252 THE BRAIN OF THE TIGER SALAMANDER 



the habenular system, though reduced in size, retains most of its 

 components. The associational connections of the habenula with the 

 tectum, dorsal thalamus, and adjacent regions justify the conclusion 

 that the complex, viewed as a whole, is adapted to insure the cor- 

 relation and integration of the activities of all parts of the olfactory 

 field with those of all other exteroceptive systems. 



The sense of smell is both interoceptive and exteroceptive. Olfac- 

 tory nervous impulses of exteroceptive type are somehow sorted out 

 from the entire olfactory field and converged into the epithalamus, 

 and those of interoceptive type are similarly converged into the 

 hypothalamus. In the former of these areas they may be correlated 

 with all relevant somatic sensory experience, and in the latter with 

 the sum total of visceral experience. The habenula discharges into 

 that portion of the motor field known to control the activities of the 

 skeletal musculature and chiefly into the interpeduncular nucleus, 

 which is imbedded within this motor field and articulated with it in 

 very complex patterns (chap. xiv). The olfacto-visceral correlations 

 of the hypothalamus may come to expression in overt action of the 

 skeletal muscles or in less obvious visceral changes. The chief nervous 

 pathways involved in the overt responses can be identified, but those 

 of visceral activities are still obscure. There is a strong connection 

 from the hypothalamus to the interpeduncular nucleus. 



Since both the hypothalamic and the epithalamic olfactory sys- 

 tems are large in all vertebrates, it was suggested by Edinger ('11, 

 p. 371) that these regulate the feeding activities, or muzzle reflexes 

 {Oralsinnapparat) — dorsally the exteroceptive components and ven- 

 trally the interoceptive. This attractive hypothesis seems to explain 

 the different courses and connections of the medial forebrain bundle 

 and stria medullaris, but it leaves out of account the equally large 

 lateral forebrain system. Moreover, the chief connections of the 

 medial and lateral forebrain bundles are localized in the hemisphere, 

 respectively medially and laterally of the ventricle; but the habenular 

 connections are drawn equally from both sides of the hemisphere. 

 The habenular connections seem to be of a different kind, something 

 added to an existing adequate provision for both visceral and somatic 

 adjustments. 



The chief efferent discharge from the habenula is to the inter- 

 peduncular nucleus, and there is a large efferent tract from the hypo- 

 thalamus to this nucleus (figs. 18, 21). It is suggested in chapter xiv 

 that the habenulo-interpeduncular system is the inhibitory com- 



