CH. XLVIII.] ASSOCIATION FIBRES AND CENTRES 



693 



myelinated before the efferent, in the central nervous system, but in the case 

 of the nerve-roots this is reversed, the anterior root fibres being myelinated before 

 the posterior. 



Held has also demonstrated the important influence of stimulus on myelination. 

 His experiments were made on cats, dogs, and rabbits, which are born blind. If 

 light is admitted to one eye by 

 opening the lid, more obvious 

 myelination is found in the cor- 

 responding optic nerve, than in 

 that of the opposite side. This 

 is not due to the irritation caused 

 by forcibly opening the lid, for if 

 the lid be opened and the animal 

 kept in the dark, no difference in 

 the myelination of the two optic 

 nerves is observable. Flechsig 

 also showed that a child born 

 at 8 months had more marked 

 myelination of its optic nerves, 

 a month later, than a child born 

 in the usual way at the ninth 

 month. 



The richness of the brain in 

 myelinated fibres increases for 

 many years after birth with the 

 progress of intellectual develop- 

 ment. Kaes states this continues 

 up to forty years of age, and that in old age the number diminishes. Myelin 

 appears to be necessary for the functional activity of nerve tracts, and its 

 development progresses pari passu with development of function; the reverse 

 change (atrophy and degeneration) is correspondingly accompanied with marked 

 disturbances of function. 



FA.C: 



-C. 



FIG. 502. Diagram of vertical section'of the brain of a child 

 5 months of age. The greater part of the white matter 

 now shows myelination, thus indicating development of 



the 



the association centres. The letters have 



meaning as in Fig. 688. (After Flechsig; Weigert 



method of staining.) 



Association Fibres and Association Centres. 



We know by common experience that any group of muscles can be voluntarily 

 contracted in reply to any form of stimulus, cutaneous, visual, auditory, etc. If, 

 for instance, the wrist is flexed in response to an auditory stimulus, the nerve 

 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. 503. This figure may be usefully compared with 

 the next (fig. 504), which shows the general plan of the projection fibres. 



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

 cortex that lie between the sensory centres he has been able to demonstrate. The 

 function of these centres is first to furnish pathways between the several centres, and 

 second 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 

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

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

 they are sometimes called the "latent 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 



