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C. JLTDSON HERRICK 



a superior colliculus for optic impressions and an inferior colli- 

 culus for auditory impressions, with a complicated system of 

 associational neurons putting these secondary centers into phys- 

 iological relation. But the final motor discharge in each case 

 may be the result of the reaction of the whole apparatus and 

 not of either the optic or the auditory center alone. 



A further complication arises from the fact that the efferent 

 tract is not simple, as diagrammed in Fig. 2 ; but it divides 

 into a descending and an ascending path. The former connects 

 directly with motor centers, including the oculo-motor, bulbar 

 and spinal motor nuclei, while the latter enters the thalamus, 

 where associations of a still higher order are effected (Fig. 3). 



^rebral hemsphere 



•nerve nerve 



Figure 3. Diagram of some conduction paths in the brain of Necturus, seen in 

 longitudinal section. From the medulla oblongata an acoustic impulse ma*y 

 be carried forward through the neurone A to the midbrain, whose neurones, B. 

 are of the type shown in Fig. 2, receiving both acoustic and optic impulses. 

 This neurone B may discharge downward to the motor nuclei of the III, V, 

 VII, etc., nerves, or it may discharge upward to a neurone of the thalamus, 

 C, which also receives descending impulses from the cerebral hemisphere. 



Here is introduced a physiological choice or dilemma; the 

 response is not a simple mechanical resultant of the interacting 

 stimuli, but its character may be influenced by variable physio- 

 logical states. The determinate type of action is replaced by a 

 relatively indeterminate or labile type. In the thalamus the 

 nervous impulse is again subjected to modification under the 

 influence of a still greater variety of afferent impulses, for these 

 centers receive all sensory types found in the midbrain, and in 

 addition important descending tracts from the cerebral hemis- 

 pheres — in lower vertebrates the latter are chiefly olfactory. In 

 fishes and amphibians few optic fibers enter the thalamus directly, 

 but most of these pass by to end in the midbrain. 



The fibers which enter the thalamus in general come from 

 other association centers and therefore may carry impulses which 

 have been already elaborated into rather complex systems of 



