FISSURES AND SULCI OF CONVEX SURFACE The convex surface of the cerebral hemisphere (fades convexa cerebri) is related to two very extensive fissures, viz., the longi tudinal and the transverse. The longitudinal fissure of the cerebrum (fissura longitudinalis cerebri) is the vertical median cleft between the hemispheres of the cerebrum (Figs. 19 and 26). It contains the falx cerebri (Fig. 1). Its floor is formed by the corpus callosum. The cerebrum is separated from the cerebellum by the transverse fissure of the cerebrum (fissura transversa cerebri, Figs. 1, 6, 20 and 33). This fissure continues forward above the mid-brain, and terminates in the cerebrum between the inter-brain and the fornix, where it is continuous, by its lateral extremities, with the chorioidal fissures of the hemispheres. The tentorium occupies the posterior part. The anterior part of the transverse fissure contains the chorioid tela of the third ventricle.
There are three great furrows in the convex surface of each cerebral hemisphere which form interlobar boundaries and con stitute very important landmarks: The fissura cerebri lateralis, the sulcus centralis, and the sulcus occipito-parietalis (Figs. 26 and 27).
The lateral fissure (fissura cerebri lateralis [Sylvii]) begins in the fossa of the same name at the base of the brain (Fig. 2r). It runs outward between the frontal and the temporal lobe, along the lesser wing of the sphenoid bone; and, turning up ward, on the convex surface, it divides three-fourths inch behind the Sylvian point into an anterior horizontal, and anterior as cending and a posterior ramus (Fig. 27). Into the frontal lobe project the small anterior rami. They are separated by the foot (posterior end) of the inferior frontal gyrus, called the pars triangularis. Below the anterior horizontal ramus is a knuckle of the same frontal gyrus which forms the pars orbitalis; and, between the ascending and posterior rami, is located the pars opercularis, constituting the connecting gyrus between the inferior frontal and central gyri. The inferior frontal gyrus forms the frontal part of the operculum (pars frontalis operculi). The operculum (operculum, a cover), covers the island. The posterior limb of the lateral cerebral fissure separates the tem poral lobe from the parietal. Near the crotch and within the fissure is situated the island. A line drawn from the Sylvian point to the subparietal point lies over the posterior ramus of the lateral fissure. The Sylvian point is one inch and a quarter (3.2 cm.) behind the zygomatic process of the frontal bone and an inch and a half (3.75 cm.) above the zygomatic arch. The
subparietal point lies three-quarters of an inch (1.75 cm.) below the parietal tubercle.
The Sulcus Centralis (Rolandi, Figs. 26, 27, 28 and 33).— Beginning just above the posterior limb of the lateral cerebral fissure, is the central sulcus, which extends upward and back ward to the longitudinal fissure of the cerebrum. Its upper extremity is about half an inch (or 5.7 per cent.) behind the middle of a line drawn from the nasal eminence to the external occipital protuberance. With this sagittal meridian the sulcus centralis forms an anterior angle of 69 to 74. The average Rolandic angle is 7' (Cunningham). The sulcus centralis is three and three-eighths inches long and forms the boundary between the frontal and the parietal lobe. It is developed in two parts a superior third and an inferior two-thirds, which join at an angle open backward, called the genu superius; both parts may present an anterior concavity. Often a concealed gyrus separates the two parts of the sulcus at the genu superius (Fig. 26). This superior genu is in line with the superior frontal sulcus and marks the probable location of the trunk center and the boundary between the arm and leg areas in the anterior central gyrus. There is a less constant angle, the genu inferius, in the lower part of the central sulcus; it is in line with the in ferior frontal sulcus and marks the lower limit of the arm area and the upper limit of the face area.
The Occipito-parietal Sulcus (Sulcus occipito-parieta/is).— If the line on the skull locating the posterior limb of the lateral cerebral fissure be extended back to the sagittal meridian its posterior end marks the location of the occipito-parietal sulcus. The sulcus is located one-sixth of an inch above the lambda in the adult, and is from one and a half to two inches above the occipital pole. The greater part of the occipito-parietal sulcus is situated on the medial surface of the cerebral hemisphere, hence, it is divided into an internal part and an external part which are continuous through the supero-medial border (Figs. zo, 26 and 34). To the extent of its depth, which is about one inch, the external occipito-parietal sulcus separates the occipital from the parietal lobe on the convex surface of the hemisphere. Cunningham considers the occipito-parietal sulcus a true fissure because in the embryo it produces a ventricular eminence, though it disappears during development.'