FISSURES. Of the five principal fissures of the brain, two have al ready been mentioned—the great longitudinal fissure, separating the hemispheres, and the great trans verse fissure, separating the cere brum from the cerebellum. Each hemisphere presents three fissures which separate the hemisphere into lobes. These are: (1) The tiss-ure of Sylvius. This begins at the base of the brain (see diagram) and pas_ses upward and backward on the external surface of the hemi sphere, where it divides into a short er ascending limb and a longer horizontal limb. It separates the frontal and parietal lobes from the tempero-sphenoidal lobe. (2) The fissure of Rolando begins a little behind the mid-point of Rolando and the posterior extremity of the brain, sends a short branch outward, while its longer branch extends downward and forward on the hiller surface of the hemisphere.
Lom,s. The frontal lobe includes that part of the brain which lies in front of the fissure of Rolando and above the fissure of Sylvius. It presents the following convolutions: (a) The ascending frontal convolution, lying just in front of the fissure of Rolando and separated from the rest of the lobe by the vertical o• preeen tral sulcus. This remaining portion of the front al lobe is divided by two longitudinal sulei into (b) the superior frontal convolution, (e) the middle frontal convolution, and (d) the inferior frontal convolution. The last is sometimes known as the convolution of Brom. These convolutions also extend more or less into the under surface of the lobe, which is also marked by a groove in which lies the olfactory bulb.
of the great longitudinal fissure and extends downward and forward to a point somewhat above and behind the bifurcation of the Sylvian fissure. It separates the frontal lobe from the parietal lobe. (3) The parieto-oecipital fis sure begins about midway between the fissure The parietal lobe is separated from the front al lobe by the fissure of Rolando, from the tempero-sphenoidal lobe below by the horizontal limb of the fissure of Sylvins, from the occipital lobe by the parieto-occipital fissure. It may be subdivided into four fairly well marked con volutions: (a) The ascending parietal, lying just behind the fissure of Rolando. Behind the as vending parietal convolution is the intra-parietal sitleits. The portion of the parietal lobe above the horizontal part of the stticus is known as (b) III(' superior-parietal convolution, and is con tinuous with the ascending pari etal. That part of the lobe be
low the horizontal arm of the sulcus is divided by a vertical silicas into (CI an anterior part, t lie supramarginal convolution, and (di a posterior part. the aniuhr convolution.
Tile occipita/ 10/11- is separated from the upper part of the pari etal lobe by the pa rieto-oceipital fissure. while it is more or less eontinuous with the lower part of die parietal lobe and with the tempero-splienoidaI 101,0 by .mall connecting convolutions. The lobe is rather indefinitely divided by two ,111311 transverse fissures into first, second. and third occipital convolut ions.
I, lobe in the middle fossa of the skull and is bounded above and in front by the Sylvian fissure. l'os teriorly. connecting convolutions imite it with the parietal and oc cipital lobes. It is divided by two horizontal saki into three cony°. lutions called from above down ward. the first. second, and third teimiero-sphenoidal ions.
The ("CP I Ott lobe or islet nil of Merl situated in the depth of the fissure of SvIvilis, and com posed of five or six rather small convolutions.
The interim! surface of the cerebrum is less distinctly marked on' into lobes Ibae the ex ternal surface, The fissures on the interim' surface are the .:111.1so•inarginal, the paricto-oe cipital, the ealearine, the collateral, and the dentate. The calloso-marginal is the longest of these fissures, begins on the superior surface just, behind the mid-point, and runs forward parallel to the curve of the surface of the brain, separating the marginal or first frontal con volution from the gyms fornicates, which lies just over the corpus callosmn. The quadrate tubule lies between the calioso-marginal sulens and the parieto-occipital silicas. The (omens or occipital lobate lies between the parieto-oe eipital suleus in front and above and the cal earine silicas below. The laminate gyms lies be low the Sylvian fissure and the dentate suleus, while below it is separated from the tempero sphenoidal lobe by the collateral silicas. The tempero-splicnoidal lobe is continuous with the already deseribed lobe of the same name in the outer surface of the cerebrum. It presents two convolutions in addition to those already de scribed on its outer surface.