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 stern, 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 out- 

 growth, the cerebral hemispheres, in the second. Hence we cannot 

 expect to find in the brain stem the regularity of arrangement 

 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 

 appearance, and may be regarded as additional sorting stations or 

 fields of conjunction for the play of afferent and efferent impulses 



which make up the nervous activities of the animal. 



407 



