The Nervous System 207 



X 314. Nerve Cells. However complicated the structure 

 of the nerve tissue in man seems to be, it is found to con- 

 sist of only two different elements, nerve cells and nerve fibers. 

 These are associated and combined in many ways. They 

 are arranged in distinct masses called nerve centers, or in 

 the form of cords known as nerves. 



Nerve cells consist of masses of protoplasm, with a large 

 nucleus and nucleolus. A number of processes branch off 

 from them, some cells giving off one or two, 

 others many. 



Certain forms of cells are found only in 

 particular parts of the nervous system. Thus, 

 we have in the spinal cord the large,- irregular 

 cells with many processes, and in the brain 

 proper the three-sided cells with a process 

 jutting out from each corner (Fig. 3). 



315. Structure of Nerve Fibers. The nerve FIG. in. 

 fibers, the essential elements of the nerves, Nerve Cells 

 somewhat resemble tubes filled with a clear, 



Spinal Cord. 



jelly-like substance. They consist of a rod, 

 or central core, continuous throughout the whole length of 

 the nerve, called the axis cylinder. This core is surrounded 

 by the white substance of Schwann, or medullary sheath, 

 which gives the nerve its characteristic ivory-white appear- 

 ance. The whole is enclosed in a thin, delicate sheath 

 known as neurilemma. 



The axis cylinder generally passes without any break 

 from the nerve centers to the end of the fibers: The 

 outer sheath (neurilemma) is also continuous throughout 

 the length of the fibers. 



The medullary sheath, on the other hand, is broken at 

 intervals of about one twenty-fifth of an inch, and at the 

 same intervals nuclei are found along the fiber, around 



