770 



THE CEREBRAL CORTEX. 



speak of as the inexcitable or latent portions, since they are dis- 

 tinguished from the rest by the fact that their excitation produces no 

 obvious response, and their excision no definite paralysis, their function 

 is at present a matter of conjecture. 1 What appears clear is, that any 



A extensive lesion, es- 



pecially bilateral, of 

 these portions, leads 

 to alterations in the 

 intellectual condition 

 of the patient, altera- 

 tions which appear to 

 vary directly with the 

 amount destroyed. 2 

 These inexcitable 

 portions of brain sur- 

 face are chief! y massed 

 in the prefrontal, in 

 the post-parietal, in 

 the temporal, and in 

 man in the occipital, 

 regions ; to which 

 must be added the 

 cortex of the island 

 of Keil. 



According to 

 Flechsig, the fibres 

 of the corona radiata 

 of all these parts be- 

 come myelinated far 

 later than those of 

 the excitable areas, 

 and very few of the 

 fibrespassdown wards 

 as projection fibres, 3 

 but nearly all belong 

 to that class of inter - 

 nuncial fibres which 

 were termed associ- 

 ation fibres by Mey- 

 nert, 4 from the fact 



1 An anatomical distinction between the excitable and latent areas consists in the fact 

 that whereas the latter all belong to the five-layered cortical type of Meynert, the excitable 

 regions, and to these must be added the gyrus fornicatus and hippocampal lobe, all possess, 

 either in addition to or in substitution for the usual five layers of cells, certain peculiarities 

 of structure, e.g. the giant cells of the Rolandic area, the small stellate cells of the visual 

 area, the special features of the hippocampal region, etc. 



- It must, however, be observed that this statement is probably true of the excitable 

 regions also. 



3 



Fig. 351. — Diagrams to show the supposed division of the 

 cerebral cortex into projection and association areas, the 

 former being shaded with dots. A, lateral ; B, mesial sur- 

 face. — Modified from Flechsig. 



3 Dejerine, however, finds that the prefrontal lobe is united to the internal nucleus 

 the thalamus by numerous fibres (Compt. rend. Soc. dc biol., Paris, 1897, p. 178) ; and oth 



nucleus of 

 ler 

 authorities have come to a similar conclusion ("Versl. Deutsch. Naturf. u. Aerzte," 

 Nenrol. Centralbl., Leipzig, 1896, S. 995). Flechsig, ibid., 1897, S. 290, finds that the 

 internal nucleus of the thalamus is developed proportionally with the development of the 



so-called association centres 



4 Article "Brain" in Strieker's "Handbook" 

 and "Neue Studien ueber die Associationsbundeln 

 Akad. d. Wisscusch., Wien, 1892, Bd. ci. 



also "Psychiatrie," Wien, 1884, 

 des Hirnmantels," Sitzungsb. d. k. 



