THE CEREBRUM. 



54i 



above, and its inferior border extends from the occipital lobe to the 

 anterior pole of the temporal lobe. 



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 the 

 Cortex. The gray matter, the cortex of the 

 cerebrum, varies from two to four millime- 

 ters in thickness. When examined 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, con- 

 nective tissue, etc., all of which are arranged 

 and interblended in a most intricate man- 

 Notwithstanding the complexity of its 



ner. 



structure, modern histologic methods have 

 enabled Cajal to divide it into four fairly dis- 

 tinct layers or zones, from without inward, as 

 follows (Fig. 249): 



i. The Molecular Layer. The most superficial 

 portion of this layer consists mainly of 

 neuroglia or glia cells, the processes of 

 which interlace in all directions, forming 

 a distinct sheath just beneath the pia. 



yrr 



FIG. 249. SECTION OF THE 

 CEREBRAI^CORTEX (MOTOR AREA) 

 OF CHILD, STAINED BY GOLGI'S 

 SILVER METHOD. A. Layer of 

 neuroglia cells. B. Layer of small 

 pyramidal ganglion cells. C. 

 Layer of large pyramidal cells. D. 

 Layer of irregular smaller cells. 

 (Piersol.) 



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 pur- 

 sue 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, den- 

 dritic processes of cells situated in the subjacent layer. The terminal 



