THE BRAIN 



SECTION XI 

 THE STRUCTURE OF THE BRAIN STEM 



THE physiology of the brain falls naturally into two main divisions, namely, 

 that of the brain stem, including the medulla, the pons, Sylvian iter, corpora 

 quadrigemina and third ventricle, and that of the cerebral hemispheres. It 

 is usual, in treating of the structure of the brain stem, to consider it as 

 a prolongation forwards of the spinal cord and as consisting, like this, of a 

 central tube of grey matter surrounded by a tube of white matter. Like the 

 spinal cord, the brain stem may be regarded as originating primitively by 

 the fusion of a series of ganglia presiding over the local reactions of their 

 respective somites. The modifications in this segmental arrangement, which 

 have occurred in the course of evolution, have been so profound that little 

 trace of the primitive segmental arrangement is to be observed. At the 

 fore end of the body have been developed the organs of special sense, which 

 are the most important in determining the reactions of the animal in response 

 to present or approaching changes in its environment. Indeed the whole 

 course of evolution is conditioned by the evolution of the brain stem in the 

 first place, and of its outgrowth, the cerebral hemispheres, in the second. 

 Hence we cannot expect to find in the brain stem the regularity of arrange- 

 ment of grey and white matter that we have studied in the cord. The typical 

 division of the grey matter into cornua becomes altogether lost. While some 

 nerves take their origin from or terminate in the central tube of grey matter, 

 in other cases the collections of nerve cells and fibres forming the nuclei 

 of the cranial nerves have become more or less separated from the central 

 axis. Moreover the central grey matter is by itself quite inadequate to deal 

 with the flood of afferent impressions entering the central nervous system 

 through the organs of special sense, or to co-ordinate these with one another 

 or with those arriving from the skin and lower part of the body. Masses 

 of grey matter, which have no representative in the cord, make their appear- 

 ance, and may be regarded as additional sorting stations or fields of conjunc- 

 tion for the afferent a*d efferent impulses which determine the nervous 

 activities of the animal. 



The general features of the structure of the brain will be best understood 

 by reference to the mode of development of this part of the central nervous 



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