CENTERS OF CORRELATION. 221 



proper and these of the substantia reticularis. Not only are the 

 neurones of the reticularis scattered throughout the four longitud- 

 inal zones, but in the brain region at least, the reticularis neurones 

 are especially related to the functional division in which they lie. 

 In all segments of the brain cephalad from the VII nerve one or 

 other of the primary functional divisions is either greatly reduced or 

 wholly wanting. Throughout this region of the brain special cen- 

 ters are formed which in most cases clearly lie within the bounds of 

 one of the primary longitudinal zones. Whether neurones belonging 

 primarily to the functional divisions have entered into the forma- 

 tion of special centers, and to what extent this may have happened 

 are questions which cannot now be decided. For the present the 

 following practical criterion may be applied ; where the structure of 

 centers and the disposition of their tracts do not follow the typical 

 arrangement of centers and tracts in one of the functional divi- 

 sions, the centers in question are treated as centers of correlation. 

 Since these centers are at least largely derived from the substantia 

 reticularis it may be stated that the most fundamental difference 

 between the brain and spinal cord is the presence in the former 

 of a relatively larger volume of this indifferent material for the 

 formation of correlating centers. It will be the purpose of this 

 and the following chapters to give in a broad and general way an 

 account of the phylogenetic history, morphology and functional 

 relations of these special brain centers. 



The reason for the use of the term centers of correlation is 

 very simple. As has already been shown (p. 8i), simple reflexes 

 are mediated by direct connections between sensory and motor 

 nuclei. Even large movements of a vague and ill-directed sort 

 may be carried out in this way. But when complex movements 

 involving the action of many muscles directed to a definite end are 

 called forth by a stimulus, the inter\'ention of secondary and 

 tertiary centers with their fiber paths is necessary for the control 

 of the muscles as to the time, extent and force of contraction. 

 The experiments with the brainless frog (p. 82) show that for 

 movements of short duration up to a certain grade of complexity the 

 tract cells of the spinal cord serve the purpose of correlating 

 centers, but for the direction of movements for a longer period 



