CENTERS OF CORRELATION. 



2I 9 



Neurones which do not properly belong to any one division 

 are at first scattered through all four divisions and in the spinal 

 cord this condition is maintained, so that the material in question 

 appears as cells scattered throughout the whole gray area. These 

 cells have come to be known as homolateral and heterolateral 

 tract cells (Fig. in). In the brain a large part of the corre- 

 sponding material is retained in its embryonic position adjacent 

 to the ventricle. The term central gray matter has been applied 

 to this circum-ventricular zone of cells. Since many of the cells 

 of the primary sensory and motor centers lie in this central gray, 



Fig. in. Tract cells in the spinal cord of the trout. Combined from two 

 figures by Van Gehuchten. 



especially in lower vertebrates, it is necessary to use some other 

 term to designate the material which serves functions of correlation. 

 Whether these cells are situated adjacent to the ventricle or are 

 scattered through the wall of the brain, their dendrites intermingle 

 with the fiber tracts which form a large part of the brain wall. 

 These areas consisting of mingled fiber tracts, cells and dendrites 

 may be called the substantia reticularis. The portion in which 

 the fiber tracts predominate is the substantia reticularis alba; that 

 which is composed chiefly of cells is the substantia reticularis 

 grisea. 



The tract cells in the spinal cord illustrate best the functions 

 which this unspecialized material first served. Since these cells 



