If we examine the cells of the gland in various states we see clearly that granules of some material, different in nature from the substance of the protoplasm itself, are being formed within them. Evidently these granules swell up during secretion and discharge their contents into the ducts. Further changes in the characters of the cell-substance, and in the nucleus, can be observed, and all these indicate that the protoplasm of the cells, as the result of stimulation, elaborates certain substances; that these substances are then washed out, so to speak, into the duct by the withdrawal of water from the cell; and that thereafter the cell absorbs fresh nutritive material from the lymph which exudes from the blood vessels, along with water. The distinctive part of the whole train of processes is, then, this elaboration of material by the cells themselves; while the concomitant changes in the calibre of the blood vessels and in the flow of blood and lymph are subsidiary ones. In the process of secretion of saliva energy is absorbed from the chemical substances of the blood to bring about the passage of water from a region of high to a region of low osmotic pressure; oxygen and nitrogen, with other elements of course, are withdrawn from the arterial blood stream for the purpose of the secretion, and carbon dioxide and other substances are given off to the venous blood and lymph. The problem thus is pushed back from the mechanical events occurring in the nervous and circulatory processes, to the physico-chemical ones occurring in the cells of the gland tubules; and it thus becomes much more obscure. It is true that we can formulate a hypothesis which describes, in a kind of way, these intra-cellular metabolic changes, in terms of physico-chemical reactions, and, without doubt, reactions of this kind must occur within the cell. But if we could test any such hypothesis as easily as the mechanical ones suggested, should we find it any more self-sufficient?[19] [19] We have not referred to “psychical secretion.” If we smell some very savoury substance our “mouth waters,” that is, secretion of saliva occurs. If we even see some such substance the same secretion occurs. All this is clear and can be “explained” mechanistically: the stimulation of the olfactory or visual organs begins a kind of reflex process. But if we even _think_ about some very savoury morsel saliva may be secreted. We must suppose now that our consciousness, something which has nothing to do, it must be noted, with energy-changes in the body, can react on the body. If we show a dog an attractive bone it will secrete saliva; if we show it again and again, the same thing occurs. But after certain such trials the dog will realise that he is being played with, and the exhibition of the bone no longer evokes a flow of secretion. Why is this? The whole process has now become more mysterious than ever.