MIDBRAIN AND THALAMUS OF NECTURUS 265 



the cerebral peduncle laterally of the fovea isthmi and appear to 

 terminate in the area of neuropil here termed the area lateralis 

 tegmenti (see pp. 242 and 295) which is also related with the 

 so-called basal optic tract and other fiber systems. The most 

 dorsal of these fibers spring from the extreme dorsal surface 

 of the pars intercalaris where there is a collection of neurons 

 with dense bushy dendrites. These axons pass downward across 

 the pars dorsalis thalami and may receive additions of both 

 myelinated and unmyelinated fibers from this part (figs. 38, 52, 

 53, tr.th.p.d s.). At the level of the pars ventralis thalami they 

 are joined by slender strands of unmyelinated fibers from the 

 gray layer of the pars ventralis thalami. 



In both Necturus and Amblystoma a strand of unmyelinated 

 fibers which seems to be a part of this system is seen to arise 

 from the habenula in company ^\dth the tractus habenulo- 

 penduncularis. It turns sharply lateral ward across the dorsal 

 aspect of the pars intercalaris to the dorso-lateral surface (figs. 

 39, 41, tr.th.p.d.s.), then ventralward and caudalward. Its 

 further course is parallel with that of the tractus habenulo-pe- 

 duncularis, but more superficial close to the outer surface of the 

 brain. There are extensive connections between the thalamus 

 and the habenula, but their details have not been worked out. 



The dorsal thalamo-peduncular tracts above described are 

 probably for the most part composed of descending fibers from 

 the sensory centers of the thalamus and epithalamus to the 

 motor centers of the pars ventralis thalami and tegmentum, 

 though other types of fibers may be mingled with them. Some 

 of these fibers may be collaterals from the mamillo-peduncular 

 fibers, comparable with the mamillo-thalamic tract of mammals. 

 Some of our Golgi preparations suggest this, but no definite 

 region can be identified as homologous with the mammalian 

 nucleus anterior thalami. 



Accompanying the slender* axons running from the pars dor- 

 salis thalami to the cerebral peduncle coarser contorted fibers 

 are seen in Golgi preparations. These are dendrites from the 

 nucleus of the tuberculum posterius which extend far forward 

 and dorsalward to break up in the neuropil of the pars dorsahs 



