9$3 THE CENTRAL NERVOUS SYSTEM 



such fibres. In this way Flechsig has distinguished thirty-six cortical 

 fields (Figs. 392, 393)- which he divides according to the time of t 

 myelination into three groups: 



1. Primary fields, ten in number, which are well provided with mye- 

 linated fibres at birth. They include the cortical centres for the 

 various sensations and also the ' motor ' area. They are connected 

 especially with the so-called projection fibres. Thus, the cutaneous 

 and muscular sense is assumed to be represented in field i, the sense 

 of smell in field 2, of vision in 4, and of hearing in 5. From field i 

 arise the fibres of the pyramidal tract, chiefly from the ascending 

 frontal convolution, while the sensory fibres from the skin and muscles 

 end mainly in the ascending parietal. This is an illustration of what 

 Flechsig considers a general rule for these primary fields viz., that 

 each primordial sensory region is connected both with an afferent 

 (cortici-petal) and with an efferent (cortici-fugal) tract. From the 

 visual area (4), e.g., arises a tract which proceeds mainly to the anterior 

 corpus quadrigeminum. 



2. Terminal fields (32 to 36 in the figures) which become myelinated 

 late, the process not beginning until at least a month after birth. 



3. Intermediate fields (n to 31) which become myelinated earlier 

 than the terminal, but later than the primary. They and the terminal 

 fields constitute par excellence association centres, which furnish fibres 

 (association fibres) connecting the centres represented in the primary 

 fields e.g., such fibres as must be continually conveying impressions 

 from the visual centre to the ' motor ' cortex when the h'and is sketching 

 a landscape. It may also be considered a function of these association 

 centres to store up the memories of previous sense impressions. Flech- 

 sig divides the association centres represented in the terminal fields 

 into (i) The great anterior association centre in the frontal lobe in 

 front of the ' motor ' area; (2) the great posterior association centre in 

 the parieto-temporal region; (3) the smaller middle or insular associa- 

 tion centre, which coincides with the island of Reil, an area which, 

 according to Sherrington and Griinbaum, is totally ' inexcitable ' as 

 regards the production of movement in the anthropoid apes. These 

 association centres are foci, from which issue and to which come the 

 long association paths. The reader must bear in mind that Flechsig's 

 conclusions as to the functions of his very numerous areas are in many 

 cases hypothetical, and can only be accepted when corroborated by 

 other methods. We are far from being able at present to subdivide 

 the functions of the cortex so minutely as is suggested by his map. 



Clinical and Pathological Observations in man agree, upon the 

 whole, with wonderful precision with the results of experiments on 

 animals ; and, indeed, before any experimental proof of the minute 

 and elaborate subdivision of the cortex had been obtained, Broca 

 had already, from the phenomena of the sick-bed and the post- 

 mortem room, located a centre for speech in the left inferior frontal 

 convolution (but see p. 971), and Hughlings Jackson had associated 

 pathological lesions of the Rolandic area with certain cases of epi- 

 leptiform convulsions. 



An extensive haemorrhage involving the Rolandic area of the 

 cerebral cortex, or an embolus blocking the middle cerebral artery, 

 causes paralysis of the opposite side of the body. An embolus of a 

 branch of the middle cerebral artery causes paralysis of the muscles, 



