BOOK XII 

 THE NERVOUS SYSTEM 



CHAPTER LXV 

 THE NEURON 



THE nervous mechanism was evolved to correlate the multicellular 

 organism with its surroundings and facilitate the proper inter- 

 action of the various organs. We have seen in previous chapters 

 how the heart, the vaso-motor mechanism, the respiratory and diges- 

 tive mechanisms, are all correlated to the body needs by the aid of 

 the nervous system. We have now to consider how, by the aid of the 

 nervous system, the animal adjusts itself to its environment. 



The unit structure of the nervous system is the neuron. It con- 

 sists of a nerve cell, with its processes. The neuron may have a 

 variety of fdrms, according to the function it subserves (Fig. 309). 

 Those engaged in the perception of the outside stimulus are small, 

 with short processes as, for example, the receptor cells, concerned in 

 olfactory and visual sensations. On the other hand, neurons which 

 conduct impulses to distant parts are supplied with one or more long 

 and a number of short processes. The cells of the anterior horn of 

 the spinal cord are an example of this type of neuron. They have 

 several processes, and. are termed multipolar. In the living cell 

 body thers may be seen a large, well-defined nucleus, and numerous 

 granules floating in a homogeneous fluid. On treatment with alcohol, 

 the cell contents are precipitated as Nissl's granules discrete masses 

 which stain with methylene blue. These are not found in exhausted 

 cells, and disappear from those cells whose axons are divided and 

 functional activity arrested. Of the processes, all but one are short, 

 and branch like the roots of a tree, till they end in small, bud-like 

 expansions, known as gemmules. These processes are known as 

 dendrons. The long process, known as the axis cylinder, or axoii. 

 comes away from a part of the cell in which there are no Nissl's 

 granules a part known as the axon hillock. The axon is character- 

 ized by its length and by the fact that it does not divide until near 

 its final terminations. 



The axon, or axis, cylinder is the essential conducting part of the 

 nerve-fibre. Many such fibres go to make up the anatomical " nerve." 

 Such fibres may be either medullated or non-medullated. 



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