21G THE BRAIN OF THE TIGER SALAMANDER 



This is typical correlational tissue interpolated between the sen- 

 sory zone above and the motor zone below, with both of which it is 

 connected by numberless short fibers, by long dendrites, and by the 

 intrinsic neuropil (fig. '24). In addition to these local connections, it 

 receives longer fibers from the cerebral hemispheres, dorsal and ven- 

 tral thalamus, hypothalanms, and tectum, many of these fibers de- 

 cussating in the anterior and postoptic commissures. There are long 

 efferent fibers which descend in the dorsal tegmental fascicles to the 

 isthmic and trigeminal tegmentum. 



This field evidently contains important components of the appara- 

 tus of sensori-motor adjustment, but how this mechanism operates 

 is unknown. This relatively homogeneous area probably loses its 

 identity in higher animals, in which its parts are specialized and dis- 

 persed in the brain stem or perhaps supplanted by the more efficient 

 adjustors of the thalamus and hemispheres. 



MOTOR ZONE 



PEDUNCLE 



At the sharp ventral cerebral flexure of the brain stem, a ventricu- 

 lar eminence marks the position of the nucleus of the tuberculum 

 posterius, which is called the "peduncle" in a restricted sense (p. 

 21). This peduncular area, which is boundetl by a shallow and 

 variable sulcus (fig. 2, s.), is an arbitrarily defined field at the ventral 

 surface of the midbrain, bounded dorsally by the dorsal tegmentum 

 and posteriorly by the isthmic tegmentum. It arises from the anterior 

 end of the embryonic basal plate of the neural tube, and nerve fibers 

 appear within it very early in embryogenesis. In the coil stage, fibers 

 are added to this tract from the ventral thalanuis and dorsal teg- 

 mentum ('37, fig. 1), and, before the early swimming stage is reached, 

 there are additions also from the tectum and other neighboring parts 

 ('37, fig. 2). This basal sector of the midbrain is the nucleus of origin 

 of the first efferent fibers of this region for lower motor centers, and, 

 as early as the S-reaction stage, fibers go out from it to the periphery 

 in the oculomotor nerve. This is before any afferent fibers are connect- 

 ed with this region. The motor mechanism develops autonomously. 



In Necturus the f . longitudinalis medialis is definitely organized in 

 the peduncle and is a well-defined anatomical structure from here 

 caudad ('36, p. 349); but in Amblystoma the fibers which compose 

 this bundle are mingled with others in tegmental fascicles numbered 

 (1), (4), (5), and (6) in the peduncle and isthmus, and below the 

 isthmus a residue of fibers from all these fascicles is assembled to 

 continue spinalward in the f. longitudinalis medialis ('36, figs. 9-16). 



