554 PHYSIOLOGY CHAP. 



fields or areas (Figs. 279 and 280) : (a) the area connected with 

 movements of the head and eyes ; (&) that connected with move- 

 ments of the face, including those of the month, cheeks, and 

 larynx ; (c) that related to the movements of the upper limbs ; 

 (d) that for movements of the lower limbs ; (e) that connected 

 with movements of the trunk and tail. These areas are not limited 

 by sulci or other structural features ; so they are not distinct, as 

 would appear from the diagram, but merge gradually one into 

 another. When the faradic stimulus is moderate and falls between 

 the limits of one of the areas, the reactions are confined to a single 

 region ; when the stimulus is strong and protracted the reactions 

 spread into the neighbouring regions ; when the stimulus falls at 

 the point of transition between two areas, muscular reactions can 

 be elicited from both regions by even a moderate excitation. 



On stimulating different points of the five areas above enumer- 

 ated, Beevor and Horsley obtained a further specialisation of the 

 reactions shown on the two figures reproduced. These intra- 

 regional localisations are more definite and pronounced in propor- 

 tion as the areas are wider and the reactions more circumscribed. 

 This holds especially for the movements of the fore-limbs and the 

 face. The reactions usually permit of distinction into a relative 

 area and a small principal area or focal point, on stimulating 

 which the given movement results with greater promptness and 

 precision. The movement is rarely simple, e.g. flexion or extension 

 of the thumb ; more frequently complex movements result, simul- 

 taneously or in succession. The reactions obtained most constantly 

 and promptly may be termed primary movements in distinction 

 from the secondary which occur rarely. 



The most salient characteristic of these reactions as Terrier 

 first pointed out is their purposive co-ordination, as though they 

 were evoked by an act of volition. The impression made is that 

 the voluntary movements most frequently carried out by these 

 animals are those most readily obtained by electrical stimulation 

 of the cortex. Thus, on stimulating certain points of the area for 

 the arm, it is easy to elicit a series of prehensile movements ; on 

 stimulating certain lower points of the area for the face, a series of 

 complex mastication movements is obtained, which are character- 

 istic of Macacus. When the reaction elicited by electrical stimula- 

 tion is not co-ordinated, it can often be shown that the surface of 

 the brain is in a condition of abnormal excitability, which causes 

 excitation to spread. 



The results which Beevor and Horsley (1890) obtained by 

 faradising the cerebral cortex of an orang-outang, and those from 

 the wider experiments of Griinbaum and Sherrington (1901-3) on 

 anthropoid apes, are of great interest, since the configuration of the 

 anthropoid brain is closely allied to that of man. 



Beevor and Horsley found that the complex excitable areas of 



