64 THE BRAIN OF THE TIGER SALAMANDER 



the motor zone are in Part II, and the pecuhar features of its forward 

 extension in the cerebral hemisphere are discussed in chapter vii. 



THE INTERMEDIATE ZONE 



The characteristics of this zone are imphcit in the preceding ac- 

 count of the sensory and motor zones. It is more elaborately devel- 

 oped and its boundaries are more clearly defined in parts of the 

 cerebrum than in the rhombencephalon. These boundaries are neces- 

 sarily arbitrary, for all parts of the brain are involved in correlation 

 and integration of bodily activities ; but throughout the length of the 

 spinal cord and brain there is a band of tissue between the sensory 

 and motor zones primarily concerned with these adjustments. At 

 lower levels I have termed this tissue the "reticular formation," and 

 here it infiltrates the other zones with no clear boundaries (for details 

 see chap. xi). At higher levels it increases in amount and is more 

 clearly segregated. It would be appropriate to include in this zone 

 most of the diencephalon and telencephalon except the specific optic 

 and olfactory terminals ; but, for reasons mentioned above, a different 

 subdivision has been adopted, primarily for convenience of descrip- 

 tion. The dorsal tegmentum, or subtectal area, is a typical represent- 

 ative of this zone in position and physiological connections. In the 

 isthmic and bulbar tegmentum the characteristics of the intermedi- 

 ate and motor zones are inextricably mingled. The habenula, hy- 

 pothalamus, and interpeduncular nucleus, as elsewhere described, 

 clearly belong physiologically to the intermediate zone; and the 

 whole cerebral hemisphere, except the olfactory bulb, might ap- 

 propriately be so classified in all Ichthyopsida. 



In the most primitive vertebrates the intermediate zone is scarcely 

 recognizable as an anatomical entity. As the action system becomes 

 more complicated in higher animals, this zone shows corresponding 

 differentiation. This specialization is more directly dependent upon 

 complication of the peripheral motor apparatus than upon sensory 

 differentiation, for, so long as the action system is largely confined to 

 mass movements, the patterning of these total activities is effected in 

 the sensory and motor zones. In tetrapods and birds more complex 

 central adjustors are required, and these are differentiated between 

 the two primary zones and anteriorly of them. With the appearance 

 of more autonomy of the local reflex systems, more efficient appara- 

 tus of integration is demanded. The final result is that in the human 

 brain the apparatus of intermediate-zone type has increased so much 



