186 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



ation tracts (A t A) connecting neighboring convolutions and long 

 tracts passing from one lobe to another. Third, the commissural 

 system, consisting of association fibers that cross the mid-line and 

 connect portions of one cerebral hemisphere with the cortex of the 

 other. These fibers make up the commissural bands, known in 

 gross anatomy as the corpus callosum, anterior white commissure, 

 fornix, etc. 



Physiological Deductions from the Histology of the Cortex. 

 Cajal* especially lays stress upon some anatomical features which 

 seem to justify certain generalizations of a physiological nature. In 

 the first place, every part of the cortex receives incoming impulses 

 and gives rise to outgoing impulses. Every part of the cortex is, 

 therefore, both a termination of some afferent path and the begin- 

 ning of some efferent path; it is, in other words, a reflex arc of 

 a greater or less degree of complexity. We may suppose that 

 every efferent discharge from any part of the cortex is occasioned 

 by afferent impressions reaching that point from some other part 

 of the nervous system. Whether or not there is such a thing as 

 absolutely spontaneous mental activity cannot be determined by 

 physiology, but on the anatomical side at least all the structures 

 exhibit connections that fit them for reflex stimulation, and many 

 of our apparently spontaneous acts must be of this character. 

 Secondly, all parts of the cortex exhibit an essentially similar 

 structure. Modern physiology has taught that different parts 

 of the cerebrum have different functions, but the differentiation 

 in structure which usually accompanies a specialization in func- 

 tion is not at first very evident. Definite differences in the 

 thickness of the layers, in the size or shape of the cells, or in the 

 character of the fibrillation, have been pointed out (see p. 229), but 

 it is perhaps something of a disappointment ,to find so little of an 

 anatomical distinction between structures whose reaction in con- 

 sciousness may be separated so widely. Numerous special studies 

 made upon the lamination of different parts of the human cortex 

 (see p. 229), and comparative observations upon the cerebral cor- 

 tex in different vertebrates, have served to give an anatomical 

 foundation for various interesting speculations which subsequent 

 work may or may not confirm, t It is pointed out that if we omit 

 the outer or molecular layer the other cells of the cortex fall into 

 three groups, namely, the granular layer (3 in Fig. 81), the supra- 

 granular layer (2), comprising the pyramidal cells external to the 



* Cajal, "Les nouvelles idees sur la structure du systeme nerveux, etc.," 

 Paris 1894. 



t'For a summary of these views consult Bolton, "Brain," 1910 and 1912, 

 or "Further Advances in Physiology," Hill, London and New York, 1909; 

 Van Valkenburg, "Folia neurobiologica," 1910, 4, 335. 



