VISION IN THE MOUSE 591 



inhibited by the draining of the nervous energy of the association 

 neurones of the higher senses in the direction of the frontal tracts 

 of the brain. Now under these circumstances, it is evident, some- 

 thing must become of the nervous currents coming from the mus- 

 cular sense organ. They seize upon certain motor neurones of the 

 Rolandic area whose connection with the systems operating to 

 reinforce the higher functions is not so direct, and consequently 

 are not so thoroughly drained of energy. Such systems are mainly 

 those which have functioned in bringing about the movements 

 which gave rise to the kinaesthetic sensations. In them the resist- 

 ance is low. Thus it comes about that motor circuits of the sec- 

 ond level are formed. The passing of the synapses in these 

 circuits is rendered progressively easier on account of the rever- 

 berating of impulses through the kinaesthetic-motor system. This 

 system and the higher systems involving attention are for the time 

 being mutually inhibitive. 



The animal in choosing between alternatives is guided mainly by 

 kinaesthetic sensations which have been registered in the nervous 

 system, just as is the case with the subconscious personality. 

 When the animal enters a compartment where it must choose 

 between situation A on the right and situation B on the left, two 

 internal factors tend to determine action. One of these is visual, 

 the other is kinaesthetic, Of these two the visual is the one em- 

 phasized by Thorndike in his experiments with cats. Observation 

 indicates that in the mouse, the visual stimuli are not of so great 

 importance in guiding the animal as the kinaesthetic. The latter 

 is relied upon wherever possible. Smell seems to be the next in 

 importance. These considerations prompt us to adopt a law of 

 parsimony in studying the senses of the mouse : we are warranted 

 in inferring a case of visual determination only when there is no 

 possibility of the muscular sense being used for the discrimination 

 in question. Training a mouse to discriminate always involves 

 training him away from a reliance on the muscular sense. This 

 law may be applied to the muscular sense and smell, or, to smell 

 and vision. There is a suggestion here of a possible criterion for 

 grading the intelligence of the animal series, viz., the relative im- 

 portance of the various senses in directing movement. 



