350 THE BODY AT WORK 



the nerve-endings of the sensory bristles of the cheek are 

 poured. 



" Nihil est in intellectu quod non prius in sensu fuerit." 

 The organ of the intellect is the cortex of the great brain, a 

 sheet of grey matter which has developed in connection with 

 the various sense-organs. The cerebral hemisphere of an infant 

 is merely an extension of the nerve-tissue associated with its 

 sense-organs. Such it remains in a microcephalous idiot. In the 

 lower animals its capacity of growth after birth is very small. 

 But in a normal child the inflow of impressions through sense- 

 organs, the experience acquired regarding itself and its sur- 

 roundings, education, whether accidental or directed, causes 

 the extension of nerve-tissue from the sensory areas into the 

 expansible intervening zones. 



There is still some uncertainty as to the nature of the sensa- 

 tions received in the excitable area. They may be termed 

 " kinaesthetic " (sensations connected with movement) without 

 more exact definition. Some physiologists consider that tactile 

 sensations, as well as the obscure sensations, originated in the 

 nerve-endings in muscles, around tendons, or on joint-surfaces, 

 are distributed to the areas, which, when stimulated, are 

 shown to represent fingers, hand, arm, and other parts of the 

 body. Others have sought, though with doubtful success, for 

 a tactile area, independent of the kinaesthetic centres. When 

 first discovered, these centres were termed " motor," and still 

 this term may be retained, on the understanding that it does 

 not imply that the exchanges which occur in the kinaesthetic 

 centres are of a different nature to those which take place else- 

 where. The region which they occupy has become the motor 

 area of the cortex because voluntary movement is possible only 

 under the guidance of sensations of movement. A sound or a 

 retinal image may prompt the movement ; but the part of the 

 temporal region, or of the occipital region in which the sound- 

 movement exchange or sight-movement exchange occurs must 

 act through the motor area by opening kinaesthetic-move- 

 ment arcs. Destruction of a part of the kinaesthetic cortex 

 causes in Man and the higher apes permanent paralysis for 

 the movements directed by the spot destroyed. In lower 

 animals the definition of the movement centres is vague, and 

 their removal produces only temporary results. Their mastery 



