THE NERVOUS TISSUES. 



CHAPTER V. 



THE NERVOUS TISSUES. 



THE nervous system is composed of three principal parts the 

 tissues originating nervous impulse, the nerve -cells ; the structures 

 serving to transmit such impulses, the nerve-fibres ; and the tissues 

 uniting and supporting the nervous elements, the. neuroglia and 

 connective-tissue framework. The nerve-cells are the primary 

 elements, being older in the development of the individual as 

 well as in the evolution of the nervous system. In certain inverte- 

 brates both generation and transmission of the impulse are performed 

 by the same cell, the peripherally situated protoplasm serving to 

 convey and expend the force originating within the more centrally 

 lying parts of the cell. Such simplicity, however, is unusual, the 

 nerve-cell soon becoming specialized and separated from the pe- 

 ripheral area with which it is connected. 



NERVE-CELLS. 



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

 in form and size, since they may be either spherical (Gasserian, 

 spinal, or other ganglia), ellipsoidal (spinal cord), pyriform (cere- 

 bellum), pyramidal (cerebrum), or stellate (spinal cord), and vary 

 from 10 to 100 // in size ; the huge cells of the spinal cord are among 

 the largest elements of the body. In general the cells of motor 

 areas are largest, those found in the convolutions bordering the 

 central fissure and in the anterior cornua of the spinal cord being of 

 conspicuous size. 



The ganglion-cells are composed of granular or striated proto- 

 plasm, containing a large round or oval vesicular nucleus within 

 which lies a prominent nucleolus ; after certain stains the protoplasm, 

 nucleus, and nucleolus present distinct tints. Many nerve-cells are 

 deeply colored, owing to the presence of considerable quantities of 

 pigment-granules around the nucleus ; a certain amount of pigment 

 within the protoplasm is almost constant. 



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

 and usually several processes, dependent upon the number of which 

 it is customary to speak of nerve-cells as unipolar, bipolar, or 

 multipolar. Since an apolar nerve-cell is, evidently, functionally 

 useless, it is doubtful whether such cells ever normally exist ; apolar 

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



