

THE NERVOUS TISSUES. 



CHAPTER 



E NERVOUS TISSUES. 



THE nervous system is composed of two principal parts the 

 tissuesr' originating and transmitting nervous impulse, the nerve- 

 _cells and the nerve-fibres, and the tissues uniting and supporting 

 the nervous elements, the neuroglia and connective-tissue frame- 

 work. While convenience warrants the consideration of the nerve- 

 cells and the nerve-fibres as distinct elements, it must be appreciated 

 that the nerve-fibres are only the extended processes of the nerve- 

 cells, and that both cells and fibres together constitute the morpho- 

 logical units, the neurones, of which, in conjunction with the sup- 

 porting tissues, the entire nervous system is composed. 



In the simplest type of nervous system the perception of external 

 stimuli and the generation and transmission of the responsive impulse 

 are performed by the same cell, the more centrally lying parts of the 

 cell serving to convey and expend the force originating within the 

 protoplasm. Such simplicity, however, is unusual, the nerve-cell 

 soon becoming specialized and removed from the peripheral area 

 with which it maintains connection by means of its process the 

 nerve-fibre. 



NERVE-CELLS. 



The nerve-cells contain a large oval nucleus, and often a conspicu- 

 ous nucleolus. After treatment with methylene-blue (Nissl's stain- 

 ing), the protoplasm is differentiated into two parts the stainable 

 and the unstainable substance. The former appears as masses of 

 deeply-tinted granules, arranged in net-works, longitudinal striae, or, 

 as in a majority of cells, as irregular combinations of both. The 

 unstainable substance constitutes the remaining portion of the cell- 

 body, and, in places, may exhibit delicate striation. A variable 

 amount of pigment is also usually present within the protoplasm. 



Nerve- or ganglion-cells of man and other vertebrates differ 

 greatly in form and size, since they may be either spherical (Gas- 

 serian, spinal, or other ganglia), ellipsoidal (spinal cord), pyriform 

 (cerebellum), pyramidal (cerebrum), or stellate (spinal cord), and 

 vary from 10 to 100 ^ in size. 



The protoplasm of every nerve-cell is prolonged into at least two 

 and frequently several processes ; since an apolar cell is use- 

 less, it is doutful whether such cells ever normally exist ; apolar 

 cells are frequently seen in preparations, but the absence of the 







