874 ANATOMY NERVOUS SYSTEM- volution. The occipital is connected with the parietal lobe by two annectent or bridging gyri, which bridge across tl transverse external part of the parieto-occipital fissure ; the FIG. 71.- Side view of the Brain in the skull. 1 depth and extent of this fissure vary in different brains in proportion to the size of these bridging convolutions. The superior annectent gyrus passes between the postero-parietal and the superior occipital convolutions, -whilst the second an nectent gyrus connects the middle occipital with the angular gyrus. Two annectent gyri also pass from the inferior occi pital convolution to the lower convolutions of the temporo- sphenoidal lobe. These lobes of the cerebrum, though named after the bones which form the vault of the skull, are not exactly co-terminous with them. The frontal lobe not only lies under cover of the frontal bone, but extends back wards under the anterior part of the parietal ; for the fissure of Rolando, which forms its posterior boundary, lies from
to 2 inches behind the coronal suture. The occipital lobe
is not limited to the upper tabular part of the occipital bone, but extends forwards under cover of the posterior part of the parietal, for the parieto-occipital fissure lies about f inch in front of the apex of the lambdoidal fissure. The temporo-sphenoidal lobe not only lies under the squamous-temporal and great wing of the sphenoid, but passes upwards under cover of the lower part of the parietal, for the Sylvian fissure passes from below obliquely upwards and backwards across the line of the squamous suture near its middle. The area covered by the parietal bone so far, 1 The above view of the brain in situ shows the relations of the sur face convolutions to the regions of the skull. E, fissure of Rolando, which separates the frontal from the parietal lobe. TO, parieto-occi pital fissure between the parietal and occipital lobes. SS, fissure of Sylvius, which separates the temporo-pphenoidal from the frontal and parietal lobea. SP, MF, IF, the supero-, mid-, and infcro-frontal sub divisions of the frontal area of the skull ; the letters are placed on the superior, middle, and inferior frontal convolutions; the inferior frontal region is separated from the middle frontal by the frontal part of the curved line of the temporal ridge ; the mid- from the supero-frontal by an antero-posterior line through the frontal eminence. SAP, the supero- antero-parietal area of the skull ; S is placed on the ascending parietal convolution, AP on the ascending frontal convolution. IAP, the infero- antero-parietal area of the skull; I is placed on the ascending parietal, AP on the ascending frontal convolution. SPP, the supero-postero-parietal area of the skull ; the letters are placed on the angular convolution. IPP, the infero-postero-parietal area of the skull ; the letters are placed on the mid-temporo-sphenoidal convolution ; the temporal ridge separates the supero- and infero-parietal regions from each other ; a vertical line drawn through the parietal eminence separates the antero- and postero- parietal regions. X, the convolution of the parietal eminence, or supra- marginal gyrus. 0, the occipital area of the skull; the letter is placed on the mid-occipital convolution. Sq, tho squamoso-temporal region of the skull ; the letters are placed on the mid-temporo-sphenoidal con volution. AS, the ali-sphcnoid region of the skull ; the letters are placed on the tip of the supero-temporo-sphenoidal convolution. The black lines mark the boundaries of different cranial regions. then, from being co-terminous with the parietal lobe of tho cerebrum, is trenched on anteriorly by the frontal, poste riorly by the occipital, and inferiorly by the temporo- sphenoidal lobe. The convolutions of the parietal lobe itself are grouped around the parietal eminence, and in the interval between it and the sagittal suture. The inner table of the cranial bones is an almost exact mould of the convolutions of these lobes ; but this is not so with the ex terior of the skull, the configuration of which is modified by the formation of ridges and processes for the attachment of muscles, by variations in the thickness of the diploe, and by the development of the frontal and mastoid air- sinuses. Hence the outer surface of the skull does not correspond in shape to the outside of the brain. The Central Lobe of tho hemisphere, more usually called the insula or island of Eeil, does not come to the surface of the hemisphere, but lies deeply within the Sylvian fissure, the convolutions forming the margin of which conceal it. It consists of four or five short convolu tions, which radiate from the locus per forates anticus, situ ated at the inner end of the fissure. This lobe is almost entirely surrounded by a deep sulcus, which insulates it from the adjacent convolutions. It lies opposite the upper part of the ali- sphenoid, where it articulates with the parietal and squa- /,. , Fia. 72. Orbital surface of the left frontal lobe Convolutions alSO an a the island of Reil ; the tip of the temporo- ov - j. nn J.I, P i nr , pr sphcnoidal lobe has been removed to display the i , . latter. 17, convolution of the margin of the longi- SUrface Of the hemi- tudinal fissure; 0, olfactory fissure, over which o-r>Tia-ro onr) rm tTio * ie lf actor y peduncle and lobe are situated; spnere, ana C , T p^ tri-radiate fissure ; 1" 1 ", convolutions on the Under Surface which orbital surface ; 1, 1, 1, 1, under surface of infero- frontal convolution; 4, under surface of ascend- restS On the tentO- jug frontal, and 5, of ascending parietal convolu- rium, but these have UODS; c central lobe or iwula. no relation to the bones of the cranial vault. They may be studied in connection with the corpus callosum or great transverse commissure, which connects the two hemispheres, and with certain fissures situated on these surfaces of the hemisphere. The small convolutions which lie behind the internal part of the parieto-occipital fissure form the inner convolutions of the occipital lobe, or the occi pital lobule (Fig. 73). Those which lie immediately in front of the same fissure belong to the inner face of the parietal lobe, and form the quadrilateral lobule. It is customary, however, to name the convolution which extends forwards from that fissure along the margin of the longitudinal fissure to tho anterior end of the hemisphere, and which then turns back to the locus perforatus anticus as the marginal convolution. This is separated by a fissure called calloso-marginal, from the callosal convolution or gyrus fomicatus, which, commencing at the locus perforatus an ticus, turns round the anterior end of the corpus callosuin, extends parallel to its upper surface, and then turns round its posterior end. It is separated from the corpus callosum by the callosal fissure, at the bottom of which the grey matter
of the gyrus fornicatus termintes in a well-defined edge.
