ORIGIN AND SIGNIFICANCE OF THE CEREBRAL CORTEX 223 



the units which we recognize in consciousness in the simplest 

 mental processes which we can observe are not simple elements 

 at all, but complexes of sensation, feeling, impulse, or what-not, 

 whose character is in some measure predetermined in each case 

 by the organization of the physiological circuit with which it is 

 genetically related. Without attempting here a justification of 

 this conclusion on psychological grounds, let us inquire how it 

 accords with what we know of cerebral structure. 



There are no afferent tracts leading to the cerebral cortex 

 directly from any peripheral sense organ or from any center 

 within the brain which is "pure," i.e., devoted to a single sensory 

 function. In other words, no simple sensory impulses ordinarily 

 reach the cortex, but only nervous impulses arising from lower 

 correlation centers, where complex reflex combinations of various 

 sensory systems are possible. The optic impulses reach the cor- 

 tex most nearly pure, i.e., with less subcortical associational 

 relation with other sensory systems (it is no accident that the 

 visual sense plays a dominant role in human cortical function) ; l 

 but even here the optic centers in the thalamus from which the 

 optic projection fibers arise are intimately related with acous- 

 tic, tactile and other important sensory centers. And in the 

 case of all of the other sensory systems, the projection fibers 

 which enter the cortex come from centers which are separated 

 from their respective sense organs by two or more association 

 centers of a high order of complexity. Each of these subcor- 

 tical associational centers may be dominated physiologically by 

 a single sensory system, but it is structurally adapted for bring- 



1 The optic apparatus is peculiar in that the cortical optic path, instead of first 

 passing through the lower reflex centers (optic tectum) on its way to the thalamus, 

 as in the case of the other sensory systems, is short-circuited in the pulvinar and 

 lateral geniculate body before the mesencephalic centers for the simpler optic 

 reflexes are reached. Sherrington's researches on sensual fusion (Integrative Action 

 of the Nervous System, chap. 10) have shown that the sensory stimuli received 

 from each eye are independently elaborated subcortically, but that the fusion 

 of the uniocular sensation complexes into a single mental image is cortical. "The 

 binocular combination must be a synthesis of a left eye with a right eye se?isa- 

 tion." "The singleness is therefore the product of a synthesis that works with 

 already elaborated sensations contemporaneously proceeding" (p. 383). The 

 separateness of the cerebral processes for the two eyes is probably correlated with 

 the necessity for accurate spatial localization in the field of vision. Similarly, some 

 elements of cutaneous sensibility, where accurate spatial localization in conscious- 

 ness is also highly developed, reach their thalamic centers very directly through 

 the medial lemniscus (Head and Holmes, Brain, vol. 34, 1911), while others pursue 

 a more indirect route through the lateral lemniscus, the latter type of connection 

 being the more primitive. 



