304 INTRODUCTION TO NEUROLOGY 



predetermined in the inborn organization of the nerve-centers 

 concerned. In the variable type of response, on the other hand, 

 the association centers involved are so arranged that many final 

 paths leading to different systems of coordinated motor centers 

 diverge from a single center of correlation. Which of these paths 

 will be taken in a given reaction, that is, which of several possible 

 different (or even antagonistic) movements will result, will be 

 determined by variable physiological factors of internal resist- 

 ance within the correlating system (fatigue, habit, the influence 

 of memory vestiges, etc.) ; accordingly, the response is not pre- 

 determined by the inborn organization of the apparatus. 



Definite, well-established reflexes generally follow distinct 

 nervous pathways between sharply limited nerve-centers. Be- 

 tween these centers there is usually found, in addition to the 

 well insulated tracts just mentioned, a more diffuse and loosely 

 organized entanglement of nerve-cells and fibers, through which 

 nervous impulses may be more slowly transmitted in any direc- 

 tion. Tissue of this character is found throughout the entire 

 length of the central nervous system, and in some places it occu- 

 pies extensive regions (especially in the medulla oblongata and 

 upper part of the spinal cord) which are termed the reticular for- 

 mation (see pp. 65, 127, 158). 



The reticular formation is the parent tissue out of which the 

 higher correlation centers have been differentiated. In the 

 spinal cord and medulla oblongata, where its character is most 

 clearly seen, it receives fibers from all of the sensory centers 

 and may discharge motor impulses into efferent centers of con- 

 tiguous or very remote regions. In the higher parts of the brain 

 the elaborate association centers of the thalamus and cerebral 

 hemispheres have been developed from such a primitive matrix, 

 and these centers are interconnected by similar undifferentiated 

 nervous tissue. 



The details of the functional connections of the reflex centers 

 of the brain stem are much more precisely known than are those 

 of the higher correlation centers of the thalamus and cerebral 

 cortex. And, in fact, it is essential that these details be fairly 

 well understood before the functions of the higher centers can 

 be investigated; for all nervous impulses which reach these higher 

 centers must first pass through the lower centers and there be 



