THE CEREBRUM 



587 



4. The quadrate lobule, or precuneus, a square-shaped convolution, is situated 



between the posterior termination of the para-central fissure and the 

 parieto-occipital fissure. It blends with the callosal convolution, on the 

 one hand, and with the parietal lobule on the other. 



5. The cuneus, a triangular or wedge-shaped convolution or lobule, is situated 



on the mesial surface of the occipital lobe 



between the parieto-occipital and calca- 



rine fissures. 



The Insula or Island of Reil. This 

 anatomic structure consists of a triangular- 

 shaped cluster of six small convolutions situa- 

 ted at the bifurcation of the Sylvian fissure 

 and concealed from view by the convolutions 

 bordering it, spoken of collectively as the oper- 

 culum. These convolutions are connected 

 with the frontal, the parietal, and the tem- 

 poral lobes. 



Structure of the Gray Matter of th 

 Cortex. The gray matter, the cortex of the 

 cerebrum, varies from two to four millime- 

 ters in thickness. When examin ed with a lens 

 of low power, it presents a laminated appear- 

 ance, due to differences in color and arrange- 

 ment of its constituent elements. With 

 higher magnification the cortex is seen to con- 

 sist of neuroglia cells, nerve-cells with special- 

 ized dendrites and axons, medullated and 

 non-medullated nerve-fibers, blood-vessels, 

 connective tissue, etc., all of which are arranged 

 and interblended in a most intricate man- 

 ner. Notwithstanding the complexity of its 

 structure, modern histologic methods have 

 enabled Cajal to divide it into four fairly dis- 

 tinct layers or zones, from without inward, as 

 follows (Fig. 244): 

 i . The. Molecular Layer. The most superficial 



portion of this layer consists mainly of 



Layer of large pyramidal cells. 

 D. Layer of irregular smaller cells. 

 (Piersol.) 



FIG. 244. SECTION OF THE 

 CEREBRAL CORTEX (MOTOR AREA) 

 or CHILD, STAINED BY GOLGI'S 

 SILVER METHOD. A. Layer of 

 i. *. ,, , neuroglia cells. B. Layer of small 



neuroglia or gha cells, the processes of pyramidal ganglion cells. C. 

 which interlace in all directions, forming 

 a distinct sheath just beneath the pia. 

 The deeper portions of this layer con- 

 tain a specialized type of nerve-cells (Cajal cells), of which there are 

 several varieties. These cells give off nerve-fibers which pursue a 

 horizontal direction for a variable distance, but in their course give off 

 collateral branches which ascend to the outer surface of the layer. 

 Among these structures are to be found, also, dendritic processes of 

 cells situated in the subjacent layer. The terminal filaments of 

 medullated nerve-fibers coming from nerve-cells in lower regions of 

 the encephalo-spinal axis are also present, but for the most part they 

 pursue a tangential direction. 



