"54 



HI/MAN ANATOMY. 



decrease in size as they approach the inner border (Foville). The substance of 

 the space proper consists of a thin sheet of gray matter containing groups of 

 nerve-cells, some of which constitute the nuclei of primary centres interposed in 

 the paths connecting the olfactory lobe with the secondary (cortical) olfactory 

 centres (page 1222). In addition to the white strands of nerve-fibres composing 

 the olfactory strue which after a longer or shorter superficial course sink into the 

 substance of the perforated space, an obliquely directed narrow ribbon-like tract, 

 the diagonal band of Broca, may be sometimes made out along the inner margin 

 of the area perforata. In front it is continuous with the subcallosal gyrus and 

 behind passes along the optic tract towards the anterior end of the hippocampal 

 convolution. The band is of interest as being probably the beginning, on the 



basal surface of the brain, of at least 



FIG. 995. a part of the fibre-tracts contained 



within the rudimentary supracallosal 

 gyrus (page 1157) that, in turn, is 

 prolonged into the gyrus dentatus. 

 The uncus is the thickened 

 anterior extremity of the gyrus 

 hippocampi, recurved around the 

 front end of the hippocampal fissure 

 (Fig. 992). Antero-inferiorly it is 

 separated from the adjacent part of 

 the temporal lobe by the inconspicu- 

 ous incisura temporalis or rhinal 

 sulcus, which in animals possessing 

 a well developed rliinencephalon 

 constitutes a definite boundary be- 

 tween this part of the hemisphere 

 and the pallium. With its deeper 

 surface the uncus is in close relation 

 with the anterior perforated space, 

 whilst postero-mesially it is connected 

 with the fimbria (page 1165) and 

 the gyrus dentatus (page 1166). 

 Although seemingly a part of the 

 limbic lobe, the comparative studies 

 of Turner and of Elliot Smith have 

 established its morphological inde- 

 pendence from the last-named lobe 

 and emphasized its relation with the 

 rhinencephalon. With the lateral 

 olfactory stria, the uncus constitutes 

 in man the feeble representation of 

 the large and conspicuous pyramidal 

 lobe, which in many animals forms the most massive part of the olfactory brain. 

 The accessory parts of the rhinencephalon include structures which, for the 

 most part, constitute collectively an elaborate path by which the olfactory cortical 

 centres are connected with each other, on the one hand, and with the optic thalamus 

 and lower levels on the other. Since these structures are by position closely asso- 

 ciated with parts of the brain still to be described, with the exception of the anterior 

 perforated space already noted (page 1153), they will be merely mentioned here, as 

 components of the rhinencephalon, their details being deferred until the related parts 

 are considered. 



The fornix (page 1158), the fimbria (page 1165) and the hippocampus 

 i l>at, r <- 1165), all seen within the lateral ventricle (page 1164), constitute important 

 paths bv which fibres pass to and from the olfactory cortical cent re. The gyrus 

 subcallosus (page 1153), the gyrus supracallosus (page 1157) and the gyrus 

 dentatus (page 1166) together form an additional arched tract, which, beginning at 

 the base of the brain, follows closely the convex surface of the corpus callosum as f; 





Atrophic ventricu- 

 lar area 



Nerve-fibre layer 



Granular layer. 



Layer of mitral 

 cells 



Molecular layer 



Olfactory 

 glomeruli 



Blood-vessel 



Olfactory fibre EBBS 



layer ^s?^L Jt ii':-- .... - , ; ---- -"-"A'^-""" 



Transverse section of olfactory bulb ; drawing includes part 

 of bulb lying ventral to atrophic ventricular area. X 90. 



