ANATOMY AND SUBDIVISION OF NERVOUS SYSTEM 113 



nerves inward to terminate in a special part of the wall of the 

 primitive neural tube, and that here a thickening of the wall of 

 the tube has taken place to provide space for the appropriate 

 terminal nucleus. It may be noticed, further, that all of these 

 structures (except a part of the olfactory centers) lie in the 

 dorsal part of the brain. An examination of the primary motor 

 centers would show that they are distributed in a somewhat 

 similar fashion along the ventral part of the brain. 



The facts just recounted give a clear picture of the pattern of 

 functional localization of the primary reflex centers in a simple 

 type of brain, and they show that all of the more obvious parts 

 of this brain except the cerebellum are in simple direct relation 

 with particular peripheral organs. In other words, nearly the 

 whole of this brain is directly concerned with simple reflexes 

 and (aside from the cerebellum) no large centers for the higher 

 types of adjustments are present. The primary reflex centers 

 are found to be arranged in accordance with essentially the same 

 pattern in the human and all other higher brains, though in these 

 cases the pattern is slightly modified and obscured by the pres- 

 ence of greatly enlarged correlation centers, of which the cerebral 

 cortex is the chief. The structure and significance of the cere- 

 bral cortex form the theme of the last three chapters of this work. 



The central nervous system of the earliest vertebrates was 

 probably a simple longitudinal tube of nervous tissue with which 

 the peripheral nerves were connected in a segmental fashion (see 

 p. 28). This is the permanent form of the spinal cord and ite 

 nerves in all vertebrates (see p. 125 and Fig. 41). In the brain 

 the enlargement of the primary reflex centers and of the corre- 

 lation centers directly related to them has changed the form of 

 the tube and disturbed the primitive segmental arrangement of 

 the cranial nerves, as is indicated in Figs. 43 and 44. Never- 

 theless, this more ancient part of the brain is sometimes called 

 the segmental apparatus, to distinguish it from two very large 

 coordination and correlation mechanisms which are of later 

 evolutionary origin, namely, the cerebellar cortex and the 

 cerebral cortex, which are termed suprasegmental structures. 

 The segmental apparatus is often called the brain stem. It 

 includes practically all of the fish brain (Figs. 43 and 44) except 

 the cerebellum, for in these animals there is no cerebral cortex. 



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