CII. L.] ELECTRICAL VARIATION IN CENTRAL NERVOUS SYSTEM 743 



They are 10 in number, and are those provided with myelinated fibres at birth ; 

 they contain the seats of the cortical representation of all the senses. To No. 1 is 

 assigned the cutaneous and muscular sense ; to No. 2 the sense of smell ; to No. 4 

 that of vision ; to No. 5 that of hearing. The functions of some of the primary 

 areas had not been determined. The principal efferent projection tracts originate 

 from the primary fields ; thus the pyramidal tract starts from part of No. 1, namely, 

 from the ascending frontal convolution. The sensory fibres connected with the 

 skin and muscles terminate in the ascending parietal convolution. The inferior 

 fornix is connected with Nos. 2 and 3. The inner bundle of the pes springs 

 from 1 B, 6, 12, 14 and 15 ; the origin of the outer bundle of the pes is doubtful. 

 From the visual area (No. 4) a tract arises which passes mainly into the anterior 

 corpus quadrigeminum ; the auditory zone (No. 5), towards which a tract proceeds 

 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 association fibres. 



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 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. 



