I 



56 THE NERVOUS SYSTEM 



cells are organs made to produce atomic movements just as the muscles 

 are instruments of molar movement. Every epithelial cell is a tiny 

 laboratory for the elaboration of some new substance, and the nervous 

 influence coming to it starts or stops its secretion. Whether or not it has 

 any qualitative control over the metabolism is still somewhat in doubt. 

 Whatever the nervous authority over a single gland-cell accomplishes, 

 it certainly (a) coordinates the actions of the myriad separate cells, and 

 (b) adapts jtheir collective action to the needs of the organism as a whole. 

 5. Concerning the trophw function of nerves, the influence over tissue- 

 nutrition, little can at present be definitely stated, save that such control 

 undoubtedly exists. Whether the nerves immediately direct the nutrition 

 of any tissue is not determined, but the function of coordinating the 

 nutrition of parts is surely performed by the nervous system. In normal 

 conditions there are some signs of this, but pathology shows us numerous 

 instances in which a disease of a brain-part results in trophic disaster in 

 some portion of the organism (e. g., acromegaly). This trophic control 

 may prove to be identical with that overglandular action, since all 

 protoplasm appears more and more to be the site of complex chemical 

 production. The nervous energy, whatever it is, actually stimulates the 

 energetic processes of protoplasm. When cut nerves are regenerated, the 

 first function to recur is the trophic function: the part slowly regains 

 its lost flesh-color, its warmth, and its firm tone. Its resistance to disease 

 increases. These, then, are the most conspicuous elements of " trophism." 

 (Sensation next appears, then reflex movement, and lastly voluntary 

 movement.) It is probably through this trophic control over tissue- 

 metabolism exerted by the central nervous system that the chronic 

 emotion of worry exerts its baleful influence over health. (See Chapter 

 XII.) 



FEATURES OF THE NEURAL STRUCTURE. 



The chief function of the nervous system is coordination, and it accom- 

 plishes this by the conduction of various influences between large or 

 small parts of the body and between the organism as a whole and its 

 environment. For this conduction fibers are necessary, and accordingly 

 we find the nervous system essentially a network of fibers or of fibrils 

 extending almost everywhere in the body. The relations of these fibers 

 or fibrils making up the neural "reticulum" are not yet fully understood, 

 nor are its relations to the nerve-cells which are so numerously connected 

 with it. It is not certain that the fibril rather than the fiber (neuraxone, 

 neurite, neuraxis, fibril-bundle, axis-cylinder) is the conducting unit 

 (that is, that each fibril bears a separate impulse), but the probability of 

 this belief is increasing. 



Throughout most of their length the supposed fibrils are gathered in 

 bundles (the fibers, axones), and these are of several different sorts, 

 classified according to their coverings. The essential fiber made up of 

 fibrils varies little, so far as is known, save in diameter. Two chief types, 



