64 INTRODUCTION TO NEUROLOGY 



shown by the neuron B of Fig. 21. The descending path 

 connects directly with motor centers, including the oculomotor, 

 bulbar, and spinal motor nuclei (Fig. 21, S), while the ascending 

 path enters the thalamus, where associations of a still higher 

 order are effected through the thalamic neuron, C. Here again 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 physiological states. 

 The invariable type of action is replaced by a relatively variable 

 or labile type (see p. 31). 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 hemispheres (in lower ver- 

 tebrates the latter are chiefly olfactory). 



The more complicated associations are effected by arrange- 

 ments of correlation tracts and centers illustrated in the simplest 

 possible form by Fig. 18, E. The mode of operation of such 

 a system may be illustrated by an example : A collie dog which I 

 once owned acquired the habit of rounding up my neighbor's 

 sheep at very unseasonable times. The sight of the flock in the 

 pasture (stimulus Rl, Fig. 18, E) led to the pleasurable reaction 

 (El) of chasing the sheep up to the barnyard. It became neces- 

 sary to break up the habit at once or lose a valuable dog at the 

 hands of an angry farmer with a shotgun. Accordingly, I 

 walked out to the pasture with the dog. She at once brought in 

 the sheep of her own accord and then ran up to me with every 

 expression of canine pride and self-satisfaction, whereupon I 

 immediately gave her a severe whipping (stimulus R2}. This 

 called forth the reaction (E2) of running home and hiding in her 

 kennel. The next day (the dog and I having meanwhile with 

 mutual forgiveness again arrived at friendly relations) we took a 

 walk in a different direction, in the course of which we unex- 

 pectedly met another flock of sheep. At sight of these the dog 

 immediately, with no word from me, put her tail between her 

 legs, ran home as fast as possible, and hid in her kennel. Here 

 the stimulus Rl led not to its own accustomed response, El, 

 but to E2, evidently under the influence of vestigeal traces of the 

 previous day's experience, wherein the activities of Cl and C2 



