CENTRAL NERVOUS SYSTEM. 257 



The term "association centres" is applied by Flechsig 1 to those portions 

 of the cerebral cortex that lie between the sensory centres which he has been 

 able to demonstrate. The functions of the association centres are first to 

 furnish pathways, more or less intricate, between the several centres, and 

 second, to retain as memories previous sense impressions, so that in acting 

 they also modify the impulses sent into them, and by these modifications 

 shade and adjust to an almost infinite degree the form of the final response. 



On looking at Figs. 109, 110, we note two well-defined areas: (1) that 

 occupying the frontal lobe and forming the great anterior association centre, 

 and (2) the area in the parietotemporal region which forms a second, the 

 posterior association centre. The third, the middle association centre coin- 

 cides with the Island of Reil, and is much less in evidence. On comparison 

 it will be seen that these regions correspond to what have been called the 

 " latent areas " of the cortex, because no evident response follows the direct 

 stimulation of them. When we compare the extent of these association 

 centres in man with that in other mammals, even the apes, we find the 

 human brain characterized by the high development of these portions. 

 Thus Flechsig feels justified in speaking of these association centres as the 

 "organs of thought," and in pointing out how by means of them the incom- 

 ing sense impressions are made to interact on one another, and in combination 

 with the memory images which are thus aroused give rise to new ideas. 



The association processes carried on by these several centres, are modified 

 by their location, so that the several centres have different and distinct values. 

 With the disturbance of these association centres are correlated the several 

 sorts of mental defects which have been gathered under the term aphasia. 



Aphasia. — The development of the ideas bearing on this subject has been 

 slow. After the publication of the great work of Gall and Spurzheim (1810— 

 19) on the brain, some pathologists (Bouillaud, 1825; Dax, 1836), especially 

 in France, were in seach of evidence touching the doctrine of the localization 

 of function. At the same time the subject of phrenology , as put forward by 

 Gall and Spurzheim, was not in good repute, and anything which looked 

 that way, even in a slight degree, was generally scouted. Broca, however, 

 published (1861) the important observation that when the most ventral or the 

 third frontal convolution in the left hemisphere (often designated Broca's eon- 

 volution) was thrown out of function, the power of expression by spoken 

 words was lost. For this reason, the name of "speech-centre" has been 

 applied to this convolution. 



Since this discovery which links the neurology of the first pari of the cen- 

 tury with that of to-day, and also forms a fundamental observation in the 

 modern doctrine of cerebral physiology, many steps have been taken. 



It was early observed that although in such cases the capacity for spoken 

 language was lost, nevertheless the muscles which were used in the act of 

 phonation were by no means paralyzed. This relation is due probably to the 

 fact that the muscles are innervated from both hemispheres and possibly also 



1 Flechsig : /.'»•. at. 

 Vol. II.— 17 



