THE CEREBRUM 263 



divided into the supra-marginal convolution, embracing the 

 short arm of this fissure, and the angular convolution con- 

 necting below with the temporal lobe. 



(c) The occipital lobe is situated posteriorly below the 

 parieto-occipital fissure and external to the median fissure. 

 It presents three convolutions, the superior, middle and in- 

 ferior. 



(d) The temporo-sphenoidal lobe is below the fissure of 

 Sylvius in front of the occipital lobe. It likewise presents 

 superior, middle and inferior convolutions. 



(e) The central lobe, or island of Reil, presents the gyrus 

 fornicatus, a convolution curving around the corpus cal- 

 losum ; the marginal convolutions beyond the calloso-mar- 

 ginal fissure from the preceding and between it and the edge 

 of the longitudinal fissure; the continuation of the parieto- 

 occipital fissure running downward and forward to meet the 

 calcarine fissure, between which is the cuneus; the internal 

 aspect of the temporal lobe, the uncinate gyrus. 



Structure. The cerebral hemispheres are composed of 

 white and gray matter, but here the gray matter is situated 

 externally. To increase its amount, with economy of space, 

 the gray matter is thrown into many convolutions, to some 

 of which reference has been made. The sulci separating 

 these convolutions have a depth in the average human brain 

 of about one inch. The thickness of the gray matter of the 

 cortex varies from 1 /i2 to % in., being thinnest in the occipital 

 and thickest in the front parietal region. 



The cells found in the superficial and deep portions of the 

 gray matter are not uniform in size or shape. In a general 

 way it may be said that they increase in size as the surface 

 is left, but in addition to the comparatively large cells in the 

 deep parts there are also numbers of small ones. Passing in 

 the same direction there are found in succession small pyra- 

 midal, larger pyramidal, and irregular branching cells. 



Fibers from the Cerebrum. Fibers pass from each cere- 

 bral hemisphere to (a) the spinal cord, (b) the cerebellum, 



