122 INTRODUCTION TO NEUROLOGY 



The isthmus is a sharp constriction which separates the brain 

 into two major divisions, the rhombencephalon behind and the 

 cerebrum in front. In the B. N. A. 'table the isthmus is regarded 

 as a transverse segment or ring; it might better be regarded 

 simply as a plane of separation between the rhombencephalon 

 and cerebrum. In the table the medulla oblongata is regarded 

 as synonymous with myelencephalon, that is, the region between 

 the pons and the spinal cord. The older usage, which is still 

 widely current, regards the medulla oblongata as including 

 everything between the isthmus and the spinal cord except the 

 cerebellum dorsally and the fibers and nuclei of the pons and mid- 

 dle peduncle of the cerebellum ventrally. This is the old or seg- 

 mental part of the rhombencephalon, and the cerebellum and 

 pons fibers related to it are added to this primitive medulla 

 oblongata. The older usage is preferable to the B. N. A. division 

 and will be adopted here, for the medulla oblongata as here 

 defined is a structural and functional unit, whose form is not 

 modified in those animals which almost totally lack the cere- 

 bellum and its middle peduncle. The midbrain (mesencephalon) 

 is the least modified part of the neural tube in the adult brain. 

 The betweenbrain (diencephalon) has three principal divisions: 

 (1) below is the hypothalamus; (2) above is the epithalamus; 

 (3) between these is the thalamus which includes the thalamus 

 and metathalamus of the table (see p. 167). The hypothalamus 

 and epithalamus are highly developed in the lowest vertebrates 

 and are related to the olfactory apparatus; in these brains the 

 thalamus proper is very small, this part increasing in size in the 

 higher animals parallel with the evolution of the cerebral cortex. 

 The thalamus proper is really a sort of vestibule to the cere- 

 bral cortex; all nervous impulses which reach the cortex, except 

 those from the olfactory organs, enter it through the thalamus. 

 The endbrain (telencephalon) includes the cerebral hemispheres 

 and a very small part of the primitive unmodified neural tube 

 to which the hemispheres are attached, this being the pars 

 optica hypothalami of the table or, better, the telencephalon 

 medium. 



If now we compare this subdivision of the human brain with 

 our rough functional analysis of the fish brain (p. 112), we notice 

 that the "ear brain" (acoustico-lateral area), "skin brain" or 



