236 THE BRAIN OF THE TIGER SALAMANDER 



these fibers in Amblystoma I have been no more successful than I was 

 with Necturus ('336, p. 15). 



DOESAL THALAMUS 



The dorsal thalamus is a well-defined part of the diencephalon and 

 is intimately related by a web of connecting fibers with all contiguous 

 parts and with many remote parts of the brain. It evidently is an im- 

 portant center of sensory correlation, but it is also the primary 

 terminal station of a considerable fraction of the fibers of the optic 

 nerve which come directly from the retina. The thalamic connection 

 of optic fibers is not a late acquisition incidental to the differentiation 

 of cerebral cortex, for it is present in all vertebrates, even the most 

 primitive types. In corticated animals the larger part of the thalamus 

 is clearly a cortical dependency, the neothalamus; but there is also a 

 paleothalamus, which is a primordial part of the vertebrate brain, 

 being present in all vertebrates, from the lowest to the highest. This 

 paleothalamic component is not lost in mammals, as is shown by the 

 surviving thalamic residue, which does not degenerate after total 

 decortication. 



The amphibian dorsal thalamus shows only vague outlines of an 

 incipient subdivision into the local nuclei so characteristic of mam- 

 mals. This is more evident in anurans than in urodeles, and in Amblys- 

 toma than in Necturus. In the urodeles, as in fishes, the undifferenti- 

 ated dorsal thalamus may be regarded as a single "nucleus sensiti- 

 vus," which acts, in the main, as a whole, without well-defined func- 

 tional localization. Nevertheless, an early stage of local differentia- 

 tion can be recognized. In Amblystoma there are three areas of gray, 

 separated by shallow and variable ventricular sulci, namely : 



1. Anteriorly the nucleus of Bellonci (fig. 2, nuc.B) produces a 

 slight ventricular eminence wedged between the middle area, the 

 ventral habenular nucleus, the eminentia thalami, and the ventral 

 thalamus. Superficially of this gray is an area of very dense and com- 

 plicated neuropil, which is intimately connected with that of all the 

 surrounding areas. It receives terminals of the optic tract, and this 

 justifies its inclusion within the sensory zone, though its other con- 

 nections are those of correlating tissue of intermediate-zone type. 

 The nucleus of Bellonci is primarily a bed-nucleus of the neighboring 

 tracts — stria meduUaris, optic and postoptic systems, and others 

 (chap, xviii). 



2. The middle part of the dorsal thalamus is marked by a low ven- 



