THE CEREBRUM 461 



hemispheres are rendered highly irregular by the presence of 

 gyri, separated from one another by intervening furrows, 

 termed sulci and fissures. The surface pattern which is pre- 

 sented by these gyri and sulci is, in its general features, the 

 same in all human brains ; but when the comparison is 

 pushed into more detail many differences become manifest, 

 not only in the brains of different individuals but also in 

 the two cerebral hemispheres of one individual. 



Of the furrows two varieties must be recognised, viz., 

 complete and incomplete. The complete fissures are few in 

 number and they consist of inwardly directed folds which 

 involve the whole thickness of the cerebral wall. They con- 

 sequently show in the interior of the cerebral cavity or lateral 

 ventricle in the form of elevations on its walls. In this 

 category are included (i) the hippocampal fissure; (2) the 

 anterior portion of the calcarine fissure ; and (3) a portion of 

 the collateral 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 coating is termed the cerebral cortex, whilst the white 

 internal part is called the medullary centre. Each gyrus 

 shows a corresponding 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 em- 

 bedded in the substance of each hemisphere in its basal part. 

 These constitute the basal nuclei, 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. 



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 



