294 INTRODUCTION TO NEUROLOGY 



impaired. In the course of a few weeks or months a new equi- 

 librium minus the lacking factors is established and the patient 

 very rapidly improves. Ultimately complete recovery may 

 occur, save for a permanent residual defect which results directly 

 from the loss of the tissue destroyed. 



The immediate shock-like interference with the activity of 

 cerebral centers not directly affected by the lesion is what von 

 Monakow means by diaschisis. Upon the restoration of the 

 nervous equilibrium this transient diaschisis effect is wholly or 

 partially lost, and the residual symptoms of defect give a fairly 

 accurate picture of the intrinsic functions of the center directly 

 attacked by the lesion. It is commonly assumed that there 

 is also during the process of gradual recovery from such a corti- 

 cal injury a certain capacity for the compensatory development 

 of other centers of the same or the opposite cerebral hemisphere, 

 so that they learn to perform vicariously the functions of the 

 lost part. 



All functions of the nervous system are facilitated by repeti- 

 tion, and many such repetitions lead to an enduring change in 

 the mode of response to stimulation which may be called physio- 

 logical habit. This implies that the performance of every reac- 

 tion leaves some sort of a residual change in the structure of the 

 neuron systems involved. These acquired modifications of 

 behavior are manifested in some degree by all organisms (see 

 pp. 22, 31), and this capacity lies at the basis of all associative 

 memory (whether consciously or unconsciously performed) and 

 the capacity of learning by experience. This modifiability 

 through individual experience is possessed by the cerebral cortex 

 in higher degree than by any other part of the nervous system ; 

 and the capacity for reacting to stimuli in terms of past experi- 

 ence as well as of the present situation lies at the basis of that 

 docility and intelligent adaptation of means to ends which are 

 characteristic of the higher mammals. It is a fact of common 

 observation that those animals which possess the capacity for in- 

 telligent adjustments of this sort have larger association centers 

 in the cerebral cortex than do other species whose behavior is 

 controlled by more simple reflex and instinctive factors, that is, 

 by inherited as contrasted with individually acquired organiza- 

 tion. This is brought out with especial distinctness by a com- 



