202 THE BRAIN OF THE TIGER SALAMANDER 



tract descends dorsally of the f. longitudinalis medialis (fig. 92). As 

 pointed out below, this position of the dorsal tract agrees with that 

 of the dorsal longitudinal fasciculus of Schiitz, and its fibers take a 

 similar course, turning laterally to spread in the trigemino-facial teg- 

 mentum and as far back as the level of the motor nucleus of the IX 

 nerve. In fact, the dorsal bulbar tract is really an extension posteri- 

 orly of the tegmental tract, for these comprise a continuous series of 

 efferents to the tegmentum. The dorsal longitudinal fasciculus of 

 Amblystoma is not a well-formed tract as in mammals, but compa- 

 rable fibers can be recognized in the deep neuropil of the brain stem. 

 In both lower and higher animals this complex seems to be concerned 

 with general central excitatory state and disposition of the individual 

 rather than with specific sensori-motor responses (pp. 208, 217). The 

 inteipedunculo-tegmental and dorsal interpedunculo-bulbar system 

 of fibers is evidently the primordium of an important component of 

 the "caudal division of the dorsal longitudinal fasciculus" as de- 

 scribed in the opossum by Thompson ('42), This morphological con- 

 clusion rests, I think, on sufficient evidence, whether or not the fol- 

 lowing physiological speculations can be confirmed experimentally. 



INTERPRETATION 



Though much remains obscure, the main features of the histologi- 

 cal structure and connections of the urodele interpeduncular nucleus 

 are now clear, and these are assembled schematically in figures 19, 

 83, and 84. The most striking feature is the ventral plaque of specific 

 neuropil containing spiral terminals of the habenulo-interpeduncular 

 tract and glomeruli. Other afferent fibers enter both the specific and 

 the diffuse interpeduncular neuropil and are so arranged as to permit 

 their activation from almost all parts of the cerebrum. All these fibers 

 come from areas of intermediate-zone type, not from primary sen- 

 sory centers. 



These afferents are of four general classes, depending on the source 

 of their activation: (1) olfactory from the hemisphere, ending in the 

 diffuse neuropil ; (2) olf acto-somatic from the habenula, ending in the 

 specific (spiral) neuropil; (3) olf acto-visceral from the hypothalamus, 

 ending in diffuse neuropil; and (4) tegmento-interpeduncular from 

 the overlying tegmentum and ending in both diffuse neuropil and 

 glomeruli. Those of the last class come from somatic sensory fields of 

 correlation with no appreciable olfactory component. The glomeruli 

 provide apparatus for nonspecific summation and reinforcement. 



