THE TELENCEPHALON. II39 



the so-called internal parieto-occipital fissure, separates the mesial surfaces of the 

 parietal and occipital lobes and ends below by joining the calcarine fissure, the 

 two sulci together forming a >- whose posteriorly directed diverging limbs in- 

 clude a wedged-shaped portion of the occipital lobe known as the cuncus. The 

 parieto-occipital fissure is continued without interruption across the upper margin of 

 the hemisphere and onto the external surface for a short distance. This outer exten- 

 sion, usually only from 12-15 mm. in length, constitutes the external parieto-occipital 

 fissure and terminates after its limited transverse course in a bowed convolution, the 

 arcus parieto-occipitalis, which surrounds and separates its end from the occipital 'part 

 of the interparietal fissure. Although sometimes ending in two short and somewhat 

 open branches, the external limit of the parieto-occipital fissure is usually relatively 

 inconspicuous; notwithstanding, the sulcus is of much importance as affording a 

 readily recognized upper limit of the conventional boundary line between the occipi- 

 tal and the parietal and temporal lobes. In the fcetal brain the parieto-occipital sul- 

 cus produces a distinct imagination of the wall of the cerebrum and corresponds, 

 therefore, to a complete fissure. In the adult brain, however, all trace of this infold- 

 ing has disappeared in consequence of the growth and thickening of the ventricular 

 wall which subsequently takes place (Cunningham). 



The collateral fissure (fissura collateral) is a well marked sulcus on the 

 inferior surface of the hemisphere. It begins behind a little to the outer side of the 

 occipital pole and extends forward, crossing the tentorial area parallel with, below 

 and lateral to, the calcarine fissure, until opposite the posterior end of the corpus callo- 

 sum, where it meets the hippocampal gyrus. It is then directed slightly outward, 

 forming the lateral boundary of the last-named convolution, over the temporal area 

 well toward the temporal pole, near which it either embraces or joins with a short 

 curved furrow, the indsura temporalis, which, in conjunction with the collateral 

 fissure, separates the lower or hippocampal part of the limbic lobe from the temporal 

 lobe. According to Cunningham, the collateral fissure is at first represented by 

 three distinct parts a posterior or occipital, an intermediate and a temporal which 

 later become one continuous furrow. Of these three primary divisions, the interme- 

 diate, and usually also the temporal, are complete fissures', producing respectively 

 the collateral protuberance and the collateral eminence seen in the lateral ventricle 

 (page 1164). The occipital portion of the fissure is never complete and, therefore, 

 does not give rise to any elevation. 



The calloso-marginal fissure (sulcus cinguli) is the most conspicuous sul- 

 cus on the mesial surface of the hemisphere, where it appears as a curved furrow 

 running above and concentric with the arched upper surface of the corpus callosum. 

 It begins in front below the fore-end of this bridge, just above the anterior perforated 

 space, sweeps around the genu of the corpus callosum and arches backward above 

 the latter structure almost as far as the splenium, where it turns upward (raraus mar- 

 ginalis) and reaches the supero-mesial border of the hemisphere a short distance be- 

 hind the overturned end of the Rolandic fissure. By its course the calloso-marginal 

 sulcus marks off on the anterior two-thirds of the mesial surface of the hemisphere 

 the marginal convolution of the frontal lobe from the callosal gyrus of the limbic lobe, 

 the somewhat uncertain posterior boundary of the latter beyond the sulcus being 

 indicated by the inconspicuous postlimbic fissure, which arches downward concen- 

 trically with the splenium. The frequent variations in the details of the calloso- 

 marginal fissure depend upon irregularities in the arrangement and fusion of the 

 three separate furrows by the union of which a continuous sulcus is formed. 



The limiting sulcus of Reil (sulcus circularis Reili) is a shallow furrow 

 that incompletely surrounds the insula and imperfectly separates this buried portion 

 of the cerebral cortex from the deeper parts of the enclosing opercula. The sulcus 

 consists of three parts a superior, separating the island from the parietal and fron- 

 tal lobes, an anterior, intervening in front between the insula and the frontal lobe, and 

 a posterior, imperfectly separating the hind part of the island from the limbic lobe. 



THE LOBES OF THE HEMISPHERES. 



The Frontal Lobe. The frontal lobe (lobus frontalis) is the largest of the 

 subdivisions of the hemisphere and includes approximately one-third of the hemi- 



