938 THE CENTRAL NERVOUS SYSTEM 



position of the lesion from the part of the body in which the fit, or the 

 aura (the sensation or group of sensations peculiar to each case, which 

 precedes and announces the attack) begins. For example, if the con- 

 vulsions commence with a twitching of the right thumb and extend over 

 the arm, or if the aura consists of sensations beginning in the thumb, 

 there is a strong presumption that the seat of the lesion is the part of the 

 arm -area known as the ' thumb-centre ' in the left cerebral hemisphere. 

 It is the seat of the convulsion at its commencement, not the regions to 

 which it may afterwards spread, that is important in diagnosing the 

 position of the lesion. For just as strong or long-continued electrical 

 stimulation of a given ' centre ' of the ' motor ' cortex may give rise to 

 contractions of muscles associated with other ' centres,' so the excita- 

 tion set up by localized disease may spread far and wide from its 

 original focus, involving area after area of the ' motor ' region first in 

 the one hemisphere and then in the other. The part of the body to 

 which a sensory aura is referred is as significant an indication of the 

 seat of the discharging lesion as is the part of the body which first begins 

 to twitch. This is one of the proofs that the ' motor ' region is not a 

 purely motor area. Disturbed action of the sensory areas on the cortex 

 may, as in the case of the motor regions, take the form either of de- 

 ficiency or of excitation. Excitation expresses itself by hallucinations, 

 the person having the impression of a sight, a sound, a smell or taste, or 

 one or other of the cutaneous sensations in the absence of the relrled 

 objects. 



Seat of Intellectual Processes Association Areas. When we 

 have deducted from the cortex of the hemisphere the whole Rolandic 

 region and the sensory centres, there still remains a large territory 

 unaccounted for. Considerable portions of the occipital, parietal, 

 and temporal lobes, nearly the whole of the island of Reil and the 

 greater part of the frontal lobe anterior to the ascending frontal 

 convolution are ' silent areas/ and respond to stimulation by neither 

 motor nor sensory sign. They correspond to the association 

 centres previously referred to. They are connected with the 

 sensory and motor areas and with each other, but are not directly 

 connected by projection fibres with the lower parts of the central 

 nervous system, as the motor area, for example, is by the pyramidal 

 path. By a process of exclusion it has been supposed that, in addi- 

 tion to, or partly in virtue of, their associative function, they are 

 the seat of intellectual and psychical operations. It is supposed 

 that the sensations aroused in the various sensory areas by the 

 impulses received from the sense organs are linked in the associa- 

 tion areas into complex perceptions. For instance, when an 

 orange is taken into the hand, the visual sensation of a yellow body, 

 the tactile sensation of a smooth round body, and perhaps the 

 olfactory sensation characteristic of an orange, are collected from 

 the sensory areas, connected and combined, or synthesized in an 

 association area to the concept of an orange. Somewhere in the 

 association areas it is to be supposed is stored the memory of past 

 experiences. The intellectual function has been more particularly 

 assigned to the frontal lobes, and with great probability, although 



