240 THE BRAIN OF THE TIGER SALAMANDER 



rather high ventricular eminence, the structure of which is similar to 

 that of the peduncle. As illustrated by previously published figures 

 ('396, p. 544 and figs. 23, 28), the large neurons of the ventral 

 thalamus have widely spread dendrites and are evidently collectors 

 of impulses from many sources (figs. 16, 17). Spinalward of the bed- 

 nuclei at the anterior end, the two parts receive the strong tr. strio- 

 thalamicus, which descends from the strio-amygdaloid field by way 

 of the lateral forebrain bundle (f.lat.t.). Short fibers descend into 

 both parts from the dorsal thalamus (tr. dorsoventralis thalami) and 

 from the pretectal nucleus by tr. pretecto-thalamicus et hypo- 

 thalamicus. There are much larger connections from the tectum, 

 some uncrossed in the brachia of the superior and inferior colliculi 

 (br.col.) and some decussating in the postoptic commissure 

 (tr.t.th.h.c.p.). 



Fibers stream backward from the ventral thalamus into the motor 

 zone of all lower parts of the brain stem and into the hypothalamus, 

 some uncrossed and some decussating in the postoptic commissure 

 and in the ventral commissure of the isthmus. These have been seen 

 especially clearly in larval brains ('39, fig. 2; '396, p. 546 and fig. 23). 

 Most of these fibers end in the cerebral peduncle and tegmentum of 

 the midbrain and isthmus ; many of them extend into the trigemino- 

 facialis region; and relatively few which enter the f. longitudinalis 

 medialis may go into the spinal cord. 



The connections just described indicate that the ventral thalamus 

 collects fibers from all parts of the cerebrum, and chiefly those con- 

 cerned with somatic responses to exteroceptive stimuli. Its efferent 

 fibers descend to those motor fields which supply the skeletal muscu- 

 lature. These fibers take a surprising variety of courses, the details of 

 which need not be recounted here ('396, p. 544), but all the more im- 

 portant pathways are links in the chain of conductors which activate 

 the skeletal muscles. For this reason this field is regarded as part of 

 the motor zone, though it has no direct connections with the periph- 

 ery. 



This ventral thalamus corresponds in all essential respects with the 

 subthalamus of mammalian neurology. In contrast with the mam- 

 mals, it is here far larger than the dorsal thalamus, as is evident upon 

 inspection of figure 2. This difference is probably due not to shrinkage 

 of the ventral thalamus in mammals but to the great enlargement of 

 the dorsal thalamus, i.e., to the addition of the neothalamus to the 

 primordial paleothalamus as the latter is seen in Amblystoma. 



