STRUCTURE OF NERVOUS MATTER. 399 



(1-2 inm.), forming the nodes of Ranvier : Axis land, band axis, axis cylinder ; this forms 

 the central part of the nerve fiber; it is continuous, subcylindrical, grayish, rather tena- 

 cious, finely h'brillated and with straight outline. The axis cylinder is the essential part 

 of the nerve, and is the only part found at its origin and termination. Medullated nerves 

 neither give nor receive anastomoses except very near their termination. In cross sec- 

 tion, a bundle of nerve fibers appears like a bunch of lead pencils, the band axis corre- 

 sponding to the lead. (2) Non-medullated fibers. These are of a pale gray appearance, 

 owing to the absence of the myeline. They possess a nucleated sheath and an axis cylin- 

 der like the medullated fibers, but differ from them in the absence of the medullary sheath 

 and from the fact that they anastomose frequently throughout their entire course ; Ran- 

 vier, A, 776. 



(B) Gray matter. Gray matter is composed of an interlacing network of nerve fibers, 

 peculiar connective tissue (neuroglia) and nerve cells, the latter being the characteristic 

 feature. The nerve cells of the brain and myelon apparently possess no proper sheath, 

 and present two forms, viz., (1) cells containing a nucleus and nucleolus, surrounded by 

 the gray or reddish brown protoplasm which gives off one or more processes, giving rise 

 to the so called multipolar cells, one or more of the poles being continued as the baud axis 

 of a nerve fiber; (2) cells much smaller than the preceding, composed apparently of only 

 a nucleus and nucleolus. The nerve cells of the ganglia possess a proper sheath, which 

 is merely an expansion of the primitive sheath of a nerve fiber. Typically, these cells 

 are pyriform and possess a pole at each end which is continued into a nerve fiber. 

 Strieker, A, 116 ; Quain, A, II, 125. 



