244 THE BRAIN OF THE TIGER SALAMANDER 



mamillary fibers are accompanied by fibers passing in the reverse 

 direction — tr. mamillo-thalamicus — the combined tract being the 

 probable precursor of the mammalian mamillo-thalamic bundle of 

 Vicq d'Azyr. 



EFFERENT CONNECTIONS 



From the whole extent of the preoptic nucleus fibers pass dorsad 

 to enter the stria medullaris thalami. Others probably ascend to the 

 hemisphere in the basal bundles. The dorsal (mamillary) part of the 

 hypothalamus is connected with the anterior part of the thalamus, 

 as just mentioned. There is probably also a mamillo-cerebellar con- 

 nection (p. 170). The other efferents from the mamillary region are 

 the very extensive and complicated systems of mamillo-tegmental, 

 peduncular, and interpeduncular fibers illustrated crudely in figures 

 18 and 21 and mentioned on page 278, where some references to 

 literature are given (for fuller description see '396, p. 551). 



The efferents from the ventral part of the hypothalamus include 

 the ascending fibers and the hypothalamo-peduncular and tegmental 

 systems already mentioned. Fibers may go out with some compo- 

 nents of the postoptic commissure to the thalamus and tectum, but 

 these have not been recognized. 



The pars magnocellularis of the preoptic nucleus gives rise to the 

 large hypophysial tract, which is one of the major features of the 

 ventral hypothalamus. In most lower vertebrates, including some 

 amphibians, these large cells are aggregated as a well-defined nu- 

 cleus, homologous with the supraoptic nucleus of mammals. But in 

 Amblystoma, as in Necturus, they are dispersed, being most nu- 

 merous above and anteriorly of the chiasma ridge. These widely 

 scattered large cells in the aggregate are here termed the nucleus 

 magnocellularis. Their long dendrites are widely branched and may 

 be activated by practically all nerve fibers of the preoptic and epi- 

 chiasmatic areas. In this mixed collection of fibers there are two sys- 

 tems which seem to be specifically related with these large cells. 



The first of these systems is the tr. preopticus (fig. 2C, 96, 97, 

 tr.po.). These fibers are axons of cells at the postero ventral border of 

 the anterior commissure ridge and the anterior part of the preoptic 

 nucleus, which descend in the thin floor of the long preoptic recess, 

 then recurve dorsally along the anterior face of the chiasma ridge, 

 where they spread in the alba among dendrites of the cells of the 

 nucleus magnocellularis. In the catfish, Ameiurus, the course is simi- 



