STRUCTURE OF THE VERTEBRATES 



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B. Units and Structure and Function 



The brain and spinal cord are composed of nerve cells and 

 their fibers, and an ectodermal supporting tissue called neuroglia. 

 The latter makes up the vast majority of the bulk of the central 

 system. The nerve cell is the neurone, the anatomical unit of 

 the nervous system. 



A neurone is composed of (1) a cell body with its nucleus 

 and cytoplasmic inclusions; (2) filamentous dendrites which re- 

 ceive the stimuli; and (3) a long axone which carries impulses 



Fig. 156. Types of Nerve Cells. Motor Cell (A) from the ventral horn 

 of the cord; Sensory Cell (B) ; and Brain Cells, (C) and (D). 



from the cell body and (4) ends in a highly branched terminal 

 arborization. Neurofibrils pass from the dendrites through the 

 cell body and out to the end of the axone. The cell body is the 

 center of metabolism. The dendrites, axone, and terminal arbor- 

 ization carry on the specialized function of the nerve tissues. 



Neurones are divided by their structure into two large groups: 

 multipolar cells which comprise the great majority, and uni- 

 polar sensory cells. The former vary enormously in structure, 

 from the rather simple type found in the nerve cord to the 

 highly complex cells of the cerebral region. In this type the 



