664 THE BRAIN. 



In order to carry out a closer analysis of the phenomena it is desirable 

 to watch or record the contraction of a particular group of muscles, or per- 

 haps better still a particular muscle, e. g., the area for extension of the hind 

 limb may be studied by help of the extensor digitorum communis of the 

 limb. When this is done, the following important facts may be observed : 

 The area of cortex having been found which gives the best movements, and 

 the stimulus being no stronger than is necessary, isolation of the area from 

 its lateral surroundings by a circular incision carried to some little depth 

 will not prevent the development of contractions in the muscle ; but these 

 do cease, even without the circular incision, if by a horizontal section the 

 gray cortex is separated from the subjacent white matter. After removal 

 of the cortex, stimulation of the white matter underlying the area produces 

 the appropriate contraction ; not only, however, is a stronger stimulus 

 necessary, but also the latent period, that is the time intervening between 

 the beginning of the application of the stimulating current and the begin- 

 ning of the muscular contraction, is appreciably shortened. The appropriate 

 contractions not only appear when the white matter immediately below the 

 cortex is stimulated, but by making successive horizontal sections and stim- 

 ulating each in turn, the effect may, so to speak, be traced through the cen- 

 tral white matter of the hemisphere 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 gray matter are 

 stimulated, the process having a marked latent period, and that as the out- 

 come of the changes induced in the gray matter, impulses pass along the 

 fibres leading down from the gray 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 de- 

 fined 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 stimulation 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 strik- 

 ing result of stimulating the area. When we study the various movements, 

 and especially perhaps when we study, by help of a graphic record, the con- 

 tractions 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 overlap- 



fing would be so great that the whole figure would appear highly confused. 

 n a similar way the excitable motor region as a whole would gradually 

 merge into, be broken up into, the unexcitable frontal, occipital, and tem- 

 poral 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 hier- 

 archy, is intermediate between such animals as the rabbit, the bird, and the 



