THE CENTRAL NERVOUS SYSTEM 855 



free from such fibres. In this way Flechsig has distinguished thirty - 

 cortical fields (Figs. 361, 362), which he divides according to the ti 



r -six 

 time 

 of myelination into three groups : 



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

 myelinated 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 illustra- 

 tion 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 ter- 

 minal 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 con- 

 veying impressions from the visual centre to the motor cortex 

 when the hand is sketching a landscape. It may also be con- 

 sidered a function of these association centres to store up the 

 memories of previous sense impressions. Flechsig 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 association 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 con- 

 clusions 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. 863), and Hughlings Jackson had 

 associated pathological lesions of the Rolandic area with certain 

 cases of epileptiform 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 para- 



