THE CEREBRUM 399 



Cerebral Gyri and Sulci. The surface pattern which is 

 presented by the cerebral gyri and sulci is, in its general 

 features, the same in all human brains ; but when the com- 

 parison is pushed into more detail many differences become 

 manifest, not only in the brains of different subjects but 

 also in the two cerebral hemispheres of one subject. 



The depressions which intervene between the cerebral 

 gyri vary in depth. Some are due to folding of the whole 

 thickness of the wall of the cerebrum, and consequently they 

 correspond with elevations of the walls of the cavities of the 

 cerebrum which are called the lateral ventricles. Such 

 depressions are called complete fissures. In this category are 

 included (i) the anterior portion of the calcarine fissure; 

 (2) a portion of the collateral fissure; and (3) the chorioidal 

 fissure. The incomplete fissures and the sulci are merely 

 furrows of varying depth which do not produce any effect 

 on the surface of the ventricular walls. 



General Structure of the Cerebral Hemispheres. Each 

 cerebral hemisphere is composed of an outside coating of grey 

 matter, spread in a continuous and uninterrupted layer over its 

 surface, and an internal core of white matter. The grey coat- 

 ing is termed the cerebral cortex^ whilst the white internal part 

 is called the medullary centre. Each gyrus shows a corre- 

 sponding structure. It has an external covering of grey 

 matter supported upon a core of white medullary matter. But, 

 in addition to the grey matter on the outside, there are certain 

 large deposits of grey matter embedded in the substance of 

 each hemisphere in its basal part, Those deposits constitute 

 the basal ganglia, and although to a certain extent they are 

 isolated from the grey matter on the surface, nevertheless, at 

 certain points, they are directly continuous with it (Fig. 186). 



By means of the gyri and sulci the grey matter on the 

 surface of the hemisphere is increased, and its close 

 association with the vascular pia mater is maintained without 

 any unnecessary increase of the bulk of the organ. The 

 vascular pia mater dips into every fissure and sulcus, and 

 opportunity is, therefore, afforded for the cortical vessels to 

 break up into twigs of exceeding fineness before they 

 enter the substance of the hemisphere. The distribution of 

 the blood to the grey cortex is in that way rendered uniform. 



Cerebral Lobes and Interlobar Fissures and Sulci. For 

 purposes of localisation and -dription, it is customary to 



