616 PHYSIOLOGY CHAP. 



the middle in the anterior perforated substance, the inner in the 

 frontal extremity of the gyrus corporis callosi. 



Brown (1879) concluded from his comparative anatomical 

 observations that there were three distinct olfactory centres. By 

 a series of careful anatomical observations, Golgi discovered that 

 the fibres of the olfactory tracts are in close relation with the 

 cells of the grey matter of the frontal lobes with which they 

 come in contact. 



The localisation of the taste-centre is at present wholly 

 unknown. Flechsig supposes, without any convincing evidence, 

 that the sense of taste is connected with the anterior part of the 

 gyrus fornicatus. But his latest researches on myelination have 

 failed to confirm this hypothesis. 



XIV. A glance at Figs. 304 and 305 (Flechsig), which repre- 

 sent the excitable areas of the cortex, shows that they extend over 

 about one-third of the surface of the human brain ; they are united 

 by projection fibres descending through the internal capsule with 

 the mid-brain and the bulbo-spinal axis, which constitute the 

 cortical sensory and motor centres. We are so far unable to 

 determine the specific function of the remaining two-thirds of the 

 cerebral cortex, which is termed latent because stimulation of it 

 gives rise to no reaction, and its excision to no permanent sensory 

 or motor disturbance. We only know that in man as well as in 

 animals extensive destruction of these inexcitable areas depresses 

 intellectual activity, proportionately with the extent of the lesion, 

 but similar effects occur after destruction of the excitable areas, 

 in addition to the sensory or motor paralysis or paresis. 



Embryological observations, particularly the work of Flechsig 

 (1880-1904), have thrown much light on this difficult subject. 

 Flechsig's method of studying the human brain during embryonic 

 development consists in ascertaining at what period different 

 bundles of fibres that make up the corona radiata, or the so-called 

 centrum ovale, acquire their myelin sheaths. The myelination 

 of any bundle of fibres is complete when the nerve elements which 

 it contains have reached their functional maturity. This maturity 

 is attained at different times by different bundles, which are 

 connected with different cortical fields. In order to bring out 

 the successive advance of myelination, Flechsig employed Weigert's 

 method, which stains all the myelinated fibres, but leaves the 

 non-myelinated fibres uncoloured. He found that in the human 

 hemisphere myelination begins at the fifth month of foetal life 

 and continues till the fourth month of extra-uterine life. 



The law of myelogenesis as formulated by Flechsig assumes 

 that functionally equivalent fibres become myelinated, that is, 

 attain their maturity, simultaneously, and fibres of different 

 functional value become myelinated at different periods. So that 

 by studying its myelogenesis the brain may be divided into a 



