THE CEREBRAL HEMISPHERES 669 



of the convex surface of the hemisphere into dorsal and lateral portions. It begins 

 on the dorso-medial border near its anterior end and, inclining gradually downward, 

 passes back to end on reaching the tentorial surface. It is continuous internally 

 with the transverse fissure usually and in front with the presylvian fissure. 



3. The presylvian fissure (F. prsesylvia) is the continuation forward of the 

 preceding. It ])asses forward, outward, and downward almost to the frontal pole, 

 and then inclines backward to end at the groove which marks the upper limit of 

 the rhinencephalon (Sulcus rhinalis). 



4. The marginal fissure (F. marginalis) extends along the dorso-medial border. 

 It begins a little in front of the middle of the border and turns around the occipital 

 pole to end on its tentorial aspect. 



5. The entomarginal fissure (F. entomarginalis) lies internal to the dorso- 

 medial border. It does not extend quite as far forward as the marginal fissure, 

 from whicii it is separated by a narrow gyrus. 



6. The ectomarginal fissure (F. ectomarginalis) lies about midway between the 

 marginal fissure and the posterior part of the suprasylvian fissure. 



7. The sulcus rhinalis is a very distinct furrow on the lower part of the lateral 

 surface which marks off the olfactory part of the brain (rhinencephalon) from the 

 rest of the hemisphere. It is undulating and is highest where it is crossed by the 

 lateral fissure (of Sylvius).' 



On the medial surface (Fig. 508) the main fissures and sulci are as follows: 



1. The calloso-marginal fissure (F. calloso-marginalis) is extensive and well- 

 defined. It is approximately i^arallel to the dorso-medial l)order of the hemisphere, 

 from which it is al)out half an inch distant. It begins in front a short distance 

 below and in front of the anterior bend (genu) of the corpus callosum and forms a 

 C-shaped curve, its posterior part extending on the tentorial surface to a point 

 Ijehind the dei:)ression for the corpora quadrigemina. It separates the marginal 

 gyri above from the gyrus fornicatus, which extends down to the corpus callosum. 



2. The transverse fissure (F. transversa) begins a little behind the middle of 

 the calloso-marginal fissure, passes obliquely upward and forward to the dorso- 

 medial border — into which it cuts deeply — and usually joins the suprasylvian 

 fissure.- 



3. The sublimbic fissure (F. sublimbica) curves over the gyrus fornicatus a 

 short distance above the corpus callosum. Its middle part is commonly indistinct, 

 and it is often divided into anterior and posterior parts. 



4. The callosal sulcus (Sulcus corporis callosi) separates the corpus callosum 

 from the gyrus fornicatus. 



The hippocampus (or cornu Ammonis) is a gyrus which curves from the deep 

 face of the pyriform lobe around the thalamus and forms the posterior part of the 

 fioor of the lateral ventricle. It can be displayed by cutting away the brain-stem 

 up to the optic tract and the interventricular foramen. Viewed from below the 

 hippocampal gyrus is seen to form a semicircular curve from the apex of the pyriform 

 lobe to the angle of divergence of the posterior pillars of the fornix, i. e., to a point 

 under the central part of the corpus callosum. It is separated deeply by the hippo- 

 campal fissure from the gyrus dentatus. Along the concave margin of the latter 

 is a band of white matter, termed the fimbria, which is the prolongation of the 

 greater part of the posterior pillar of the fornix into this region. The ventricular 

 surface of the hippocampus (Fig. 511) is covered with a thin layer of white 

 matter, the alveus, which is also derived from the posterior pillar of the fornix, and 



1 Just above this point is a lobe which is homologous with the insula of man. When the 

 overhanging gyri which partly conceal it — forming the operculum — are removed, there are dis- 

 closed several short, deeply placed gyri (Gyri breves). 



^ By some authors this is regarded as the homologue of the cruciate fissure of the dog, but it 

 seems Ukely that the latter is represented by a short and inconstant sulcus situated further forward. 



