60 THE BRAIN OF THE TIGER SALAMANDER 



In the sensory zone of the medulla oblongata there are two elon- 

 gated synaptic pools of neuropil, into which terminals of the sensory 

 root fibers converge (chap. xi). One of these receives terminals of all 

 somatic sensory systems; the other lies more ventrally and internally 

 in the visceral lobe and receives terminals of the visceral sensory and 

 gustatory systems (figs. 9, 89). The secondary fibers which emerge 

 from these pools are distributed locally to the motor zone of the 

 medulla oblongata, downward to the spinal cord, and upward to 

 higher levels. The last take different courses, some to the cerebellum, 

 some to the tectum and thalamus, and some to the hypothalamus. 

 Each of these pathways discharges into a higher synaptic pool of 

 neuropil, where its terminals are in physiological relation with ter- 

 minals of other related sensory systems. The relations to which refer- 

 ence has just been made are in terms of the types of response to be 

 evoked. Thus the tectum becomes the dominant regulator of somatic 

 adjustments to exteroceptive stimulation, the hypothalamus be- 

 comes the regulator of visceral responses to olfacto-visceral stimula- 

 tion, and the cerebellum provides regulatory control of the action of 

 the skeletal muscles. The dorsal thalamus is ancillary to the tectum 

 and shows a very early stage in the evolution of the ascending sen- 

 sory projection systems to the cerebral hemispheres. 



These local differentiations, each with characteristic structure and 

 connections, are receptive fields for the several systems of peripheral 

 sensory fibers, though some of them receive few peripheral fibers and 

 are concerned chiefly with sensory correlation. 



THE MOTOR ZONE 



The motor zone as here defined includes the peripheral motor 

 neurons and those areas of the brain stem concerned with the or- 

 ganization of motor impulses in patterns of synergic action. It in- 

 cludes the following histologically different parts: (1) corpus striatum 

 (paleostriatum); (2) anterior part of the ventral thalamus; (3) pos- 

 terior part of the ventral thalamus; (4) nucleus of the tuberculum 

 posterius ("peduncle" in the restricted sense); (3) isthmic tegmen- 

 tum; (6) trigeminal tegmentum; (7) a poorly defined tegmental field 

 extending farther posteriorly through the length of the medulla 

 oblongata into continuity with the ventral gray column of the spinal 

 cord. 



The floor plate of the embryonic neural tube probably ends an- 

 teriorly at the fovea isthmi (fig. 2B, f.i.), and the adjacent basal 



