THE SYSTEMS OF FIBERS 279 



Aniblystoma has no differentiated corpus inamillare; its primor- 

 diuni is in the dorsal part of the hypothahiniiis, from which efferent 

 fibers go out dorsalward to the peduncle, forward to the ventral 

 thalamus, and backward to the tegmentum. They are dispersed in 

 the alba of the hypothalamus, and as they leave it those for the 

 thalamus and tegmentum accumulate rostrally of those for the pe- 

 duncle. Some of them decussate in components 1 and 2 of the com- 

 missure of the tuberculum posterius ('36, fig. 2). Afferent fibers to the 

 hypothalamus are known to be present in the mamillo-peduncular 

 and mamillo-thalamic tracts, and this may be true also of the mamil- 

 lo-tegmental. 



Mamillo-peduncular fibers have wide distribution in the alba of 

 the nucleus of the tuberculum posterius, including the neuropil of the 

 area ventrolateralis pedunculi (figs. 6, 18, 27-30, tr.mam.ped.; '36, 

 p. 338, figs. 3, 8; '39/;, p. 338, figs. 6-12, 22, 35, 42, 89; '42, fig. 39; 

 Necturus, '336, p. 246; '346, p. 422). The accompanying pedunculo- 

 mamillary fibers transmit visceral sensory, gustatory, and optic im- 

 pulses received by the area ventrolateraHs pedunculi to the hypo- 

 thalamus. The mamillo-thalamic tract and the accompanying thala- 

 mo-mamillary tract put the mamillary region into reciprocal rela- 

 tions with the anterior part of the thalamus, as has been fully illus- 

 trated ('396, figs. 22, 35; '42, fig. 39; Necturus, '336, p. 247; '346, p. 

 423, figs. 2, 8, 9). 



The fibers of tr. mamillo-tegmentalis arise in company with those 

 of the two preceding systems and form a loose fascicle rostrally of 

 those of tr. mamillo-peduncularis (figs. 27-31; '36, p. 338, figs. 3, 

 8-21; '396, p. 552, fig. 43; '42, fig. 3). Most of these fibers enter 

 ventral tegmental fascicles of group (3), as shown in figures 92 and 

 94, some of them first decussating in the two ventral components of 

 the commissure of the tuberculum posterius. These fascicles lie ven- 

 trolaterally of those of group (4), which contains thicker and more 

 heavily myelinated fibers from^ the ventral hypothalamus. They ter- 

 minate in the alba of the isthmic tegmentum, chiefly through the 

 f. tegmentalis profundus. Mingled with these hypothalamic fibers are 

 some thicker well-myelinated fibers of tr. pedunculo-tegmentalis. 

 Some of the latter, and probably some of the hypothalamic fibers 

 also, descend for long distances in the f . longitudinalis medialis. 



Ventral fascicles (4). — These are mixed bundles derived chiefly 

 from the postoptic commissure. The largest component is tr. hypo- 



