2l8 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER XIV. 



CENTERS OF CORRELATION. 



The whole nervous system has been treated thus far as consisting 

 of four main divisions, each of which is connected with a special 

 set of functions. For a review and summary of these functional 

 divisions the student should turn to the outline given in Chapter 

 V. The central portions of the functional divisions constitute 

 four longitudinal zones of the brain and spinal cord. The four 

 zones lie in the same relative position throughout the whole length 

 of the central nervous system, except in certain segments of the 

 brain where one or other zone is largely or wholly wanting. It 

 may be supposed that the speciahzation of these four zones has 

 taken place within a relatively undifferentiated neural tube which 

 was possessed by the remote ancestors of vertebrates and that in all 

 existing vertebrates the functional divisions are quite distinct 

 from one another. The process of specialization of the functional 

 divisions has left over, so to speak, a certain amount of material 

 which has come to serve the purpose of connecting the functional 

 divisions with one another and the centers of one segment with 

 those of another. The elements which properly belong to the 

 primar}^ functional divisions as described in previous chapters 

 may be summarized as follows, (i) In the afferent or sensory 

 columns: (a) receptive neurones, which receive the end-branches 

 of afferent fibers; (b) intrinsic neurones, whose neurites whether 

 long or short are confined to the given column; and (c) extrinsic 

 neurones, whose neurites go beyond the given column to carry im- 

 pulses to other centers. These other centers are primarily located 

 in the same column from which the neurites spring but they fall 

 under the category of the centers of correlation which are here 

 under consideration. The neurones (b) and (c) may both be at 

 the same time receptive neurones. (2) In the efferent columns: 

 motor or excito-glandular neurones whose neurites go to the 

 periphery as fibers of efferent nerves. 



