CEREBRUM. 493 



hension. Thus, Meynert's estimate is 612 millions; Donaldson's 

 1200 millions; while Thompson's is 9200 millions. 



Structure of the White Matter. The white matter of the 

 cerebrum consists of medullated nerve-fibers which, though in- 

 tricately arranged, may be divided into three systems: viz., the 

 commissural, the association, and the projection. 



1. The commissural system. The fibers which compose this system 



unite corresponding areas of the cortex of each hemisphere. 

 The fibers from the frontal, parietal, and occipital lobes cross 

 in the median line and form a band of transversely arranged 

 fibers, the corpus callosum. The fibers which unite the corre- 

 sponding areas of the temporo-sphenoid lobes cross in the 

 anterior commissure. All the commissural fibers are the axons 

 of nerve-cells in the cortex, the terminals of which are to be found 

 in the cortex of the opposite side. 



2. The association system. The fibers which compose this system 



unite neighboring as well as distant parts of the same hemi- 

 sphere, and may therefore be divided into long and short fibers. 

 They associate the inexcitable or association areas with the 

 excitable or projection areas. 



3. The projection system. The fibers composing this system unite 



certain areas of the cortex of the cerebrum with the basal 

 ganglia, the pons, medulla oblongata, and spinal cord. They 

 may be divided into: (i) afferent fibers which have their origin 

 in the lower nerve-centers at different levels and thence pass to 

 the cortex; and (2) efferent fibers which have their origin in the 

 cortex and thence pass to the lower nerve-centers, terminating 

 at different levels. The former are also termed the cortico- 

 afferent or cortico- petal ; the latter, cortico-efferent or cortico-fugal. 

 The afferent fibers, the so-called sensor tract, which transmit 

 nerve impulses coming from the general periphery and the sense- 

 organs, pass through the tegmentum as the mesial and lateral fillets, 

 and thence to the cortex directly by way of the internal capsule, or 

 indirectly through the intermediation of the thalamic and subthalamic 

 nuclei. The distribution of these fibers to the various areas of the 

 cortex will be found in following paragraphs. 



The efferent fibers of the so-called motor tract which transmit 

 motor or volitional nerve impulses from the cortex to the pons, 

 medulla, and spinal cord, emerge from the layer of pyramidal cells 

 of the gray matter of the anterior or the pre-central convolution, 

 the paracentral lobule and immediately adjacent areas. From this 

 origin the axons descend through the white matter of the corona 

 radiata, converging toward the internal capsule, into and through 

 which they pass, occupying the anterior two-thirds of the posterior 

 limb or segment. Beyond the capsule they continue to descend, 



