24 THE BRAIN OF THE TIGER SALAMANDER 



Here they are interpolated between the primary sensory and motor 

 apparatus of the medulla oblongata and spinal cord below and the 

 great olfactory field and suprasegmental apparatus of the cerebral 

 hemispheres above. 



In all lower vertebrates the roof of the midbrain, the tectum, is the 

 supreme center of regulation of motor responses to the exteroceptive 

 systems of sense organs. The hypothalamus is similarly elaborated for 

 regulation of olfacto- visceral adjustments. The patterning of motor 

 responses for both these groups of receptors is effected in the cerebral 

 peduncle and tegmentum. In the region of the isthmus, between the 

 tectum and the primary vestibular area of the medulla oblongata and 

 above the tegmentum, the cerebellum was elaborated as the supreme 

 adjustor of all proprioceptive systems. 



At the rostral end of the brain, within and above the specific 

 olfactory area of the cerebral hemisphere, there gradually emerged a 

 synthetic apparatus of control, adapted to integrate the activities of 

 all the other parts of the nervous system and to enlarge capacity to 

 modify performance as a result of individual experience. In the low- 

 est vertebrates this "suprasensory" and "supra-associational" ap- 

 paratus, as Coghill termed it, is not concentrated in the cerebral 

 hemispheres, but it is dispersed, chiefly in the form of diflfuse neuropil. 

 In the amphibian cerebral hemispheres this integrating apparatus is 

 more highly elaborated than elsewhere, with some local differentia- 

 tion of structure. The hemispheres are larger than in fishes, and the 

 primordia of their chief mammalian subdivisions can be recognized. 

 A dorsal pallial part is distinguishable from a basal or stem part of 

 the hemisphere, though the distinctive characteristics of the pallium 

 are only incipient. There is no cerebral cortex, and, accordingly, the 

 mammalian cortical dependencies in the thalamus, midbrain, and 

 cerebellum have not yet appeared. The primordial thalamus is con- 

 cerned chiefly with adjustments within the brain stem, though 

 precursors of the thalamic radiations to the hemispheres are present. 



VENTRICLES 



The lateral ventricles of the cerebral hemispheres have the typical 

 form except at the interventricular foramen, where the amphibian 

 arrangement is peculiar. The anterior and hippocampal commissures 

 do not cross as usual in or above the lamina terminalis, but in a more 

 posterior high commissural ridge ; and between these structures there 

 is a wide precommissural recess, into which the interventricular 



