CHAPTER XIX 



THE GREAT AFFERENT SYSTEMS 

 EXTEROCEPTIVE PATHWAYS TO THE CEREBRAL CORTEX 



As has been intimated elsewhere, it is chiefly those nervous impulses, which 

 are aroused by stimuli acting upon the body from without, that rise above the 

 subconscious level and produce clear-cut sensations. The importance of these 

 sensations in our conscious experience is no doubt correlated with the fact that 

 it is through the reactions, called forth by such external stimuli, that the organism 

 is enabled to respond appropriately to the various situations in its constantly 

 changing environment. To meet these complex and variable situations cor- 

 rectly requires the nicest correlation of sensory impulses from the various sources 

 as well as their integration with vestiges of past experience, and it is in connec- 

 tion with these higher correlations and adjustments that consciousness appears. 

 The responses initiated by interoceptive and proprioceptive afferent impulses 

 are more stereotyped and invariable in character ; and these reactions are for the 

 most part carried out without the individual being aware either of the stimulus 

 or the response. 



It is known that the cerebral cortex is the organ within which occur at least 

 the majority of those complex and highly variable correlations and integrations 

 which have consciousness as their counterpart. A single object may appeal 

 to many sense organs, and our perception of that object involves a synthesis of 

 a corresponding number of sensations and their comparison with past experience. 

 For example, when I meet a friend and grasp his hand in greeting, my perception 

 of him includes not only the image of his face but also the sound of his voice 

 and the warm contact of his hand. Thus thermal, tactile, auditory, and visual 

 sensations may be fused in the perception of a single object, and this involves an 

 integration of the corresponding afferent impulses within the cerebral cortex. 

 Accordingly, it becomes of special interest to trace the course of these afferent 

 impulses from the various exteroceptive sense organs to their cortical receptive 

 centers. 



As we shall see, the outer world has for the most part a crossed representation 

 in the cerebral cortex. Cutaneous stimuli, received from objects touching the 

 302 



