306 INTRODUCTION TO NEUROLOGY 



functionally defined motor areas, each group of muscles which 

 cooperate in definite reaction complexes (termed synergic 

 muscles, see p. 35) being excited from a definite part of the motor 

 cortical iield, whose motor tract is anatomically distinct through- 

 out its entire further course from the cortex to the periphery. 

 Between the sensory projection centers and the motor areas are 

 interpolated the association centers, and these are so arranged 

 that all correlation, integration, and assimilation of present 

 sensory impulses with memory vestiges of past reactions are 

 completed, and the nature of the response to be made is deter- 

 mined before the resultant nervous impulses are discharged into 

 the motor centers. Only such of the motor areas will be excited 

 to function as are necessary for evoking the particular reaction 

 which is the appropriate (that is, adaptive) response to the total 

 situation in which the body finds itself. This arrangement of 

 association centers in relation to a series of distinct motor areas 

 provides the flexibility necessary for complex delayed reactions 

 whose character is not predetermined by the nature of the con- 

 genital pattern of the nervous connections. 1 



The thalamus, as we have seen (p. 163), has its own intrinsic system 

 of association centers which discharge downward into the cerebral pedun- 

 cles, and this is the primary reflex apparatus of this part of the brain The 

 thalamo-cortical connections arose to prominence later in the evolutionary 

 history, though feeble rudiments of these are present in lower brains. 

 Parallel with the enlargement of these cortical connections a special part of 

 the thalamus was set apart for them, and from the Amphibia upward in the 

 animal scale this dorsal part of the thalamus assumed increasingly greater 

 importance. This part is termed by Edinger the neothalamus, and makes 

 up by far the larger part of the thalamus in the human and all other mam- 

 malian brains. It occupies the dorso-lateral part of the thalamus proper and 

 comprises most of the great thalamic nuclei (lateral and ventral nuclei, 

 pulvinar and lateral and medial geniculate bodies). The primitive in- 

 trinsic reflex thalamic apparatus in man is a relatively unimportant area 

 of medial gray matter and the subthalamic region (corpus Luysii, lattice 

 nucleus, etc., not to be confused with the hypothalamus which lies farther 

 down in the tuber cinereum and mammillary bodies). 



The neothalamus, accordingly, serves as a sort of vestibule to the cortex, 

 every afferent impulse from the sensory centers (except the olfactory sys- 

 tem) being here interrupted by a synapse and opportunity offered for a wide 

 range of subcortical associations. The olfactory cortex (hippocampal for- 

 mation) has a similar relation to subcortical correlation centers in the olfac- 

 tory area in the anterior perforated space, septum, etc. 



l /The paragraphs which follow (pp. 306-311) are reproduced with slight 

 modification from The Journal of Animal Behavior, vol. iii, 1913, pp. 228- 

 236. 



