THE CENTRAL NERVOUS SYSTEM 851 



other hand, disturbances of movement on the right side were 

 very noticeable up till its death. It learned again to use the 

 right limbs in locomotion ; but, although they were not markedly 

 weaker than those of the left side, their movements had a certain 

 clumsiness, which was associated with a permanent diminution 

 in the sensibility of the skin of these limbs. Muscular sensibility 

 was also lessened. In acts requiring the use only of one hand, 

 the right was never willingly employed, and it evidently cost 

 the animal a great .effort to use it in such movements, but by 

 special training it learnt again to give the right hand when 

 asked for it, and to make use of it for other purposes. The 

 movements with which the motor areas are concerned are essen- 

 tially, skilled movements, and we may suppose that it is more 

 difficult for a monkey to educate again a centre for such complex 

 and elaborate manoeuvres as are performed by its hand than 

 for a dog to regain normal control of the comparatively simple 

 movements of its paw. In man in cases of hemiplegia, when 

 the patient lives for some time, a certain amount of recovery 

 usually takes place, especially in young persons, in the paralyzed 

 leg, but much less in the paralyzed arm. 



In the lower monkeys the ' motor ' area was formerly 

 stated to extend behind the sulcus centralis into what in man 

 would be called the ascending parietal convolution (postcentral 

 gyrus), and also to be more extensively represented on the 

 mesial surface of the hemisphere than in the higher apes 

 (Figs. 353, 354). Such observations, however, require to be 

 reinterpreted in view of the results of Sherrington and Griin- 

 baum, especially as they were carried out by the bipolar method 

 of stimulation, with both electrodes on the cortex. This method 

 does not admit of such strict localization of the stimulus as the 

 unipolar method. The most recent work with the unipolar 

 method has indicated that in the lower apes also excitation of the 

 gyrus postcentralis does not cause movements (C. and O. Vogt). 



It is in the light of the results obtained in monkeys, and by 

 the aid of histological, embryological, clinical, and pathological 

 observations, that the ' motor ' areas in man have to a great 

 extent been mapped out. 



The histological differentiation of the various cortical regions recently 

 demonstrated by Brodmann and by Campbell are of especial interest 

 (Figs. 356-360). It has long been customary to divide the cortex 

 into layers, although the number and the boundaries of these layers 

 are somewhat arbitrarily fixed. Brodmann distinguishes six layers : 

 (i) A zonal or peripheral layer, containing many nerve-fibres and 

 neuroglia cells, but few nerve-cells ; (2) a layer containing ' granules ' 

 and small pyramidal cells (external granular layer] ; (3) a layer of 

 medium and large pyramidal cells (pyramidal layer) ; (4) a layer of 

 small irregular cells (internal granular or stellate layer) ; (5) a 



542 



