94 THE BRAIN OF THE TIGER Sx\LAMANDER 



bulb, except for the possibility suggested (p. 54; '21a) that specific 

 impulses pass from the vomeronasal organ to the accessory bulb and 

 amygdala. The nervus terminalis is more evidently specific, but of its 

 functions nothing is known. All other descending olfactory impulses 

 seem to be physiologically homogeneous. From this it follows that 

 local differentiations in the hemispheres are due mainly to differences 

 in the distribution of the various systems of ascending nonolfactory 

 fibers or to differences in the destination of descending fibers arising 

 from various parts. Evidently both factors are involved. 



The connections between the hemispheres and the lower levels are 

 assembled in the basal forebrain bundles and the stria medullaris 

 thalami. The former are roughly comparable with the subcortical 

 components of the mammalian extrapyramidal systems plus large 

 numbers of ascending fibers, and the details of their connections are 

 given in chapter xx. The analysis of these systems of fibers provides 

 the key to the interpretation of the hemispheres of Amblystoma; and 

 similar analysis in reptiles, birds, and mammals is essential for an 

 understanding of the history of cortical evolution. 



The fibers of the stria medullaris system are efferent or commis- 

 sural, passing from all parts of the hemisphere to the habenula or 

 decussating in the habenular commissure and returning to the hemi- 

 sphere. The arrangement of the components of the stria is shown in 

 figure 20, and this is essentially similar to that of mammals. The 

 stria medullaris, in fact, is remarkably similar in all vertebrates; and 

 these fibers evidently belong to a different category from those of the 

 basal bundles, which contain ascending and descending fibers in ar- 

 rangements that vaiy widely from species to species as we pass from 

 lower to higher vertebrates. These variations seem to be especially 

 significant for interpretation of morphogenesis. In this section, ac- 

 cordingly, attention will be directed to the latter systems, and the 

 reader is referred to chapters xiv and xviii for the interesting details 

 of the habenular connections. 



The basal bundles comprise three groups of fascicles, the descend- 

 ing components of which are shown diagrammatically in figure 6: 

 (1) dorsally and laterally the lateral forebrain bundles (f.lat.t.) ; (2) 

 ventrally and medially the medial bundles (f.med.t.) ; (3) the olfacto- 

 peduncular tract (tr.ol.ped.), lying between the two preceding and 

 with connections which are intermediate between them at both ends. 

 Fibers ascend in these bundles from two sources: (1) from the 

 somatic sensory field of the dorsal thalamus, by way of the thalamo- 



