158 THE CEREBRAL HEMISPHERES. 



and secondarily convoluted gyrus dentatus which covers the under surface of the splenium in 

 some mammals (G. Retzius). 



Hidden beneath the posterior part of the callosal gyrus. between this and the subsplenial 

 part of the gyrus dentatus, a small projection of the cortex is usually to be found, about 1 cm. 

 long and 2 mm. to 4 mm. broad, marked by 1 to 4 semicircular eminences. This has been described 

 (by A. Retzius originally, and more recently and fully by Zuckerkandl) under the somewhat 

 misleading name of callosal r/yrus (Balkenwindung), but is not to be confounded with the 

 gyrus cinguli which has long been so denominated. It is best developed in osmatic mammals. 



The fimbria (fig. 102, y?) represents the white matter of the hemisphere, which 

 here comes to the surface along the side of the dentate gyrus. It is continuous 

 with the alveus which covers the hippocampus major within the lateral ventricle, 

 and this is continuous with and forms part of the central white matter. The 

 fimbria itself, however, appears to be actually formed of fibres which are prolonged 

 into it posteriorly from the pillars of the fornix : anteriorly it is continued into the 

 white matter of the uncus. In section the fimbria appears as a somewhat club- 

 shaped expansion of the alveus, exhibiting a hook-like mesial prolongation (fig. 1 10,/). 

 This is the section of a thin lamina (tcenia fimbrife), continuous with the epithelium 

 covering the choroid plexuses which here invaginate the ventricular epithelium. 



The only sulcus within the limbic lobe which remains to be described (the 

 callosal sulcus and the fimbrio-deniate sulr-us having been already noticed) is the 

 hippocampal or dentate fissure (fig. 102, h}. This begins as a shallow furrow 

 just above the posterior end of the splenium corporis callosi, between the fasciola 

 cinerea and the end of the gyrus fornicatus, it then lies between the remainder of 

 the dentate gyrus above and the isthmus gyri fornicati and hippocampal gyrus below, 

 and becoming deeper as it passes forwards ends in the bend between the hippocampal 

 gyrus and its uncus. This fissure causes the elevation of the hippocampus major 

 or cornu Ammonis in the descending horn of the lateral ventricle (fig. 110). 



Gyrus fornicis. The fornix which has already been in the main described (p. 129) also 

 belongs to the limbic lobe as here defined. It contains an association-bundle uniting the 

 hippocampus major with the olfactory lobe. This bundle, the olfactory bundle of the cornu 

 Ammonis (Zuckerkandl), best developed in osmatic mammals, is contained within the body of 

 the fornix, but leaves the anterior pillar to pass in front of the anterior commissure and then 

 to run in the subcallosal gyrus to the vallecula Sylvii. Here it divides into two parts, one 

 (pars olfactoria) passing by the anterior circumference of the lamina perforata anterior to the 

 inner root of the olfactory tract, and the other (pars temporalis) at the hinder border of the 

 anterior perforated lamina to the point of the hippocampal gyrus. This is the band which has 

 been described (by F. Arnold and Broca) as continuing the peduncle of the corpus callosum 

 with the hippocampal gyrus. 



In the monkey there is a band of grey matter running along the upper surface of the 

 fornix, on each side near the middle line, wedged in between it and the corpus callosum. 

 This closely resembles the supracallosal gyrus above the callosum (see below), and appears to 

 be part of another rudimentary gyrus which may be termed gyrus infracallosus, or, including 

 certain patches of grey matter which occur here and there in the substance of the fornix, 

 gyrus fornicis. 



The cingrulum, which also belongs to the limbic lobe, is a tract of association-fibres run- 

 ning in the gyrus hippocampus and gyrus fornicatus. The bundle has for the most part a 

 longitudinal course within the white matter of these gyri ; but the fibres probably commence 

 in the cortex, and since they constantly tend, as the whole bundle courses longitudinally, to 

 diverge into the adjacent white matter of the hemisphere, it appears probable that they 

 connect the hippocampal and callosal gyri with the cortex of the outer surface of the hemi- 

 sphere (Beevor). A bundle of fibres having a somewhat similar course in the anterior 

 descending part of the gyrus fornicatus is usually described as part of this tract ; but according 

 to Beevor it is doubtful if these fibres can be considered to belong to the cingulum ; at all 

 events, their continuity with the remainder of that tract could not be traced. 



Gyrus limbicus. The longitudinal striae of the corpus callosum (p. 127) belong to a thin 

 lamina of grey matter which extends over the surface of the corpus callosum from the lower 

 edge of the gyrus fornicatus, and is much better developed in osmatic mammals than in Primates 

 (Valentin, Jastrowitz). The striag, together with this grey matter, represent a degenerated con- 

 volution (supracallosal gyrus, Zuckerkandl), which is continuous posteriorly with the fasciola 

 cinerea, i.e., with the dentate gyrus. Together with another degenerated gyrus in front, which is 

 represented only by the prolongation of the stria medialis (gyrus geniculi of Zuckerkandl) these 



