282 



THE GROWTH OF THE BRAIN. 



system are such that it reacts more perfectly to a series 

 of slight stimuli than to a small number of strong 

 ones. When in an afferent nerve an impulse is aroused 

 under these conditions it passes on to a cell-body, and 

 there initiates changes which, though in the main 

 similar in different cells, have probably local peculiarities 

 according to the elements involved. These changes 

 cause a second set of impulses, which may leave the 



FIG. 63. A record of the variation in the contraction of a muscle, 

 and the electric condition of the pyramidal fibres upon stimu- 

 lation of the cerebral cortex by electricity. (Gotch and 

 Horsley.) I, contractions of the muscles, first tonic then 

 clonic ; II, variations in the electrical condition of the pyra- 

 midal fibres, as recorded by a capillary electrometer; III, 

 record of the electric stimulus applied to the cortex. The 

 duration of the stimulus is measured by the length of the 

 elevation in the record. 



cell-body with a rate and force quite different from the 

 first. The rapidity with which the impulses follow one 

 another has been recently studied with success. When 

 during an epileptic attack of cortical origin the muscles 

 of a limb are thrown into contraction, they are first held 

 in a tonic spasm, which in turn gives way to a clonic 

 series. The record of the muscular contractions during 

 such an attack would be similar to that expressed by 

 the accompanying curve (I), Fig. 63. The curve (II), 



