NERVOUS SYSTEM 



847 



points in the histology, general anatomy, and physiology of the frog. 

 And the task is made the more difficult on account of all the early studies 

 having been made upon the human brain before our better stains made 

 it possible to understand the finer structure of nerve cells and fibers. 

 The result is that the names of the various parts of the brain have been 

 derived from fanciful resemblances, often very confusing. 



We shall attempt to study the entire central nervous system in 

 terms of function rather than in terms of structure, and the latter only 

 ^in its development, as then, and then only, are we able to place a valid 

 interpretation upon our findings. 



Following are several terms without which no progress in this study 

 can be made: 



A center is any group of nerve-cells which performs a single func- 

 tion. (This does not imply, however, that all of this particular function 

 is located in this one center alone. There may be several, or many, per- 

 forming similar functions.) 



It is these centers which form a sort of switch-board for the redis- 

 tribution of various nervous impulses. 



Afferent fibers are those which conduct toward the centers. 

 Efferent fibers are those which conduct away from a center. 

 Peripheral nerves (those running from and toward the central sys- 

 tem) are naturally mixed 

 nerves in that they carry both 

 afferent and efferent fibers. 



Inhibitory fibers are those 

 which check an action. 



White matter (substantia 

 alba) is that portion where the 

 nerve fibers are covered with 

 white myelin sheaths. 



Gray matter (substantia 

 grisea) is that portion where 

 there is a mass of nerve-cell- 

 bodies uncovered with myelin 

 sheaths. 



Brain nuclei are the gray 

 centers within the brain, which 

 are divided in turn into: 



Primary centers which are 

 those directly connected with 

 the peripheral nerves, either as 

 terminal nuclei of afferent 

 fibers or as nuclei of origin of efferent fibers, and 



Correlation centers, which are those in which the impulse received is 

 redistributed after meeting with other impulses at a common center. 



Fig. 474. 

 Five types of reflex arcs. 



