ISTHMUS 189 



6. The nucleus isthmi, which is large in the frog (Larsell, '24), is 

 probably represented in Amblystoma by a few outlying cells in the 

 dorsal alba of the isthmus, which are more numerous in the larva 

 ('42, pp. 244, 254). If present here, it is an insignificant vestigial 

 structure. 



7. The neuropil associated with the superior visceral-gustatory nu- 

 cleus (fig. 33, nucris.n.) is an important field of sensory correlation. 

 It is penetrated by dendrites of its nucleus ('42, fig. 43) and by ter- 

 minals of the ascending secondary visceral tract ('25, fig. 19) and 

 also by fibers from several other sources, including the lemnisci and 

 collaterals of tr. tecto-bulbaris rectus (fig. 37; '42, fig. 67). This com- 

 plex is very large in fishes (the Uebergangsganglion of Mayser) and 

 is present, though small, in the frog (Larsell, '24). 



8. The neighboring large cells of the motor tegmentum are un- 

 doubted precursors of several mammalian nuclei of this region, in- 

 cluding the substantia nigra and the locus coeruleus. 



PHYSIOLOGICAL INTERPRETATION 



The afferent fibers which terminate in the isthmus come, directly or 

 indirectly, from practically all parts of the brain, and they carry 

 nervous impulses activated from all the major functional systems. 

 The efferent discharge is also widely spread, but the chief distribu- 

 tion seems to be to upper levels of the medulla oblongata which are 

 concerned with movements of the musculature of the head. 



It is obvious that the isthmus contains a motor adjustor of prime 

 importance. Its distinctive features can perhaps best be appreciated 

 by comparing it with some of the other important centers of adjust- 

 ment. The tectum of the midbrain and the dorsal thalamus, which 

 in this animal is ancillary to it, comprise the dominant apparatus of 

 correlation, on the sensory side, of all exteroceptive and propriocep- 

 tive systems. In the habenula the olfactory organ as an exteroceptor 

 is tied in with the other exteroceptive systems. In the hypothalamus 

 the olfactory system is similarly related with the visceral and gusta- 

 tory systems. All fields of sensory correlation discharge into the cor- 

 pus striatum, ventral thalamus, and peduncle, where they are in- 

 tegrated and co-ordinated primarily in the interest of mass move- 

 ments of the skeletal musculature, such as those involved in locomo- 

 tion. Efferent fibers from all the centers just enumerated and from 

 others converge into a common pool in the isthmic tegmentum. Since 

 apparatus adequate for executing the primary mass movements of 

 the trunk and limbs is found elsewhere — primarily in the midbrain. 



