CH. L.] 



MYELOGENETIC FIELDS 



741 



impulses pass first to the auditory area, then by certain fibres to the cerebral cells 

 which control the muscles of the arm. The fibres which connect the two areas are 

 termed association fibres. A diagrammatic view of the principal bundles of 

 association fibres is given in fig. 436, p. 699. 



The term "association centres" is given by Flechsig to those portions of the 

 cortex that lie between the sensory centres. The function of these centres 

 is first to furnish pathways between the several centres, and secondly to retain 

 as memories previous sense impressions, so that in action they may modify the 

 impulses sent into them, and by these modifications adjust to an almost infinite 

 degree the form of the final response. 



The association centres comprise a very large area of the cortex, and are 

 divided into three: (1) The great anterior association centre in the frontal 

 lobe ; (2) the posterior association centre in the parieto-temporal region ; (3) the 



FIG. 454. Lateral view of the left cerebral hemisphere of man (after Donaldson). V is the cortical area 

 damage to which produces " word blindness " ; it is situated in the angular gyrus, and is called the 

 visual word centre. H is the area in the superior temporal convolution, called the auditory word 

 centre, damage to which produces "word deafness." S is Broca's convolution, damage to which 

 produces loss of audible speech (motor aphasia); this is the motor area for the movements 

 of the tongue, vocal cords, etc., concerned in speaking; Bastian terms it the glosso-kincesthetic area. 

 The area W, called by Bastian the cheiro-kinwsthetic area, is the corresponding region concerned in 

 hand movements, damage to which abolishes the power of writing (agraphia). 



middle association centre ; this is smaller and coincides with the island of Reil. 

 These regions are in fact those in which no evident response follows excitation ; 

 they are sometimes called the " latent or silent or inexcitable cortex." The human 

 brain is characterised by the high development of these parts, and as already 

 explained they are doubtless, as Flechsig terms them, the organs of thought. 



The importance of the association of ideas, which has for its anatomical basis 

 the association of cortical centres, will be at once grasped when one considers such 

 complex actions as speaking, reading aloud, or writing from dictation. The 

 accompanying diagram (fig. 454) shows the position of the main centres involved, 

 particulars of which will be found in the small text beneath the figure. 



In reading aloud, the impressions of the words enter by the eyes, reach that 

 portion of the visual sphere known as the visual word centre, travel across to the 

 auditory word centre by association fibres, where the memory of their sounds is 

 revived'; another tract of association fibres connects this to the sensori-motor area 

 in Broca's convolution called by Bastian the alosso-kincesthetic area, whence motor 

 impulses originate which finally reach the muscles concerned in pronouncing the 

 words originally seen. 



Writing from dictation is just as complex ; the course of the impulses is by 

 the auditory channels to the auditory word centre, then by association tracts to the 

 visual word centre, where the shapes of the letters composing the words are 

 revived ; another association tract carries the impulse thence to the sensori-motor 



