FISSURES OF CEREBRUM 681 



THE CEREBRAL HEMISPHERES 



The cerebral hemispheres constitute al)<)ut 87.-") per cent, of the entire brain, 

 and viewed fioin ahove, piesent an ovate torni, broader behind than in front. 

 They cc^ntain the lateral ventricles in their interior. They are separated from one 

 another Ijv the great longitudinal tissure, the floor of which is formed l)y the corpus 

 callosum, a great commissure which connects each hemis])here with its fellow. 

 Each hemisphere presents for examination four surfaces, supero-external, inferior, 

 internal, and tentorial, and two extremities or poles, an anterior and a posterior; the 

 anterior pole presenting an edge flattened internally and bevelled externally; the 

 posterior forming a blunt rounded point which is directed l)ackwai-ds with an 

 iiichnation downwards. With the exception of a small ]ii)rtion of tlie inferior sur- 

 face, the cerebral hemis])heres are entirely covered with fissures or sulci, which 

 mark oft' intervening elongated elevations termed convolutions or gyri. By this 

 means the surface area of the brain and consequently the proj^ntion of grey to white 

 matter is very greatly increased. The general arrangement of the convolutions in 

 the two hemispheres of the same brain is tolerably syimnetrical, but minor differ- 

 ences always occur. These differences are more particularly seen when there is any 

 striking deviation from the normal arrangement, as such alniormalities are usually 

 contined to one hemisphere. The supero-external surface is convex, presenting 

 a shar])er curve in the transverse than in the antero-])osterior direction. It is 

 marked oft" from the internal surface at the great longitudinal fissure l)y a well- 

 defined edge. It is divided for aljout its middle tliird into a superior and an inferior 

 segment by the horizontal limb of the fissure of Sylvius which lies above tin- 

 temporo-sphenoidal lobe. The internal surface is flattened, and is separated from 

 the opposite hemisphere by the falx cerebri and the subarachnoid space. The infe- 

 rior surface is divided into two parts by the stem of the fissure of Sylvius; the ])0s- 

 terior part is formed ])v the anterior end of the temporal lo1)e which is rounded and 

 l)roniinent, constituting the temi)oral pole. In front of the Sylvian fissure is the 

 orbital area of the frontal lobe, wliicli is concave and adapted to the orltital i)late 

 of the frontal l)one. Tlie tentorial surface is directed downwards and inwards; it 

 is ct)ncavc and rests upon the tentorium cerebelli. 



Fissures. — Under this term are included: (a) narrow intervals formed by thr 

 a))])roximation of parts primitively widely sejjarated, and (b) fissures due to 

 infoldings of the ventricular wall for a part or the whole of its thickness. Only 

 tw(j fissures are included in the former category, viz. the great longitudinal and the 

 great transverse fissures of the cerebrum, (o) The great longitudinal fissure 

 extends from the frontal to the occipital pole of the cerel>rum and se])aratt's thr 

 hemispheres from one another, except where they are joined Ijy the corjuiscallo.-um. 

 It contiiins the falx cere])ri and the anterior cerebral vessels. The great trans- 

 verse fissure will l)e described witli the anatomy of the lateral and third ventricles. 

 ill) Till- ri'inaining fissures present a very distinct division into (•om]>lete and 

 incomplete. The complete fissures are formed by an infolding of the entire thick- 

 ness of the ventricular wall, so that the reverse of each complete fissure a])]>ears 

 as a bulging into the cavity of the ventricles. The complete fissures comprist^ 

 the dentate fissure, the collateral fissure, the calcarine fissure, and parieto-occii>ital 

 fissure. The fissure of Sylvius is sometimes descriJjed as a complete fissure, ' but 

 the itrojection into the hemisi)here cavity which corresi)onds with it (viz. the 

 corj)Us striatum) is not formed l)y an infolding of the mantle wall, but as an 

 elevation on tlie floor of the prosencci)halon. The i^urface area corresjionding to 

 this internal projection does not keep i)ace witli the mantle, as the latter grows 

 around it, and in consequence the Sylvian dcpri'ssinn makes its ai)]»earance.' 

 (Cunningham. ) 



Interlobar fissures. — Certain fissures are termed interlobar because tliey have 

 been selected to determine the l)Oundaries of the various lobes into which tiie 

 hemispheres have been (somewhat artificially) divided. These are the parieto- 

 occipital fissure and the fissures of Sylvius and of Rolando. 



The parieto-occipital fissure a])]iears as a well-marked vertical sulcu,s on 

 the i)0.sterior i)art of the inner face t>f the hemisphere, and is continued outwards 



