Page:The New International Encyclopædia 1st ed. v. 14.djvu/429

NERVOUS SYSTEM AND BRAIN. 379 NERVOUS SYSTEM AND BRAIN. which il is M'paiatcd liy tlio falx ceifbri ; and an under suilati'. wliieh is somewhat concave and rests upon tlu- anterior and middle fossa' of the skull and upon the tentorium ocrelxdli. The surface of each heniispliere is nuirked 1)}' nu- merous elevations and de|)ressions, the former known as convolutions or ^ri. the latter as fissures and sulci. These convolutions serve to increase the actual surface of the lirain without increasing; the actual size of the orijan, and the number and e.xtent of the convolutions are in direct relation to the intellectual development, increasing tliroughout the ascent of the mammalian scale, and reaching their highest com- plexity in civilized num. FissrRE.s. 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. Eacli lu'misphere presents three fissures which separate the heniispliere into lol)es. These are: (1) The fisjuro of Sylvius. Tliis begins at the base of the brain (see diagram) aiul ])asses upward and backward on the external surface of the hemi- .sphere. where it divides into a short- er ascending limb and a longer horizontal linili. It separates the frontal and ])arietal lobes from the tcmpero-sphenoidal lolie. (2) The fissure of Rolando begins a little behind the mid-point of Kulando and the jiosU'rior extremity of the brain, sends a short branch outwaid, while its longer branch extends downwanl and forward on file inner surface of the hemisphere. LoHEs. The frontal lobe includes that part gnd ofCaUoso CONV<ILUTION8 AND FISSURES OF THE OUTER SURFACE OF THE CBREDRAL HEMISPHERE. 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 or precen- tral sulcus. This remaining portion of the front- al lobe is divided by two longitudinal sulci into (b) the superior frontal convolution, (c) the middle frontal convolution, and (d) the inferior frontal convolution. The last is sometimes known as the convolution of Broca. 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. Parieto-occipttajb J^issure UPPER SURFACE OP THE BRAIN. THE ARACHNOID HAVING BEEN REMOVED. 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 ]iariptal lobe. (3) The parieto-occipital fis- sure begins about midway between the fissure Vol. XIV.— 25. CONVOLUTIONS AND FISSCHES OF THE INNER SURFACE OP THE CEREBRAL HEMISPHERE. The parietal lobe is separated from the front- al lobe by the fissure of Rolando, from the tcmpero-sphenoidal lobe below by the horizontal limb of the fissure of Sylvius, from the occipital lobe by the parieto-occipital fissure. It may be subdivided into four fairly well marked eon- volutions: (a) The ascending parietal, lying just behind the fissure of Rolando. Behind the as-