Let us not exaggerate then medical theories; let us regard nature in diseases as she is in a state of health, in which the solids elaborate the fluids and are at the same time excited by them. There is a reciprocal action, every thing succeeds each other, every thing is connected together. Our abstractions hardly ever exist in nature. We usually adopt a certain number of general principles in medicine, and we accustom ourselves afterwards to deduce from these principles, as necessary consequences, all the explanations of diseases. There is in physical phenomena a regularity and uniformity which never deceive. In morals even, there is a certain number of principles acknowledged by all men, which direct them and regulate their actions; hence a constant uniformity in our manner of considering moral and physical phenomena; hence the habit of going always from the same principles in reasoning upon them. We have carried this habit into the study of the living economy, without considering that it incessantly varies its phenomena, that under the same circumstances they are hardly ever the same, that they are continually increased and diminished and have a thousand different modifications. Nature seems at every instant to be irregular, capricious and inconsequent in their production, because the essence of the laws which preside over these phenomena, is not the same as that of the physical laws. I would observe that the experiments the result of which I have just given for the secreted fluids, differ from those which I published the last year, and in which these fluids have always been fatal, the instant they were forced towards the brain by the carotid. This is a phenomenon general to all the irritating fluids, whether drawn from the economy, or foreign to it; they destroy life when they arrive at the cerebral organ, by a direct injection and without having undergone any alteration, whilst we can inject them with impunity into the veins, as the experiments of the physicians of the last age have proved. We can even without danger, as I have observed, introduce them into the arterial system, on the side opposite to the brain, as in the crural artery, for example. Do the fluids mixed with the black blood rid themselves of some principles by respiration, before they arrive at the brain, or is the preceding phenomenon owing to other causes? I know not. I would only observe that every thing which is not arterial blood, as the black blood and even serum, produces death when forced into the carotid. Water alone is injected with impunity. When the irritating principles are much diluted in this fluid, their contact is less injurious. I have seen very light coloured urine not produce death. _Structure of the Excretories._