Fundamental Structure 19 



centre, if the various stimuli received are of different kinds. 

 A correlation neuron may send its impulses to several other 

 elements, which may include both efferent cells and other 

 correlation neurons. Thus one correlation centre may 

 influence another, and it itself may be controlled by yet 

 another, and so on. Hence we have lower and higher func- 

 tional levels in the central nervous system, the lowest being 

 the simple reflex level, the higher the correlation levels, of 

 which there are many, and the higher levels exerting a direct- 

 ing or controlling influence over the lower ones. To a certain 

 extent these correspond roughly with the structural levels, 

 the simple reflexes being confined to the lower parts of the 

 brain and to the spinal cord, w^hile only the lower correlation 

 centres are found near these, the higher ones occurring in 

 more or less regular succession towards the upper or anterior 

 part of the brain, and the highest being in the cerebral cortex. 



Any part of the nervous system which is composed chiefly 

 of cell bodies and dendrites or unmyelinated axons appears 

 rather grayish in the fresh condition and hence is known as 

 gray matter, while the presence of a large number of axons 

 with myelin sheaths (myelinated fibres) gives a region a clear 

 white colour, so that it has received the name of white matter. 



In all parts of the system, neurons of similar function 

 tend to be associated topographically. Hence we find more 

 or less definite groups of cells, or masses of gray matter, 

 which are concerned with particular functions and which are 

 given the name of nuclei. The axons proceeding from a 

 particular nucleus or group of nuclei also associate them- 

 selves into definite groups whicK are called fibre-tracts, when 

 all the fibres have similar connections. A bundle con- 

 taining fibres belonging to more than one tract is called a 

 fasciculus. It is the task of neuro-anatomy to identify the 

 various nuclei and tracts, showing how they are related, both 

 structurally and functionally, and to trace out in detail the 



