GENERAL PHYSIOLOGY OF THE CEREBRUM. 177 



arising from these parts do not enter the capsule to make connec- 

 tion with the motor and sensory paths, below, but pass to other 

 parts of the cortex, forming a part of the system of association 

 fibers. Second, the association system, which may be defined as 

 comprising those fibers which connect one part of the cortex with 

 another (Fig. 77). There are short association tracts (A, A) con- 

 necting neighboring convolutions and long tracts passing from one 

 lobe to another. Third, the commissural system, consisting of as- 

 sociation 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 recognized clearly that different 

 parts of the cerebrum have different functions, but the differentia- 

 tion in structure which usually accompanies a specialization in 

 function is not very evident. Definite differences in the thickness 

 of the layers, in the size or shape of the cells, have been pointed out, 

 but it is perhaps something of a disappointment to find so little 

 of an anatomical distinction between structures whose reaction in 

 consciousness is so widely separated. It would seem that the 

 structural peculiarities must lie chiefly in the ultimate chemical 

 composition and physical properties of the protoplasm. In the 

 third place, the central nervous system throughout the verte- 

 brates is constructed upon the same lines, a mechanism of intercon- 

 necting neurons. There is a vast difference in the mental activity 

 of a frog and a man, but the cortex of the cerebrum shows a funda- 

 mental similarity in structure in the two cases. The chief difference 

 that comparative anatomy is able to show is that in the higher 



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

 Paris, 1894. 

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