1038 CORTICAL MOTOR REGION. [BOOK in. 



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 system of fibres to the spinal cord and motor spinal roots. 

 The anatomical considerations advanced in a previous section 

 lead us to suppose that the fibres in question belong to the great 

 pyramidal tract, on which we have so much insisted ; 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 outside 

 which stimulation never produces extension of the hind limb. All 

 that is meant is that extension of the hind limb is the salient and 

 striking result of stimulating the area. When we 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 belonging 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 similar 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 localisa- 

 tion 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 localisation may be observed, but far less 

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

 hardly recognisable in the frog. It will not be profitable to dwell 

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

 monkey, leading up as they do to those of man, call for special 

 notice. 



655. When in a monkey, in an individual for instance 

 belonging to the genus Macacus, the surface of the cerebrum is 



