96 THE BRAIN OF THE TIGER SALAMANDER 



nified; but only in mammals are the several functional systems well 

 segregated and separately localized, and here this localization is cor- 

 related with the differentiation of the sensory projection areas of the 

 neopallium and the related nuclei of the dorsal thalamus. The course 

 of the evolutionary differentiation of the cerebral hemisphere has 

 been determined, in its main features, by the penetration of these 

 nonolfactory fibers into its medial and lateral walls and the elabora- 

 tion of related centers for the reception and correlation of these sen- 

 sory impulses and appropriate motor discharge. 



The entire ventrolateral wall of the hemisphere of Amblystoma is 

 an olfacto-striatum. Its anterior end is under strong olfactory influ- 

 ence and is regarded as the primordium of the head of the caudate 

 nucleus (figs. 98, 99). Posteriorly, the large amygdala (fig. 97) also 

 receives many olfactory fibers. The middle sector receives fewer 

 olfactory fibers and is a terminal station of the thalamo-frontal tract. 

 From it fibers descend to the ventral thalamus and peduncle. It is, 

 accordingly, regarded as paleostriatum, or somatic striatum, pri- 

 mordium of the mammalian lentiform nucleus. 



The striatal gray of the middle sector is obscurely divided into 

 dorsal and ventral nuclei, separated by a shallow sulcus striaticus 

 (fig. 99, s.st.). Small cells of putamen type and larger cells of globus 

 pallidus type are mingled in both nuclei, so that these structures are 

 not separated as in mammals; yet their connections, as described in 

 chapter xx, suggest that the ventral nucleus is the precursor of the 

 globus pallidus. Both these nuclei receive fibers from the overlying 

 piriform area, some of which descend into the lateral forebrain bundle 

 (figs. 6, 111). The ventral nucleus is continuous with and intimately 

 connected with the primordial caudate ('27, p. 298), and its efferent 

 fibers go chiefly to the cerebral peduncle. The dorsal nucleus receives 

 thalamo-frontal fibers and is in intimate relation with the amygdala 

 and the piriform area. Its descending fibers have wide distribution to 

 the tectum, thalamus, dorsal part of the peduncle, and dorsal, 

 isthmic, and bulbar tegmentum as far back as the V nerve roots (fig. 

 QJ.lat.t.d.). In Necturus (fig. HI; '336, p. 197; '33e) there is a com- 

 plicated system of associational connections between the striatum 

 and the piriform area; these are present also in Amblystoma, but the 

 details have not been described. 



Most of the white substance of the striatal field is occupied by a 

 very dense and sharply circumscribed intermediate neuropil of pe- 

 culiar texture (p. 53 and figs. 98, 99, 108, 109, 113; '27, p. 300; '42, 



