168 INTRODUCTION TO NEUROLOGY 



Some of these centers are seen in cross-section in Fig. 79. The 

 preceding analysis of the diencephalon, which differs in some 

 respects from that of the B. N. A. (p. 121), is summarized in the 

 accompanying table (p. 167), which includes also a few of the 

 more important fiber tracts connected with each nucleus. 



In front of the thalamus lie the corpus striatum and olfactory 

 centers (see Fig. 45, p. 114), and above these last two is spread 

 the great expanse of the cerebral cortex or pallium. The corpus 

 striatum consists of masses of gray matter separated by sheets 

 of white matter, an arrangement which gives a striated appear- 

 ance in section. 



In studying the comparative anatomy of the cerebral hemi- 

 spheres we find the corpus striatum well developed in some lower 

 vertebrates which lack the cerebral cortex, and very highly de- 

 veloped in others, like reptiles and birds, where the cortex is 

 present, though very small. In these animals the corpus stri- 

 atum appears to be a reflex center of great importance and of 

 higher order than the thalamus; and the dfferentiation of this 

 apparatus seems to have been a necessary precursor of the elabo- 

 ration of the cerebral cortex as we find it in the mammals. 



The functions of the mammalian corpus striatum are very 

 obscure. It is connected by both ascending and descending 

 fibers with various nuclei of the thalamus and cerebral peduncle, 

 and also with the cerebral cortex. Ramon y Cajal is of the 

 opinion that the mammalian striatum functions chiefly to re- 

 inforce the descending motor impulses which leave the cerebral 

 cortex, these systems of fibers giving off collateral branches as 

 they traverse it, and the striatum itself sending important de- 

 scending tracts into the thalamus and cerebral peduncle. 



The white matter of the corpus striatum consists partly of the 

 fibers already mentioned as passing between it and the thalamus 

 and cortex, but chiefly of fibers passing between the cortex and 

 deeper parts of the brain stem, having no functional connection 

 with the striatum itself. These are called projection fibers. 

 They are partly ascending and descending fibers passing between 

 the thalamus and the cortex (the optic, auditory, and somesthetic 

 projection systems, or radiations, which have already been 

 mentioned, p. 165), and partly descending motor projection 

 fibers of the cortico-spinal or pyramidal tract (p. 140 and Fig. 



