Behind the sulcus centralis, or .fissure of Rolando, and above the ramus posterior of the fissure of Sylvius, stretches the parietal lobe, while below the last named fissure lies the temporal lobe. Posteriorly, both lobes pass into the occipital lobe without a definite boundary. As a conventional boundary, we may adopt a line that unites the dorsal end of the parieto-occipital fissure, which incises the upper border of the hemi sphere, with the incisura praeoccipitalis. The fissura is a deep cleft on the hind part of the median surface of the hemisphere (Fig. 22), which incises the upper border of the hemisphere and extends a short distance on its dorso-lateral aspect. It is readily identified as a deep incision on the upper border of the hemisphere about midway between the central fissure and the occipital pole, rather nearer the latter. The incisura praeoccipitalis appears as a slight notch on the lateral border of the hemisphere, approximately at the junction of the middle and posterior thirds (Fig. 19).
Lobus parietalis. The sulcus postcentralis extends behind and more or less parallel with the central fissure. This furrow is sometimes continuous, and sometimes subdivided into two parts, the sulcus postcentralis superior and inferior. Each sub division may retain its individuality, or at the same time join the sulcus interparietalis. When the superior postcentral fissure is independent, it usually exhibits variations in form and size, sometimes being unbranched and paralleling the central fissure, but often forming a three- or four-limbed furrow. As does the precentral, so the postcentral fissure at times anastomoses with the central fissure ; the inferior postcentral fissure, moreover, may connect with the Sylvian fissure.
The sulcus interparietalis begins, mostly with a bifurcation, behind the upper end of the inferior postparietal fissure. By the junction of this sulcus with one or other of the postcentral fissures, a veritable vortex of furrows, a fissure-star, is formed. The sulcus interparietalis pursues an arched course backward, beneath the dorso-lateral end of the parieto-occipital fissure, and usually opens out into the sulcus occipitalis trans?ersus. Occasionally the interparietal fissure passes across the transverse occipital furrow and continues backward as the sulcus occipilalis superior. The interparietal fissure is often made up of several parts ; during its course isolated fissures are given off upward as well as downward. A short furrow, known as the sulcus parietals transversus (Bris saud), extends upward, towards the border of the hemisphere, from in front of the dorsal end of the parieto-occipital fissure. Often two furrows pass downward. One
sulcus runs behind the ascending end-branch of the ramus posterior of the Sylvian fissure and is called the sulcus intermedius primus (Jensen). It often extends as a continuation of the upper transverse parietal fissure, but may be strongly developed and, indeed, may establish a connection between the interparietal fissure and the ascending end of the superior temporal fissure. The other sulcus is given off farther backward, runs behind the ascending end of the superior temporal fissure and is known as the sulcus intermedius secundus (Eberstaller). Both of these sulci intermedii may also exist as independent fissures.
By means of the foregoing fissures the following convolutions are defined : the gyrus centralis posterior lies behind the sulcus centralis, bounded below by the fissura cerebri lateralis and behind by the fissura postcentralis. Above the sulcus interpari etalis lies the lobulus parietalis superior, while beneath this fissure extends the lobulus parietalis inferior. This lower parietal lobule presents two special convolutions, the gyms supramarginalis and the gyms angularis. The gyrus supramarginalis encloses the ascending terminal stem of the ramus posterior of the Sylvian fissure and is bounded by the sulcus intermedius primus behind. The gyrus angularis surrounds the ascending end of the superior temporal fissure, and is bounded in front by the sulcus intermedius primus and behind by the sulcus intermedius secundus.
Lobus temporalii. One of the most constant fissures is the sulcus temporalis superior. It begins in front at the temporal pole, extends backward and upward parallel to the fissura cerebri lateralis and ends, as a rule, in the gyms angularis by running upward behind the ascending terminal branch of the fissura cerebri lateral's. At times one finds a forking into an ascending and a descending branch. The sulcus temporalis medius runs below the superior temporal fissure. The fissure is seldom continuous, usually being made up of several parts. Below the middle temporal fissure, and on the basal surface, extends the sulcus temporalis inferior. The three temporal convolutions are defined by these fissures. The gyrus temporalis superior extends below the sulcus cerebri lateralis and above the superior temporal fissure ; the gyrus temporalis medius lies between the superior and middle temporal sulci ; and the gyms temporalis inferior is located below the inferior temporal fissure. The surface of the upper temporal con volution facing the Sylvian fissure presents the gyri temporales transversi, also known as the convolutions of Heschl, which are weakly developed in the front half and more strongly behind.