LIMBIC LOBE. 155 



The precentral lobule of the insula is formed of a few short convolutions (gyri breves) 

 (fig. 108, 1, 2, 3) converging from the base of the triangle but falling short of the apex of the lobule, 

 which is smooth and forms a slight prominence (pole of the island, p). These convergent gyri 

 are usually three in number (anterior, middle, and posterior), being subdivided in this manner 

 by secondary and somewhat shallow sulci, having the same direction. The most marked of 

 these sulci lies between the middle and posterior gyri breves, and has been named sulcus pre- 

 centralis insula;. A fourth small gyrus is described by Eberstaller as lying deeply underneath 

 the orbital operculum. This he has termed the gyrus brevis accessorius. This accessory gyrus, 

 and usually also the anterior of the three gyri breves, look forwards, lying upon an anterior 

 surface of the insula, which is sometimes nearly at a right angle with the lateral or general 

 surface, in other brains hardly marked off from it. Its grey matter is continued over the 

 anterior limiting sulcus into that of the orbital gyrus. The grey matter of the rest of the 

 precentral lobule is continuous superiorly with that of the frontal lobe, inferiorly with that 

 of the anterior perforated space. The inferior transitional part has been termed the threshold 

 of the island (limen insula!, Schwalbe). 



The postcentral lobule (gyrus longus insula- of Giacomini (fig. 108, 4, 5) ) is also often 

 subdivided at its upper end by a longitudinal furrow (sulcus postcentralis insulce*) into two parts, 

 anterior and posterior. Its grey matter is continuous below with the tip of the hippocampal 

 gyrus, laterally with that of the first temporal gyrus, above and posteriorly over the limiting 

 sulci with the grey matter of the parietal and temporal opercula. 



The three principal furrows of the insula (sulcus centralis, s. precentralis, and s. post- 

 centralis insular), which radiate from the vallecula Sylvii, have been compared with the three 

 similarly radiating fissures of the mantle, viz., the fissure of Rolando, the precentral fissure, 

 and the intraparietal fissure. They are not, however, directly continuous with those, being 

 arrested at the sulcus limitans insulte. Sometimes the fissure of Rolando is continued by a 

 small sulcus (transverse precentral) almost into the s. centralis insulse. Below and anteriorly 

 the sulcus centralis insulas comes into close relationship with the extremity of the calloso- 

 marginal sulcus. 



LIMBIC LOBE. The callosal gyrus, with its continuation the hippocampal 

 gyrus, were separated by Broca, on morphological grounds, from the other parts 

 of the hemisphere, as a distinct lobe, intimately united in front and below 

 with the olfactory lobe ; and the name of grande lobe limbique was given by 

 him to these combined lobes. To this lobe of Broca, Schwalbe added the other 

 central parts of the mesial wall of the hemisphere, viz., the lamina septi lucidi and 

 the dentate convolution, as well as the fornix ; looking upon these as representing 

 an inner encircling convolution concentric with the gyrus fornicatus and uncinatus, 

 and naming the whole lobe thus reconstituted, the " falciform lobe," the olfactory 

 lobe being, however, excluded. 



To the parts included by Schwalbe there must now be added a rudimentary 

 supracallosal gyrus (represented in man principally by the longitudinal strise of 

 the corpus callosum), which is continuous with the dentate convolution posteriorly, 

 and with the rudimentary gyrus geniculi (see p. 1 58) anteriorly. The lobe must 

 further include the peduncles of the corpus callosum (see p. 127), which appear to 

 represent a rudimentary gyrus (gyrus subcallosus, Zuckerkandl), and also another 

 rudimentary gyrus underneath the corpus callosum, partly imbedded in and 

 intimately connected with the fornix, to which the name of gyrus infracallosus or 

 gyrus for nicis may be applied. 



As thus constituted the limbic lobe is bounded circumferentially mainly by two 

 important fissures, the calloso-marginal above, and the anterior part of the collateral 

 below, while the less constant postlimbic sulcus separates it behind from the 

 parietal lobule, and the anterior end of the calcarine cuts into it just below the 

 splenium. 



The circumferential boundary is least distinct opposite the quadrate lobule, from which it 

 is only separated by the small postlimbic sulcus, which in man is only occasionally continued 

 into the calloso-marginal. But in many animals the fissure which forms the external boundary 

 is uninterrupted or nearly so, and forms the limbic fissure of Broca. This might perhaps 

 more appropriately be termed the outer limbic fissure, since the callosal sulcus and the 

 hippocampal fissure form together a second crescentic fissure within the limbic lobe, which 

 may be distinguished as the inner limbic fissure. The internal limit of the lobe is formed by 

 the Jissura choroidea (Schwalbe), through which the choroid plexus is invaginated into the 



