CH. XLVII1.] CENTRES FOR SPEECH AND WRITING 



695 



complex actions as reading aloud or writing from dictation. The accompanying 

 diagram (fig. 505) shows the position of the main centres involved, particulars of 

 which will be found in the small text beneath the figure. 



FIG. 505. Lateral view of the left cerebral hemisphere of man (after Donaldson), v is the cortical area, 

 damage to which produces "word blindness " ; it is situated in the angular gyrus, and is called the 

 visual word centre. H is the area m the superior temporal convolution, called the auditory word 

 centre, damage to which produces "word deafness." s is Broca's convolution, damage to which 

 produces loss of audible speech (motor aphasia) ; this is the sensori-motor area for the movements 

 of the tongue, vocal cords, etc., concerned in speaking; Bastian terms it the glosso-kinfesthetic area. 

 The area w, called by Bastian the chein-kincesthetic area, is the corresponding region concerned in 

 hand movements, damage to which abolishes the power of writing (agraphia). 



In reading aloud, the impressions of the words enter by the eyes, reach that 

 portion of the visual sphere known as the visual word centre, travel across to the 

 auditory word centre by association fibres, where the memory of their sounds is 

 revived'; another tract of association fibres connects this to the sensori-motor areas 

 in Broca's convolution called by Bastian the glosso-kincesthetic area, whence motor 

 impulses originate which finally reach the muscles concerned in pronouncing the 

 words originally seen. 



Writing from dictation is just as complex ; the course of the impulses is by 

 the auditory channels to the auditory word centre , then by association tracts to the 

 visual word centre, where the shapes of the letters composing the words are 

 revived ; another association tract carries on the impulse to the sensori-motor area 

 connected with the movements of the hand (Bastian's cheiro-kincesthetic area) in the 

 middle region of the Rolandic cortex, and finally the movement of writing is 

 accomplished. The diverse symptoms exhibited by patients suffering from various 

 forms of aphasia can be all explained by more or less extensive damage either to 

 the centres themselves or to the association tracts which connect them. 



The association fibres of the spinal cord are described on p. 618. 



In the development of a neuron, four stages can be distinguished : (1) Cells 

 without processes ; (2) the appearance of simple branches, the axon developing 

 most rapidly ; (3) the formation of collaterals ; (4) the appearance of the medullary 

 sheath. In the cerebral convolutions the fibres become myelinated in a strictly 

 regular sequence; some convolutions have their fibres medullated three months 

 before birth, while in others complete myelination has not occurred six months later. 

 Fibres of equally great importance become medullated at the same time ; those of 

 primary importance first, and so on. In this way, myelogenetic cortical fields can 

 be mapped out, which retain their contours for some time. Thirty-six of such fields 

 were made out by Flechsig, and can be divided chronologically into three groups, 

 primary, intermediate, and terminal. 



The primary fields are darkly shaded in the accompanying diagrams (figs. 506 

 and 507). 



