways throughout the nervous system, figure 81, A 



NERVE FIBERS. 



While the nerve fiber is really to be considered as a process of the nerve- 

 cell, it is convenient to describe it separately. Nerve fibers are of two kinds, 

 medullated or white fibers, and non-medullated or gray fibers. 



Medullated Fibers. Each medullated nerve fiber is made up of 

 the following parts: An external sheath, called the primitive sheath, neuri- 



S.N. 



FIG. 83. Diagram Showing the Arrangement of the Neurons or Nerve Units in the Architecture 

 of the Nervous System. (Raymon y Cajkl.) A, Pyramidal neurone of cerebral cortex; B, anterior- 

 horn motor cell of spinal cord ; D, collateral branches of A ; E, medullary neurone with ascending 

 axone: F, spinal-ganglion neurones; G, sensory axones of F\ I, collaterals of F in the cord. 



