6 
PROFESSORS V. HORSLEY AND E. A. SCHAFER 
movements may be marked out.'" It will be convenient, in the first place, to describe 
those portions of these areas which are seen upon the external surface of the brain 
(see Diagram I.), and subsequently those which ai'e met with upon the mesial surface.t 
The arm-area occupies a portion of the cerebral surface which is triangular in 
shape, being broad behind and narrow in front. It comprises most of the upper half 
of the ascending pai-ietal and ascending frontal gyri, from a little below the level of 
the sagittal part of the precentral fissure below, nearly to the margin of the 
hemisphere above, together with the adjacent part of the frontal lobe below the small 
antero-posterior sulcus marked x. In front of this sulcus it bends round to the 
mesial surface and is continuous with a part of the marginal gyrus, excitation of which 
also produces movements of the shoulder and arm, and which must therefore be 
regarded as also belonging to the arm-area. 
Diagi'am I. 
Over a large part of this area the actual movement which is obtained is the raising 
and protraction of the arm and hand, described by FEfiEiEK, as resulting from excita- 
tion of the area marked (5) in his diagram. In this movement most of the muscles of 
the shoulder and arm share in some degree or other, the exact part taken by one or 
other group, and consequently the effect produced, varymg with the portion of the 
* Although it is convenient for purposes of description to represent the areas and centres of the 
motor region as if tbey were sharply marked ofE from one another, there is no distinct evidence that 
this is really the case ; but, on the contrary, if we are to accept the results of excitation (and, according to 
LuciANi, they are confirmed in this particular by the results of localised ablation), there are no such sharp 
lines of demarcation, but every centre and area overlaps to a greater or less extent the surrounding areas. 
t More precise details regarding the character of the movements evoked on excitation of the several 
parts of each area are given by Professor Schafee in an article " Ueber die motorischen Rindencentren 
des Affengehims " in ' Beitrage zur Physiologic, C. Ludwig gewidmet,' Leipzig, 1887. 
