THE PHYSIOLOGY OF THE BRAIN. 623 



cerebral cortex : it is therefore probable that in man some of 

 the lower and simpler mental faculties are associated also with 

 those parts. There are, however, great and obvious chances 

 of error in arguing from the actions of the lower animals as to 

 their mental state : and these are increased by the compara- 

 tively small proportion the cerebral cortex bears to the whole 

 cerebro-spinal centre in these animals when compared with its 

 ratio in man, showing its less importance in the management 

 of their actions. Hence the most useful observations are 

 those made of late years on apes and monkeys and on men 

 suffering from local brain disease. By utilizing these it has 

 been possible to map out certain areas of the brain surface 

 as having special, though possibly not absolutely unshared 

 association, with volitional movement and with groups of 

 sensations and sensory interpretations. In addition to facts 

 obtained by removal or local disease of parts of the brain we 

 have others obtained by electrical stimulation of certain 

 parts of the cortex, which although quite insensible to cut- 

 ting or mechanical irritation does in some places respond to 

 application of the interrupted or constant electric current. 

 The more important results obtained are indicated in a 

 general way in Figs. 179 and 180, representing respectively the 

 outer and inner surfaces of the right cerebral hemisphere; 

 these diagrams should be compared with the more detailed 

 figures in Chapter XI. 



The shaded area beginning on the top of the brain and 

 extending down the sides of the fissure of Rolando or central 

 fissure, Ro, and beyond its ventral end is the motor area of 

 the cortex. It also extends to the inner side of the hemi- 

 sphere, as shown in Fig. 179. Electric stimulation of dif- 

 ferent parts of this area causes movements of leg, arm, or 

 face as indicated. Removal of the region marked " arm " in 

 the monkey causes motor paralysis and some loss of sensi- 

 bility in the arm on the opposite side of the body. It is 

 also followed by degenerations extending from the re- 

 moved region of cortex through the internal capsule to 

 some pyramidal fibres in the pes and thence back through 

 the pyramids to the crossed pyramidal and direct pyram- 

 idal tracts in the cord as far as the cervical enlarge- 

 ment. Localized disease of this area in man is followed 

 by paralysis of voluntary movements of the opposite 'irm 

 and by similar degenerations. Similar statements are true 



