HEMISPHERES OF THE BRAIN. 



653 



animals), such as the dog and fox. To the lobe thus defined the following parts 

 must be added, viz. : the laminae of the septum lucidum, together with the 

 fornix and its fiinbriae, which may be regarded as forming an inner or deep arch ; 

 the peduncles and longitudinal striae of the corpus callosum, together with the 

 gyrus dentatus, which form a middle arch, while the outer arch is constituted by 

 the callosal and hippocampal convolutions : the first two arches are separated 

 from each other by the corpus callosum. 



Convolutions of the Limbic Lobe. (1) The callosal convolution, gyrus forni- 

 catus, or gyrus cinguli is an arch-shaped convolution, lying in close relation to 

 the superficial surface of the corpus callosum, from which it is separated by a slit- 

 like fissure, the callosal fissure. It commences below the rostrum of the corpus 

 callosum, curves round in front of the genu, extends along the upper surface of 

 the body, and finally turns downward behind the splenium, where it is connected 

 by a narrow isthmus with the gyrus hippocampi. It is separated from the 

 marginal convolution bv the calloso-marginal sulcus, from the quadrate lobe by 

 the post-limbic sulcus, and from the subcalcarine convolution by the calcarine 

 fissure. 



(2) The hippocampal convolution (gyrus hippocampi) is bounded above by the 

 hippocampal or dentate fissure, and below by the anterior part of the collateral 



FIG. 349. The Island of Reil. Left side. The overlapping parts of the hemisphere have been removed. 

 (Dalton.) 1, 2, 3, gyri breves ; 4, 5, gyrus longus, bifurcated at its upper extremity. 



fissure. Behind, it is continuous superiorly, through the isthmus, with the callosal 

 convolution, and inferiorly with the subcalcarine or lingual convolution. Its 

 anterior extremity is recurved in the form of a hook, and is named the uncus. 

 Running in the substance of the callosal and hippocampal convolutions, and con- 

 necting them together, is a tract of arched fibres, named the cingulum. The outer 

 root of the olfactory tract passes into the anterior extremity of the hippocampal 

 convolution, and the inner root into the commencement of the callosal convolution, 

 so that these two convolutions, with the addition of the olfactory tract, present a 

 racquet-like appearance the olfactory tract constituting the handle and the two 

 convolutions the circumference of the blade. 



(3) The dentate convolution (formerly named the dentate fascia) is situated above 

 the gyrus hippocampi, from which it is separated by the hippocampal or dentate 

 fissure. It is covered by the fimbria, and is a narrow, elongated convolution, the 

 free surface of which presents a notched or toothed appearance, hence its name. 

 Posteriorly it is prolonged as a delicate lamina, the fasciola cinerea, around the 

 splenium of the corpus callosum, and becomes continuous on the upper surface 

 of that body with its mesial and lateral longitudinal striae. Anteriorly it is pro- 

 longed into the notch produced by the recurving of the uncus, where it forms a 

 sharp curve ; from here it can be traced as a delicate band (band of Giacomini) 

 over the uncus, on the outer surface of which it is lost. 



