Chap. 16 CONDUCTION AND COOKDINAilON NERVOUS SYSTEM 281 



sometimes of extraordinary length. Currents of energy, the impulses, move 

 along these fibers and by means of them messages are flashed to other neurons. 

 The nervous system contains millions of neurons like electric wires which 

 are protected and insulated from one another except at the tips of their 

 fibers (Fig. 16.3). The body of the neuron is relatively large. Their unique 

 Nissl granules or bodies (named for the neurologist, Nissl) disappear when 



Fig. 16.2. The neuromotor system of a paramecium. The unified action of the 

 cilia on the surface of its body and food passage is controlled through the extraor- 

 dinarily fine fibrils that connect them. Changes proceed rapidly over these fibrils 

 and they conduct them as our own nerve cells do. A paramecium takes in food 

 because its cilia "agree" to wave it into the mouth. An outline of paramecium 

 with the mouth, gullet and posterior end of the body in the same position as in 

 the greatly magnified view. A cut through the body. The sharp lines of the con- 

 ducting fibrils are shown in the right half of the mouth and gullet and a particle of 

 food trapped among the fibrils at the lower end. These fibrils are visible only with 

 skilled preparation. To the naked eye the whole animal is only a minute white 

 fleck in the water. (Courtesy, Lund. Univ. of California Pubs, in Zoology, Vol. 39, 

 1935.) 



the neurons are fatigued or injured by toxins as in poliomyelitis (infantile 

 paralysis), but are restored by rest or removal of the harmful agent if the 

 damage is not already too severe. There are two kinds of fibers, dendrites 

 through which nerve impulses come into the cell and the axon through which 

 they leave it (Fig. 16.3). In their evolution as in their embryological develop- 

 ment neurons originate from epithelial cells from which processes grow out- 

 ward. 



Dendrites are commonly short with treelike branches. But there are ex- 

 ceptions: certain of the neurons whose cell bodies are located in the ganglia 

 of the spinal nerves may have a dendrite several feet long. These dendrites 

 carry incoming messages of sensation from skin, muscle, and other parts of 

 the body and compose a section of all branches of the spinal nerves. Such 

 dendrites are always figured in diagrams of a reflex arc (Figs. 16.9, 16.1 1 ). The 



