108 INTRODUCTION TO NEUROLOGY 



it should be borne in mind that this does not imply that this 

 function resides exclusively in that place. These functions are 

 all more or less complex and the "center" is usually the region 

 where various nervous impulses are received and redistributed; 

 it is, therefore, roughly analogous with the switchboard of an 

 electric plant. 



The nerve-fibers which conduct nervous impulses toward a 

 given center are called afferent, and those which conduct away 

 from the center are called efferent with reference to that center. 

 Most of the peripheral nerves are mixed, in the sense that they 

 carry both afferent and efferent fibers with reference to the 

 central nervous system. The efferent fibers may excite move- 

 ment in muscles (motor fibers) or secretion in glands (excito- 

 glandular fibers); other efferent fibers which check the action 

 of the organ to which they are distributed are called inhibitory 

 fibers. The afferent fibers of the peripheral nerves are often 

 called sensory fibers, though it must be borne in mind that theif 

 excitation is not always followed by sensations or other conscious 

 processes. 



The vertebrate nervous system when examined in the fresh 

 condition is found to be made up of white matter (substantia 

 alba) and gray matter (substantia grisea), the white matter 

 containing chiefly nerve-fibers with myelin sheaths (see p. 46) 

 and the gray matter nerve-cell bodies and unmyelinated fibers. 

 The centers are, therefore, generally gray in color and the inter- 

 vening parts of the central nervous system are white. 



A group of nerve-cells constituting a center as above described is often 

 called a "nucleus," a term which has nothing to do with the nuclei of the 

 individual cells (see p. 39) of which the center is composed. Some critical 

 writers use the word "nidulus" (originally suggested by C. L. Herrick) or 

 "nidus" (Spitzka) for such a center, thus avoiding the ambiguity in the use 

 of the word nucleus. The term "ganglion" is also sometimes used for nuclei 

 or centers within the brain (ganglion habenulae, ganglion interpedunculare, 

 etc.), but this usage is objectionable, for the use of the word ganglion in 

 vertebrate neurology should be restricted to collections of neurons outside 

 the central nervous system, such as the ganglia of the cranial and spinal 

 nerves and the sympathetic ganglia. 



A nucleus from which nerve-fibers arise for conduction to some remote 

 part of the nervous system is called the nucleus of origin of these fibers; 

 conversely, a nucleus into which nervous impulses are discharged by fibers 

 arising elsewhere is the terminal nucleus of those fibers. Any correlation 

 center is, therefore, a terminal nucleus for its afferent fibers and a nucleus 

 of origin for its efferent fibers. 



