CEREBRUM. 491 



3. The temporo-occipital gyrus is bounded by the collateral fissure 



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

 the anterior pole of the temporal lobe. 



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



between the posterior termination of the calloso-marginal fis- 

 sure and the parieto-occipital fissure. It blends with the gyrus 

 fornicatus, 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 calcarine fissures. 



The Insula or Island of Reil. This anatomic structure con- 

 sists of a triangular shaped cluster of six small convolutions situated 

 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 temporal lobes. 



Structure of the Gray Matter or the Cortex. The gray matter, 

 the cortex of the cerebrum, varies from two to four millimeters in 

 thickness. When examined with a lens of low power, it presents a 

 laminated appearance, due to differences in color and arrangement 

 of its constituent elements. With higher magnification the cortex 

 is seen to consist of neuroglia cells, nerve-cells with specialized 

 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 manner. Notwithstanding the com- 

 plexity of its structure, modern histologic methods have enabled 

 Cajal to divide it into four fairly distinct layers or zones, from without 

 inward, as follows (Fig. 222): 



1. 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. The deeper portions of this layer contain a specialized 

 type of nerve-cell (Cajal cells), of which there are several varie- 

 ties. 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. 



2. The Layer oj Small Pyramidal Cells. This layer consists mainly 



of nerve-cells, the majority of which are pyramidal in shape 

 and of small size. Other cells, however, are present, which 



