740 FUNCTIONS OF THE CEREBRUM [CH. L. 



the efferent projection fibres and the association fibres are myelinated later. Thus 

 in the human foetus the peripheral nerves and nerve-roots become myelinated in the 

 fifth month of intra-uterine life ; of the tracts in the cord, those of Burdach and Goll 

 (exogenous fibres springing from the cells of the spinal ganglia) are the first to be 

 myelinated ; next come the tracts of Flechsig (dorsal cerebellar) and of Gowers 

 (ventral cerebellar) : these are endogenous fibres springing from cells within the cord. 

 All these tracts are afferent. The pyramidal tracts, the great efferent or motor 

 channels, are not myelinated until after birth. The whole afferent tract is myelinated 

 at birth ; these fibres have in utero been exercised in conveying impressions to the 

 afferent reception centres, the stimuli arising from contact of the fretal integuments 

 with the maternal tissues. There is also early myelination around the calcarine 

 fissure in the visual sphere, and in connection with the areas related to other special 

 senses. This is shown in figs. 452 and 453, where the condition at birth and that 

 some months later are compared. 



Ambronn and Held confirm Flechsig in finding that the afferent fibres are 

 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 



C.F. 



FA.C.- 



Fia. 453. 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 association centres. The 

 letters have the same meaning as in Fig. 452. (After Flechsig ; Weigert method of staining.) 



light is admitted to one eye by opening the lid, more obvious myelination is subse- 

 quently found in the corresponding optic nerve than in that of the opposite side. 

 This is not due to the irritation caused by forcibly opening the lid, for it 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 development. 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 part passu with development of function ; the reverse 

 change (atrophy and degeneration) correspondingly accompanies marked disturb- 

 ances of function. 



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 



