764 THE CEREBRAL CORTEX. 



hippocampal gyrus, the hippocampus major, and the dentate convolu- 

 tion x (but not the amygdala) ; (4) a convolution termed the " bordering 

 gyrus," 2 which closely encircles the corpus callosum, and is connected 

 at both ends with the olfactory lobe. Very little of this convolution 

 is present in primates, the dorsal part only being represented by the 

 " nerves of Lancisi," and the ventral part by the lamina of the septum 

 lucidum, the peduncle of the corpus callosum, the fornix, and the 

 fimbria ; (5) the anterior part of the callosal convolution (gyrus forni- 

 catus) ; 3 (6) the anterior commissure. 



The main connection of the olfactory nerve fibres is with the cortex of the 

 same side of the brain. On physiological grounds, it appears to be doubtful 

 whether there is any crossed connection between the two olfactory lobes, but 

 anatomically such has been affirmed through the medium of some of the 

 fibres of the anterior commissure (Meynert). This commissure is stated to be 

 chiefly a means of connection between the hippocampal gyri, especially their 

 anterior parts, and it also appears to connect the olfactory tracts with one 

 another. 4 According to the observations of Flechsig, 5 the nerve fibres of the 

 olfactory tract, which begin to be myelinated towards the end of the ninth 

 month of foetal life, pass (1) partly to the whole of the hinder border of the 

 base of the frontal lobe and to the front part of the gyrus fornicatus ; (2) 

 partly to the uncus and to the adjacent inner pole of the temporal lobe, these 

 two regions coming together below the island of Reil. From the frontal 

 olfactory region, fibres pass by way of the stria tecta to the middle segment of 

 the gyrus fornicatus, others into the septum lucidum, and then by way of 

 the cingulum to the cornu ammonis. From the temporal olfactory region a 

 strand of association fibres also goes into the cornu ammonis. From both 

 regions, tracts (corticifugal) pass to the globus pallidus of the lenticular nucleus 

 and to the thalamus. 



Ramon y Cajal 6 has traced fibres of the external auditory root, which arise 

 as axons of the mitral cells of the olfactory bulb, into the uncus, where they 

 terminate by arborising in the grey matter. Cajella " describes the fibres of the 

 mesial root as passing to end in the trigonum olfactorium by arborising amongst 

 peculiar groups of pyramidal cells which are found in this region, and to which 

 the name " olfactory islands " has been applied. According to the observations 

 of Elliot Smith in monotremes, 8 the axons of the mitral cells pass partly 

 direct to the fimbria and fascia dentata, arborising amongst the cells of the 

 fascia dentata (the axons of these cells ultimately go to the hippocampus) ; 

 partly to the septum lucidum and peduncle of corpus callosum (gyrus sub- 

 callosus of Zuckerkandl; precommissural area of Elliot Smith), where they 



1 Hill, Phil. Trans., London, 1893, vol. clxxxiv. p. 389, states that in anosmatics the 

 fascia dentata varies directly with the development of the olfactory organs. 



2 See Quain's "Anatomy," vol. iii. pt. 1 ; also Zuckerkandl, " Uebe'r das Riechcentrum," 

 Stuttgart, 1887. 



3 There is, however, .as Broca pointed out, an exception in the case of Cetacea, "in which, 

 although the sense of smell is rudimentary or absent, the anterior portion of the callosal 

 convolution is specially well developed" (Ferrier, "Functions of the Brain," p. 314). 

 According to Ferrier, it is only the uncinate (anterior) portion of the hippocampal gyrus 

 (hippocampal lobule) the development, of which is correlated with that of the sense of 

 smell. In man and in the dolphin (both anosmatics) the hippocampus is comparatively 

 well developed. In reptiles, which have a strong olfactory sense, the cerebrum is almost 

 entirely represented by the hippocampal region (Edinger, Anat. Anz., Jena, 1893, Bd. 

 viii. S. 305). 



4 Ganser, Arch. f. Psyehiat., Berlin, 1878, Bd. ix. See also Dana, "On the central 

 tracts of the^ olfactory nerves," New York Med. Jovrn., 1889. 



"Die Localisation der geistigen Vorgange insbesondere der Sinnesempfindungen," 

 Leipzig, 1896. 



fe "Neue Darstellung vora histol. Bau des Centralnervensystems." 



7 "La region olfatoria," Madrid, 1893. 



8 Anat. Anz., Jena, 1895, Bd. x. S. 470. 



