Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/931

Rh BEAIX.] dinal fissure, which subdivides it into two hemispheres. Inferiorly there is a continuity of structure between the two hemispheres across the mesial plane, and if the two hemispheres be drawn asunder by opening out the longi tudinal fissure, a broad white band, the corpus callosum, may be seen at the bottom of the fissure passing across the mesial plane from one hemisphere to the other. The outer surface of each hemisphere is convex, and adapted in shape to the concavity of the inner table of the cranial bones ; its inner surface, which bounds the longitudinal fissure, is flat and is separated from the opposite hemisphere by the falx cerebri; its under surface, where it rests on the tentorium, is concave, and is separated by that membrane from the cerebellum and pons. From the front of the pons two strong white bands, the crura cerebri or cerebral peduncles, pass forwards and upwards to enter the optic thalami in their respective hemispheres. Winding round the outer side of each crus is a flat white band, the optic tract. These tracts converge in front, and join to form the optic commissure, from which the two optic nerves arise. The crura cerebri, optic tracts, and optic commissure enclose a lozenge shaped space, which includes a, a grey layer, called pons Tarini, which, from being perforated by seve ral small arteries, is often called locus perforatus posticus; b, two white mammillae, the corpora albicantia ; c, a grey nodule, the tuber cinereum, from which, d, the infundi- bulum projects to join the pituitary body. Immediately in front of the optic commissure is a grey layer, the lamina cinerea or lamina terminalis of the 3d ventricle; and between the optic commissure and the inner end of each Sylvian fissure is a grey spot perforated by small arteries, the locus perforatus anticus. The peripheral part of each hemisphere, which consists of grey matter, exhibits a characteristic folded appearance, known as the convolutions or gyri of the cerebrum. These convolutions are separated from each other by fissures or sulci, some of which are considered to subdivide the hemi sphere into lobes, whilst others separate the convolutions in each lobe from each other. In each hemisphere of the human brain five lobes are recognised : the temporo-sphe- noidal, frontal, parietal, occipital, and the central lobe or insula. Passing obliquely on the outer face of the hemi sphere from before, upwards and backwards, is the well- marked Sylvian fissure, which is the first to appear in the development of the hemisphere. Below it lies the tem- poro-sphenoidal lobe, and above and in front of it, the parietal and frontal lobes. The frontal lobe is separated from the parietal by the fissure of Rolando, which extends on the outer face of the hemisphere from the longitudinal fissure obliquely downwards and forwards towards the Sylvian fissure. About two inches from the hinder end of the hemisphere is the parieio-occipital fissure, which, commencing at the longitudinal fissure, passes down the inner surface of the hemisphere, and transversely outwards for a short distance on the outer surface of the hemi sphere; it (separates the parietal and occipital lobes from each other. The Temporo-Sphenoidal Lobe presents on the outer surface of the hemisphere three convolutions, arranged in parallel tiershovn. above downwards, and n&medsuperior, middle, and inferior temporo-sphenoidal convolutions. The fissure which separates the superior and middle of these convolutions is called the parallel fissure. The Occipital Lobe also con sists from above downwards of three parallel convolutions, named superior, middle, and inferior occijntal. The Frontal Lobe is more complex ; immediately in front of the fissure of Rolando, and forming indeed its anterior boundary, is a convolution named ascending frontal, which ascends obliquely backwards and upwards from the Sylvian to the longitudinal fissure. Springing from the front of this con- ANATOMY 873 volution, and passing forwards to the anterior end of the cerebrum, are three convolutions, arranged in parallel tiers from above downwards, and named superior, middle, and inferior frontal convolutions, which are also prolonged on to the orbital face of the frontal lobe. The Parietal Lobe is also complex ; its most anterior convolution, named ascending parietal, ascends parallel to and immediately behind the fissure of Eolando. Springing from the upper end of the back of this convolution is the postero-parietal convolution, which, forming the boundary of the longitudinal fissure, extends as far back as the parieto-occipital fissure; springing from the lower end of the back of this convo lution is the supra - marginal convolution, which forms the upper boundary of the hinder part of tho Sylvian Far FIGS. G9 and 70. Profile and vertex views of cerebrum. Fr, the frontal lobe; Par, pai ietal ; Oc, occipital; Ts, temporo-sphenoidal lobe ; SS, Sylvian fissure; RR, fissure of Rolando; PO, parieto-occipital fissure; IP, intra -parietal fissure; 1 P, Parallel fissure; SF and IF, supero- and infero-f rental fissures; 1, 1, 1, inferior, 2, 2, 2, middle, and 3, 3, 3, superior frontal convolutions; 4, 4, ascending frontal convolution ; 5, 5, 5, ascending parietal, 6, postero-parietal, and 6, 6, angular convolutions; A, supra-marginal, or convolution of the parietal eminence; 7, 7, superior, 8, 8, 8, middle, and 9, 9, 9, inferior temporc- sphenoidal convolutions; 10, superior, 11, middle, and 12, inferior occipital convolutions; a, /3, &amp;lt;y, d, four annectent convolutions. fissure ; as this gyrus occupies the hollow in the parietal bone, which corresponds to the eminence, it may appro priately be named the convolution of the parietal eminence. Continuous with the convolution of the parietal eminence is the angular convolution, which bends round the posterior extremity of the Sylvian fissure. Lying in the parietal lobe is the intra-par ietal fissure, which separates the convolution of the parietal eminence from the postero-parietal con- I. no