GIL XLVIII.] ELECTKICAL CHANGES IN BRAIN 697 



that leads from the internal corpus geniculatum, sends an outgoing tract into the 

 column of Tiirck, and thus motor functions of the upper part of the body are 

 possible as a direct result of auditory impressions. In fact in every case each 

 primordial sensory zone is connected with a well-defined pair of tracts, one proceed- 

 ing to it (cortico-petal) and the other from it (cortico-fugal). It is thus impossible to 

 speak of a purely motor or a purely sensory area. 



The terminal areas (Nos. 31 to 36, unshaded in the diagrams) do not begin to 

 be myelinated until at least a month after birth. These and the majority of the 

 intermediate areas (Nos. 11 to 31, lightly shaded in the diagrams) show few or no 

 projection fibres * even 8 months after birth. They comprise, in fact, the association 

 centres, and are rich in long associatipn fibres. 



The view of Monakow, Dejerine, and others, that the fasciculus longitudinalis 

 inferior (E in fig. 503, p. 694) and the cingulum (c in the same figure) are long associa- 

 tion tracts is denied by Flechsig; they connect primordial zones, and are regarded 

 by him as projection fibres, the former connecting the lateral corpus geniculatum 

 with the cortical field of vision, and constituting the real optic radiation. 



Electrical Variation in Central Nervous System. 



Du Bois Keymond found that the spinal cord, like a nerve, 

 exhibits a demarcation current between its longitudinal surface and 

 a cross-section, and that a diminution of this current occurs on 

 excitation (negative variation). Gotch and Horsley investigated the 

 currents of the cord very thoroughly. If the Eolandic area of the 

 cortex is stimulated, and a portion of the thoracic region of the 

 spinal cord is led off to a galvanometer, a persistent negative varia- 

 tion followed by a series of intermittent variations is observed ; this 

 exactly corresponds to the tonic spasm followed by clonic con- 

 tractions which occur in the muscles excited by this means. 



The galvanometer in the hands of these observers also proved to 

 be a valuable instrument for determining the paths taken by nervous 

 impulses in the cord. One example will suffice : If the central end 

 of one sciatic nerve is stimulated, the chief electrical variation in the 

 cord is noticed to be obtained when the same side of the cord is led 

 off to the galvanometer, but a certain amount of electrical variation 

 is obtainable from the opposite side of the cord. This coincides with 

 the fact ascertained by other methods, that the main sensory 

 channel is on the same side of the cord as the entering nerves, but 

 that there is a certain small amount of decussation below the level 

 of the bulb. 



Electromotive changes also occur during activity in the cortex 

 cerebri, but they have not been much studied, and we do not know 

 whether they have their seat in the grey or in the underlying white 

 matter. 



Sleep. 



The conditions that favour sleep are : 



(1) A diminution of the impulses entering the central nervous 

 system by the afferent channels. This is under our voluntary 



* That they never have any projection fibres at all is denied by most observers, 



