160 



THE CEREBRAL HEMISPHERES. 





aspect (where it fits into the olfactory sulcus), and therefore triangular in section. 

 It measures about 2 centimeters in length and 2| millimeters in breadth, being 

 narrowest anteriorly where it passes out of the bulb, and broadening posteriorly as it 

 bifurcates to form the roots. As already stated it encloses a central grey substance 

 formed of neuroglia. 



The trigomim olfactoriiim and the area of Broca are in fact parts of one 

 and the same area of grey matter which forms the base of the anterior olfactory 

 lobule, and is traversed by the roots of the olfactory tract. This area is separated 

 from the peduncle of the callosum and the posterior olfactory lobule by the fissura 

 prima, and the mesial root of the olfactory tract in passing over its ventral surface 

 subdivides it superficially into its two parts. Of these the area of Broca receives 

 many fibres from the mesial root as this passes over the surface, whilst the trigonum 

 olfactorium receives others which are directly prolonged into it from the posterior 



Fig. 112. OLFACTORY LOBE OF THE 

 HUMAN BRAIN (His). 



Bu, olfactory bulb ; T, tract; Tr.o., 

 trigone ; R, rostrum of corpus callosum ; 

 p, peduncle *of corpus callosum, passing 

 into, g.s, gyms subcallosus (diagonal 

 tract, Broca) ; Br, Broca's area ; F.p, 

 lissura prima ; F.s, fissura serotina ; C.a, 

 position of anterior commissure ; L.t, 

 lamina terminalis ; Ch, optic chiasma : 

 T.o, optic tract ; p.olf, posterior olfactory 

 lobule (or anterior perforated space) : 

 m.r, mesial root ; I. r, lateral root of 

 tract. 



end of the tract, and which some- 

 times form a distinct middle root 

 of the tract. The ultimate des- 

 tination of these fibres which pass 

 into the trigone is not accurately 

 known, but some appear to join 

 the anterior commissure and to 

 be conducted by this into the 

 posterior part of the temporal 



lobe, and others, according to Meynert, may pass across in this commissure to the 

 temporal lobe and hippocampal region of the opposite side, an " olfactory chiasma " 

 being thereby produced. The evidence in favour of this crossed connexion is, 

 however, at present insufficient. Fibres from the posterior end of the olfactory 

 tract also pass directly into the white matter of the frontal lobe : these have been 

 named the upper or dorsal root of the olfactory tract (Henle). 



The outer or lateral root passes as already stated over the outer part of the 

 substantia perforata anterior, and becomes lost to view in the depth of the vallecula 

 Sylvii. In osmatic mammals it can be traced without difficulty into connection 

 with the anterior part of the hippocampal gyrus, and, according to Luys, with 

 the nucleus amygdalae. The inner or mesial root is recurved sharply around the 

 posterior limit of Broca's area, and its fibres pass partly into this, partly into the 

 callosal gyrus. The olfactory tract has thus a relation to the limbic lobe which has 

 been compared to that of the handle of a tennis-racquet to the ring of the blade, the 

 two extremities of the curved limbic lobe being continuous respectively with the inner 

 and outer roots of the olfactory tract (fig. 109). The combined olfactory and limbic 

 lobes may be spoken of collectively as the rhinencephalon ; it is marked off from the 

 rest of the hemisphere by the combined calloso-marginal, postlimbic and collateral 

 fissures forming together the limbic fissure, and in its development it varies directly 

 with that of the olfactory organ. This co-relation in development applies more 



p.olf. 



T.o. 



