EVOLUTION AND SIGNIFICANCE OF CEREBRAL CORTEX 307 



From these anatomical considerations it follows that no simple sensory 

 impulse can, under ordinary circumstances, reach the cerebral cortex 

 without first being influenced by subcortical correlation centers, within 

 which complex reflex combinations may be effected and various automatisms 

 set off in accordance with then; preformed structure. These subcortical 

 systems are to some extent modifiable by racial and individual experience, 

 but their reactions are chiefly of the invariable or stereotyped character, 

 with a relatively limited range of possible reaction types for any given 

 stimulus complex. 



It is shown by the lower vertebrates which lack the cerebral cortex that 

 these subcortical mechanisms are adequate for all of the ordinary simple 

 processes of life, including some degree of associative memory. But here, 

 when emergencies arise which involve situations too complex to be resolved 

 by these mechanisms, the animal will pay the inevitable penalty of failure 

 perhaps the loss of his dinner, or even of his life. 



In the higher mammals with well-developed cortex the automatisms and 

 simple associations are likewise performed in the main by the subcortical 

 apparatus, but the inadequacy of this apparatus in any particular situation 

 presents not the certainty of failure, but rather a dilemma. The rapid 

 preformed reflex mechanisms fail to give relief, or perhaps the situation pre- 

 sents so many complex sensory excitations as to cause mutual interference 

 and inhibition of all reaction. There is a stasis in the subcortical centers. 

 Meanwhile the higher neural resistance of the cortical pathways has been 

 overcome by summation of stimuli and the cortex is excited to function. 

 Here is a mechanism adapted, not for a limited number of predetermined 

 and immediate responses, but for a much greater range of combination of 

 the afferent impressions with each other and with memory vestiges of pre- 

 vious reactions and a much larger range of possible modes of response to any 

 given set of afferent impressions. By a process of trial and error, perhaps, 

 the elements necessary to effect the adaptive response may be assembled 

 and the problem solved. 



It is evident here that the physiological factors in the dilemma or problem 

 as this is presented to the cortex are by no means simple sensory impres- 

 sions, but definitely organized systems of neural discharge, each of which is a 

 physiological resultant of the reflexes, automatisms, impulses, and inhibi- 

 tions characteristic of its appropriate subcortical centers. The precise form 

 which these subcortical combinations will assume in response to any par- 

 ticular excitation is in large measure determined by the structural connec- 

 tions of these centers inter se. And the pattern of these connections is 

 tolerably uniform for all members of any animal race or species. This 

 implies that it is hereditary and innate. This is the underlying basis of 

 instinct. 



The connections between the cortical centers, on the other hand, are 

 much less definitely laid down in the hereditary pattern. The details of 

 the definitive association pattern of any individual are to a greater degree 

 fixed by his particular experience. This is the basis of docility and the 

 individually modifiable or intelligent types of behavior. The typical cor- 

 tical activities, even when physiologically considered, are far removed indeed 

 from those of the brain stem. 



It should be emphasized, however, that the differences between the cor- 

 tex and the lower centers of the brain stem, so far as these can be deduced 

 from a study of structure and from physiological experiment, are relative and 

 not absolute. Indeed, the general pattern of the regional localization of the 



