46 THE BRAIN OF THE TIGER SALAMANDER 



complex as a whole is believed to have two chief functions: (1) Here 

 are organized the patterns of the local reflexes of the musculature of 

 the head, particularly those concerned with feeding. (2) The small- 

 celled central nucleus is a special differentiation of the periventricular 

 gray, which serves, in addition to the specific functions just men- 

 tioned, a more general, nonspecific, totalizing function; that is, it is a 

 part of that integrating apparatus which appears in mammals as the 

 dorsal tegmental nucleus and the related fasciculus longitudinalis 

 dorsalis of Schiitz (p. 208). The details of structure are given in 

 chapter xiii. 



The isthmic tegmentum occupies a strategic position between the 

 primitive bulbo-spinal mechanisms and the higher cerebral adjustors; 

 it plays a major role both in the patterning of local reflexes and in the 

 integration of all bodily activities. This mass of tissue, which in 

 urodeles is at a low level of differentiation, in higher animals is split 

 up and distributed so that in mammals the identity of the isthmic 

 tegmentum as an anatomical entity is lost in the adult brain, though 

 the isthmic sector is plainly marked in the early embryonic stages. 



6. INTERPEDUNCULAR NUCLEUS 



The interpeduncular nucleus also is unusually large in urodeles. 

 It is not interpeduncular but interisthmic, extending from the fovea 

 isthmi back to the level of the V nerve roots. The histological texture 

 is extraordinary. A well-defined, trough-shaped column of cells 

 borders the ventral angle of the ventricle, with dendrites extending 

 downward through the ventral commissure, to arborize with tufted 

 endings in a ventromedian band of neuropil (figs. 65, 66, 82, 83, 91). 

 The axonic components of this interpeduncular neuropil come from 

 various sources: (1) terminals of the fasciculus retroflexus, which 

 take the form of a flattened spiral (fig. 50); (2) terminals of tr. 

 tegmento-interpeduncularis from small cells of the overlying teg- 

 mentum with tufted endings, which join with the dendritic tufts of 

 the interpeduncular nucleus to form small glomeruli (figs. 60-66, 84) ; 

 (3) collaterals of thick fibers of tr. tegmento-bulbaris from the large 

 cells of the tegmentum with similar tufted endings in the glomeruli 

 (fig. 68); (4) collaterals of tr. interpedunculo-bulbaris, which also 

 enter glomeruli (figs. 83, 84) ; (5) terminals of tr. mamillo-inter- 

 peduncularis with dispersed free endings (figs. 60, 61); (6) similar 

 terminals of tr. olfacto-peduncularis (fig. 59); (7) less numerous 

 terminals from several other sources. The slender, unmyelinated 

 axons of the interpeduncular cells branch freely in the interpeduncu- 



