THE ORIGIN AND SIGNIFICANCE OF CEREBRAL CORTEX 99 



increasingly a structure which may be interpreted as leading to in- 

 creased degrees of indeterminacy." 



A fundamental feature of cortical functions is that they are delayed 

 reactions (p. 78; '!24r, p. 271); there is, first, the arrest or inhibition 

 of some lower and more primitive patterns of behavior of reflex or 

 instinctive type. This allows time for cortical reorganization of the 

 component factors of the situation, conditioning of reflexes, or other 

 modifications of the stereotyped patterns of response. The cortex, 

 accordingly, is lifted up away from the lines of through traflic in the 

 brain stem, and the dorsal convexity of the evaginated cerebral hemi- 

 sphere is conveniently located, with ample space for indefinite en- 

 largement. 



During the phylogenetic development of cortex, ascending and 

 descending pallial projection fibers are added to the pre-existing sys- 

 tems of the underlying stem. They do not entirely supplant them, for 

 even in mammals, where cortical projection systems are highly de- 

 veloped, the subpallial parts of the hemisphere retain their own 

 diencephalic connections. 



Primitively, as in cyclostomes, the entire cerebral hemisphere is 

 little more than an olfactory bulb and a secondary olfactory nucleus. 

 In Amblystoma the hemispheric evagination is more extensive, and 

 there is a large increment of ascending nonolfactory fibers; yet here 

 the pallial part of the hemisphere receives the largest olfactory tracts, 

 and all of it is essentially an olfactory nucleus. 



The olfactory reflexes seem to be adequately provided for in the 

 stem portion of the hemisphere. I have suggested ('33) that here, and 

 especially in the corticated mammals, the olfactory sense, lacking 

 any localizing function of its own, co-operates with other senses in 

 various ways, including a qualitative analysis of odors (desirable and 

 noxious) and also the activation or sensitizing of the nervous system 

 as a whole and of certain appropriately attuned sensori-motor sys- 

 tems in particular, with resulting lowered threshold of excitation for 

 all stimuli and differential reinforcement or inhibition of specific 

 types of response. The olfactory cortex (and its predecessors in lower 

 vertebrates) may, then, serve for nonspecific facilitation of other 

 activities, in addition to its own specific olfactory functions. This 

 facilitation may involve both general excitatory action and general 

 inhibition. That the latter is present is indicated by the observation 

 of Liggett ('28) that anosmic rats are more active than the normal 

 controls. The organization of the olfactory system as a whole in all 



