GENERAL PHYSIOLOGY OF THE CEREBRUM. 175 



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. 78). 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 arid 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. 



The structure of the cortex is probably more complicated than would 

 appear from the above description. Numerous recent writers (Held, Apathy, 

 Nissl, Bethe, Hatai, et al.) have called attention to a very fine network peri- 

 cellular or Golgi net which envelops the cell body and the dendritic branches 

 of the neurons. Most of these observers consider that this delicate network is 

 of a nervous nature. It fills up the spaces between the nerve cells and makes, 

 therefore, a connection of exceeding complexity, and a histological feature 

 whose details must be worked out by improved technical methods. Nissl has 

 called attention to the fact that in the cortex the nerve cells, axons, dendrites, 

 neuroglia cells and fibers, and blood-vessels found are not sufficient to fill up the 

 whole space of this layer, and that there must exist an in-between substance, 

 which he speaks of specifically as "the gray." It seems probable, as contended 

 by Bethe, that this in-between substance is the network just referred to, whose 

 delicacy is such that it escapes detection by ordinary histological methods. If 

 this standpoint is correct it is evident that histology has a new field opened to 

 it in the study of the brain, and the structural features that may be revealed 

 by future study will doubtless add much to our knowledge of the mechanism 

 involved in brain activity. 



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 



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

 Paris, 1894. 



