254 THE BRAIN OF THE TIGER SALAMANDER 



functions. The habenula, accordingly, may be regarded as a bed- 

 nucleus interpolated in one of the main pathways from higher centers 

 of correlation, where olfaction as an exteroceptive function is in- 

 tegrated with other functions of this type and transmitted to lower 

 centers in the motor zone, where patterns of response are organized. 

 The hypothalamus is a similar way-station for through traffic from 

 the same higher centers to lower motor fields, with olfacto- visceral 

 functions dominant. 



Most of the afferent connections of the habenula are in the stria 

 medullaris, and most of the efferent connections in the f. retroflexus. 

 Before describing this chief thoroughfare of through traffic, mention 

 should be made of two systems of fibers which are in intimate func- 

 tional relation with the stria medullaris, though not component parts 

 of it. These are the fornix and stria terminalis. 



In mammals this name is given to a complicated system of fibers 

 which descends medially from the hippocampal formation to the 

 underlying brain stem in two groups separated by the anterior com- 

 missure. Both groups descend within or adjacent to the lamina ter- 

 minalis dorsally and rostrally of the interventricular foramen. The 

 postcommissural fornix, commonly called columna fornicis, passes 

 downward between the foramen and the anterior commissure and 

 thence across the thalamus to end chiefly in the mamillary body. 

 The precommissural fornix is a complex system of more loosely 

 arranged fibers descending in front of the anterior commissure to the 

 gray of the septal nuclei and neighboring parts and continuing spinal- 

 ward in the medial forebrain bundles to preoptic nucleus, hypothala- 

 mus, and (according to some descriptions) as far as the cerebral 

 peduncle. 



Primordia of the two main divisions of the mammalian fornix are 

 obviously present in the amphibian brain, but the topography of the 

 region of the lamina terminalis is here so different that these tracts 

 take peculiar courses. The anterior and hippocampal commissures 

 cross, not in the lamina terminahs, but in a commissural ridge behind 

 the interventricular foramen (fig. 2; '27, p. 235; '35, p. 299). To reach 

 this crossing, the fibers of the massive hippocampal commissure 

 swing downward behind the foramen (figs. 72, 76), and here they are 

 accompanied by the fibers of tr. cortico-habenularis medialis and tr. 

 cortico-thalamicus mediahs (fig. 20, nos. 8 and 9), which, as pointed 



