THE FUNCTIONS OF THE CEREBRAL CORTEX 285 



the visual sphere. Each of these focal spheres is surrounded by 

 other areas which receive projection fibers, though in less 

 abundance, and also numerous association fibers from other 

 parts of the cortex. These marginal fields are, therefore, to be 

 regarded as association centers, each of which is under the 

 dominant physiological influence of the adjacent focal projection 

 center. These are sometimes called visual psychic, auditory 

 psychic fields, etc., after the adjacent projection centers; but 

 these terms are objectionable as implying the old phrenological 

 notion of localization of specific psychological faculties. 



Each sensory projection center which receives afferent fibers 

 of course sends out association fibers to other parts of the cortex. 

 Some of these fibers may be very short, reaching only to the 

 adjacent marginal fields (these are arcuate fibers, see Fig. 121, 

 f.p.) ; other much longer association fibers may assist in forming 

 the great associational tracts of the subcortical white matter. 

 The association centers themselves are likewise connected by 

 fiber tracts of bewildering complexity, so that every part of the 

 cerebral cortex is in direct or indirect physiological connection 

 with every other part. All of these parts are, therefore, able to 

 influence the motor centers of the precentral gyms, from which 

 alone voluntary motor impulses can be discharged from the 

 cortex to the lower motor centers of the brain stem and spinal 

 cord. 



The relations of the tactual and somesthetic sensory projection fibers to 

 the postcentral and precentral gyri have been variously described, and some 

 further consideration of the functional connections of these fibers may here 

 be appropriate. From a large body of anatomical, experimental, and clinical 

 evidence it was formerly assumed that the cortical motor centers are co- 

 extensive with those for the general somatic sensory projection systems of 

 cutaneous and muscular sensibility, the projection centers of both the 

 sensory and motor fibers related to each region of the body being located 

 on both the anterior and posterior sides of the central sulcus or fissure of 

 Rolando, that is, in both the precentral and postcentral gyri. Most of the 

 diagrams of cortical localization in all but the most recent manuals are 

 based upon this view of the case. But recent work has shown definitely 

 that the motor centers are confined to the region in front of this sulcus. 

 Here only are found the giant pyramidal cells of Betz which give rise to 

 most of the fibers of the pyramidal tract. It may, therefore, be regarded as 

 definitely established that motor projection fibers do not arise from the 

 postcentral gyrus, as formerly supposed. 



Sensory projection fibers, however, are known to pass from the general 

 somatic sensory centers in the ventral and lateral nuclei of the thalamus to 



