THE ORIGIN AND SIGNIFICANCE OF CEREBRAL CORTEX 95 



frontal tract (figs. 19, 111, tr.ih.f.) in the lateral forehrain bundle, 

 and (2) from the visceral field of the hypothalamus, by way of the 

 medial bundle (fig. 113). The first of these bundles connects with the 

 lateral wall of the hemisphere, the second with the medial wall. In 

 conformity with this, the descending path from the lateral wall goes 

 by way of the lateral forebrain bundle to the somatic motor field in 

 the peduncle, and the path from the medial wall goes by way of the 

 medial bundle to the visceral field in the hypothalamus (figs. 6, 11, 

 111, 112, 113). These connections inaugurated the different types of 

 differentiation seen in lateral and medial walls of the hemisphere, a 

 difference in type which becomes more pronounced in higher ani- 

 mals. The amygdala is of intermediate type, with somatic and 

 olfacto-visceral connections of both afferent and efferent fibers 

 (fig. 19). . _ 



All parts of the hemisphere are under olfactory influence, with 

 olfacto-visceral correlations effected medially and olfacto-somatic 

 laterally. The motor responses are radically different, and this dif- 

 ference is probably the basic determining factor in shaping the struc- 

 tural plan, not only of the olfactory connections, but of the organiza- 

 tion of the hemisphere as a whole. 



The hypothalamus is disproportionately large in lower vertebrates 

 as compared with higher, and the olfacto-visceral functions of the 

 hemisphere are correspondingly magnified. This doubtless accounts 

 for the fact that differentiation in the pallial field is further advanced 

 on the medial (hippocampal) side than elsewhere (fig. 99) and also 

 for the fact that efferent projection fibers from this part of the 

 pallium (fornix system, p. 254) appear in large numbers very early in 

 phylogeny. 



Fibers ascend from the dorsal thalamus to the hemisphere in fishes 

 and in all higher animals. In Amblystoma these thalamo-frontal 

 fibers arise in the generalized nucleus sensitivus, ascend in the lateral 

 forebrain bundle, and end in the amygdala and middle part of the 

 corpus striatum (figs. 19, 111, tr.th.f.). They are comparable with 

 thalamo-striatal fibers of mammals and are precursors of mamma- 

 lian sensory projection fibers, though in urodeles none have been 

 seen to reach the pallial part of the hemisphere. This tract is small, 

 and there is no evidence that different sensory systems are separately 

 localized within it; its terminal nucleus, the corpus striatum, is cor- 

 respondingly small and simply organized. The thalamo-frontal sys- 

 tem is larger in reptiles and birds, where the striatal complex is mag- 



