618 PHYSIOLOGY CHAP. 



exclusively to the cortex. Dejerine recognised that the pre-f'rontal 

 lobe, which represents an association area, contains a bundle of 

 projection fibres running to the thalamus, and particularly to its 

 nucleus interims. In the parietal lobe again, and especially in the 

 angular gyrus, there are, according to Dejerine, projection fibres 

 that run to the pulvinar and posterior part of the lateral nucleus 

 of the thalamus, which degenerate after lesions of those regions of 

 the cortex. These are projection fibres whose function is not to 

 conduct sensory and motor impulses, but to associate the cortical 

 with the sub-cortical psychical centres. 



Monakow, on the other hand, observed that the sensory and 

 motor centres are also provided with association fibres, and indeed 

 contain more association than projection fibres. But even if we 

 accept the accuracy of this fact, which Flechsig denies, it does not 

 follow that the structural difference between the projection centres 

 and association centres is not sufficiently marked to enable them 

 to be readily distinguished and identified by simple enibryological 

 features. There is, of course, no absolute difference between the 

 two classes of centres, but merely a relative and gradual difference. 

 It would be a mistake if the terms sensory and motor centres on 

 the one hand and purely psychical centres on the other were taken 

 to exclude all representative or ideative capacity from the former. 

 But it is only reasonable to suppose at least it is a probable 

 hypothesis that the latter have more important psychical functions 

 than the former. 



This hypothesis appears to be supported by comparative 

 anatomy and physiology, which show that the surface of the in- 

 excitable association centres of the cerebral cortex increases 

 progressively in proportion as the intelligence of the animal rises. 

 In the lower mammals, as the rodents, there are no association 

 centres, and consequently the sensory and motor centres are in 

 contact ; in carnivora the association centres are little developed 

 and hard to identify by Flechsig's method; they increase con- 

 siderably from the lower apes to the anthropoids ; and finally in 

 man they extend over the greater part of the cerebral cortex. 



If we study the chronological order in which the nerve-fibres 

 of the different cortical fields become myelinated, as shown in 

 Figs. 299, 300, we find another argument in support of the view 

 that the association centres have a higher psychical function than 

 the sensory and motor centres. Myelination in fact commences 

 with the ascending cortical afferent fibres which reach the 

 sensory areas of the cortex ; next the cortico-motor bundles 

 descending from the motor cortical centres become myelinated ; 

 and lastly the arcuate fibres, which serve to bring the different 

 cortical fields into inter -communication, obtain their myelin 

 sheaths. The association centres are ontogenetically the last 

 to attain anatomical maturity, for the very reason that they have 



