CHAP, ii.] THE BRAIN. 743 



speak, be traced through the central white matter of the hemi- 

 sphere down to the internal capsule. We may conclude from 

 these results, that when the current is applied to the surface of 

 the cortex, certain parts of certain structures in the grey matter 

 are stimulated, the process having a marked latent period, and 

 that as the outcome of the changes induced in the grey matter, 

 impulses pass along the fibres leading down from the grey 

 matter to the internal capsule, and so by the pedal fibres of the 

 cms to the spinal cord and motor spinal roots. Anatomical 

 considerations lead us to suppose that the fibres in question 

 belong to the great pyramidal tract ; and as we shall see, all our 

 knowledge confirms this view. 



It must not, however, be supposed that the several areas 

 stimulation of which produces each its distinctive movement, are 

 in the dog sharply defined from each other ; when the term area 

 for extension of the hind limb is used it must not be supposed 

 that the area can be defined by an outline, within which stimula- 

 tion produces nothing but extension of the hind limb, and out- 

 side which stimulation never produces extension of the hind limb. 

 All that is meant in that extension of the hind limb is the salient 

 and striking result of stimulating the area. When AVC study the 

 various movements, and especially perhaps when we study, by 

 help of a graphic record, the contractions of various individual 

 muscles resulting from the stimulation of various parts of the 

 motor region, we find not only that the areas for particular 

 movements or particular muscles are very diffuse, but that the 

 several areas largely overlap each other. If for instance we 

 were to map out on the same diagram the several areas belong- 

 ing to four or five muscles of different parts of the body, such 

 as the extensors of the digits of the fore and of the hind limb, 

 the flexors of the same, and the orbicular muscle of the eyelid, 

 that is to say, the several areas within which in turn stimulation 

 of the cortex produced contraction of the particular muscle, the 

 overlapping would be so great that the whole figure would 

 appear highly confused. In a simular way the excitable motor 

 region as a whole would gradually merge into, be broken up 

 into, the unexcitable frontal, occipital and temporal regions, in 

 front, behind and below. In other words, the localization in the 

 cortex of the dog is to a marked degree imperfect. 



In this respect the dog, corresponding to its position in the 

 animal hierarchy, is intermediate between such animals as the 

 rabbit, the bird, and the frog, on the one hand, and the more 

 highly developed monkey on the other; and that is one reason 

 why we have taken the dog first and dwelt so long upon it. In 

 the rabbit, a similar localization ma)' be observed, but far Ir-^s 

 definite, far more diffuse; it becomes still less in the bird, and 

 is hardly recognizable in the frog. It will not be profitable to 

 dwell on the details of these lower animals; but the phenomena 



