Let us turn now from the consideration of visual perception to that of auditory perception in the horse. We saw that the fact that Hans was able to respond to commands which were only inwardly enunciated, that is, commands which were merely thought of but not spoken, was not proof of great acuity of hearing, but rather that hearing was not at all involved. If Hans had been deaf he would, none the less, have promptly obeyed the commands. Blind and near-sighted horses try to overcome their deficiency by means of the sense of hearing, and hence show a pronounced play of ears. In the case of the Osten horse, however, attention has been diverted from auditory stimuli in the process of habituation to visual signs, and as a result ear-movements are almost completely wanting. One is not of course permitted to deny _a priori_ that perhaps some associations might have been formed between objects and the vocal signs belonging to them, e. g., between the colored cloths and the names of the colors if both had been presented together oftener than was the case. But there is a dearth of reliable observation as to how far auditory associations of this sort may be established in horses. Usually the following is cited. Horses learn to start off, to stop, and to turn about in response to calls. They are able to distinguish properly between the expressions "right" and "left", or equivalent terms. Upon command they will start to walk, to trot or to run. And they also know the name by which they are usually called. All authors agree that cavalry horses understand the common military commands; one writer even avers that they excel the recruits in this respect.[69] Some believe that in riding schools the horses pay closer heed to the calls of the riding-master than to the control of unpractised riders, even when the two are at variance with one another.[70] My experience with the Osten horse and a number of other pertinent observations aroused in me the suspicion that much that is called or spoken in the process of managing a horse may possibly be just so much labor lost. In consequence I made a series of relevant experiments. I have thus far tested twenty-five horses of different kinds, from the imported Arabian and English full-blood, down to the heavy draft-horse. The experiments were made partly in the courtyard of military barracks, partly in the circus, and partly in a riding-school or in private stalls. I am specially indebted for kind assistance to Messrs. von Lucanus, Busch, and to H. H. Burkhardt-Foottit and E. Schumann, the two excellent trainers connected with the Busch Circus. During these tests, the horses were always amid circumstances familiar to them, whether free or bridled, under a rider or hitched to a wagon. All aids or signals, except the calls, were eliminated in so far as it was possible.