66 THE BRAIN OF THE TIGER SALAMANDER 



nuncials are so arranged that functional systems of afferents, which 

 normally co-operate to effect a particular type of motor response, are 

 more intimately associated. Thus the tectum opticum receives most 

 of the lemniscus fibers of all somesthetic systems and minimum num- 

 bers of olfactory and visceral systems. This basic pattern as seen in 

 Ambly stoma is changed in mammals, where higher associational cen- 

 ters have taken over most of the functions of correlation. 



Motor co-ordination is effected primarily in the motor zone, which 

 is so organized as to activate synergic groups of muscles in appropri- 

 ate sequence with inhibition of their antagonists. This grouping 

 may be adapted for mass movements or for local reflexes. Internu- 

 clear tracts connect the various parts of the sensory zone directly 

 with appropriate parts of the motor zone. More refined analysis and 

 conditioning of motor responses are effected through the intermedi- 

 ate zone, and the tissues of the latter group are greatly enlarged and 

 complicated in higher brains. 



The activities of stimulus-response type which have just been con- 

 sidered are so interconnected with internuclear tracts and the inter- 

 stitial neuropil as to facilitate the integration of all local activities in 

 the interest of the requirements of the body as a whole. Every local 

 part of the brain is a component of the apparatus of general integra- 

 tion, and some of these parts have this association as their dominant 

 function. In Ambly stoma most of this suprasensory and supramotor 

 tissue is dispersed as interstitial neuropil. In mammals, higher types 

 of associational tissue have been differentiated locally, notably in the 

 cerebral cortex and its dependencies, with corresponding enhance- 

 ment of those synthetic functions which are manifested as condition- 

 ing, educability, and reasoning. Parallel with these changes there is 

 an enormous increase in accumulated reserves of potential nervous 

 energy, which come to expression as spontaneity, memory, and crea- 

 tive imagination. 



A survey of the nerve fibers of Amblystoma as a whole in view of 

 the principles just expressed shows that they may be classified in four 

 groups: (1) the peripheral afferent systems and associated inter- 

 nuclear correlating tracts within the sensory zone (lemniscus sys- 

 tems, etc.) ; (2) the peripheral efferent systems and the related co- 

 ordinating fibers of the motor zone; (3) the central internuclear sys- 

 tems intercalated between the preceding two and so interconnected 

 as to yield appropriate responses to ordinary recurring stimuli; and 

 (4) infiltrating these mechanisms of stimulus-response type, a differ- 



