THE GENERAL FUNCTION OF THE CEREBRUM 



659 



the line of Gennari, which really corresponds to the outer stripe of 

 Baillarger. 



It will be seen, therefore, that the cortex of the cerebrum presents 

 a definite histological structure which, however, does not remain the 

 same in its different regions. Certain minor differences appear here 

 and there, which enable us to tell from which particular area a certain 

 section has been taken. In making this distinction, we should .be 

 guided by (a) the thickness of the entire cortex, (6) the relative thick- 

 ness of its different zones, (c) the type of cells found in each layer, 

 and (d) the character of the radial and transverse striae of fibers. Thus, 

 it is to be noted that the thickness of the human cortex varies from about 

 4 mm. in its motor .region to about 2 mm. in other parts. In the 



FIG. 329. NEUROGLIA CELLS OF CORTEX CEREBRI. GOLGI METHOD. (G. Retzius.) 



former area are found the large pyramidal cells of Betz which are char- 

 acteristic motor elements. In addition it is to be observed that the 

 layer of pyramidal cells is very thick, while the granular layer is thin. 



The visual area of the occipital lobe is characterized by a very 

 prominent granular layer which, as has been stated above, is really 

 divided into two by a broad band of transverse fibers. The distinguish- 

 ing feature of the association areas of the frontal, parietal and oc- 

 ciptal lobes is the highly developed outer layer of pyramidal cells. 



Classification of the Tracts of the Cerebrum. The fibers of the 

 cerebral white matter are arranged in three distinct groups, namely: 



(a) Those which connect the hemispheres with the outlying 

 structures of the nervous system, i.e., with the thalamus, pons, medulla 

 and spinal cord. 



1 Brodmann, Vergl. Localisationslehre der Grosshirnrinde, Leipzig, 1909; also 

 Ramon y Cajal, Stud, iiber die Hirnr. des Menschen, Leipzig, 1906, or Lewan- 

 dowsky, Handb. der Neurologic, Jena, 1910. 



