THE STRUCTURE OF THE NERVE SYSTEM 333 



nerve force originates, and which maintains the 

 specific functions of the body expressed in motion, 

 trophic changes, or stimuli of a chemic, mechanic 

 (touch, sound), thermal, or photic nature. 



Neurones are so arranged to be receptive and 

 receive (afferent) impulses from other parts of the 

 body. These are termed sensor neurones. Others 

 are emissive and return (efferent) impulses; the latter 

 are called motor neurones; if connected with muscle 

 cells, excitomotor, and with gland cells, excito- 

 glandular. 



The neurones or nerve cells compose the cellular 

 element of the whole nerve system, and the balance 

 is the supporting white fibrous tissue and neuroglia 

 derived from the supporting membrane or from the 

 sheaths of vessels. 



The Structure of a Neurone or Nerve Cell. It consists 

 of a cell mass or body containing a nucleus, and within 

 the latter a nucleolus. From this cell body are given 

 off protoplasmic processes of two different varieties: 

 (1) dendrites, and (2) axone, or axis-cylinder. 



The Bodies of Nerve Cells. They vary in size, 

 they measure 4 to 135 microns or more in diameter, 

 and when it is considered that 1 micron is equal to 

 "2 5~ToT f an inch, their minute size may be appreciated. 

 Neurones are classified according to the number of 

 processes arising from the body, as unipolar, bipolar, 

 and multipolar. 



The Dendrites. -- They are processes extending 

 from the body of the cell in large numbers. They 

 consist of the same structure as the cell, and thus 

 increase the functional surface or expression of the 

 cell. The dendrites never communicate with the 

 branches of the same process direct, or anastomose 

 with the terminals of dendrites from adjoining nerve 

 cells. The function of the dendrites is supposed to 

 be conductive and receptive for nerve impulses. 



