Consequently, the observation of Bidder and Schmidt was perfectly correct. It cannot, however, be said that it received general recognition in physiology, or that it was sufficiently appreciated. There are authors who could never convince themselves of its reality, and in many physiological text-books it is not once mentioned. By way of explanation, we shall now consider how this matter must be dealt with by those who wish to observe the effect. It is only under certain conditions that it can be seen. Firstly, the animal must be healthy and vigorous; it must have a perfectly uninjured gastric mucous membrane; and this, from the description in the case of many authors who obtained a negative result, was not the case. Secondly, the success of the experiment, as stated above, is dependent upon the intensity of the desire for eating, and this, again, is dependent upon how freely and how long beforehand the dog had eaten, and also upon what it is tempted with, whether with a dish that excites its desire or leaves its interest unawakened. It is known that dogs have very different tastes, just as men have. Thirdly, one may find among the dogs positively careless, indifferent creatures, incapable of being perturbed in this way by anything which has not actually reached their mouths, and patiently waiting till the food is given them. Hence for success in the experiment, eager, impressionable, and excitable animals are necessary. Fourthly, one has to reckon with the sense and cunning of the dog, a factor which is not lightly to be disregarded. Often the animals perceive at once that they are only being teased with the food, become annoyed thereat, and turn away offended at what is being done before them. We must, therefore, so arrange matters as if the animals were not going to be disappointed but fed in reality. If attention be paid to these conditions the experiment of “psychic excitation of the gastric secretion,” as we usually term it, will be found to be as reliable as the experiment of sham feeding. When one is occupied for a length of time with the study of the gastric secretion under different conditions, one becomes convinced of what a dangerous source of error this psychic excitability may become in the different experiments. We must constantly fight, so to speak, against this factor, keep it ever in view, and guard against it. If the dog has not eaten for a long time, every movement,--the going out of the room, the appearance of the attendant who ordinarily feeds the animal--in word, every little triviality may give rise to excitation of the gastric glands. The minutest attention is necessary in order to avoid such sources of error, and we should not be far wrong if we said that much which has been ascribed in former investigations to the effect of this or that agency was in reality a result of unobserved psychic influence. Consequently, in order to verify our own conclusions concerning the