[23] These expressions _dose_, _sum_, _quantity_ of sensibility are incorrect, inasmuch as they exhibit this vital faculty under the same point of view as the physical forces, as attraction, for example; and as they present it to us as susceptible of calculation, &c.; but, from a want of words for one science, it is necessary, in order to make it understood, to borrow them from the other sciences. There are expressions, like the words to _solder_, to _glue_, to _unglue_, &c. that are used for the want of others in the osseous system, and which really give very inaccurate ideas, unless the mind corrects the sense. [24] If the urine, during a perfect erection, does not go out of the bladder, it is because the contraction of the muscles of the perineum, and especially of the levator ani, prevents it. If these muscles are relaxed, though the turgescence of the corpus cavernosum and of the urethra remains the same, the urine flows out without any other obstacle than what arises from the contraction of the canal produced by the swelling of its parietes. [25] These different excretory ducts do not exhibit in the mammalia any contractility. There is no stimulus which can produce it in them; I have tried them all in vain. In birds, on the contrary, the ureters and the pancreatic and biliary canals are contractile, and their motions, which return at intervals, are too well marked to be mistaken. It appears that the contractility of the excretory canals in the abdomen, is connected in these animals with the absence of the diaphragm. We know in fact that this muscle in the mammalia, assists by the pressure which it exerts, the course of the secreted fluids, and renders useless the existence of a peculiar motion in the canals which contain them. If it be however pretended that this motion exists in them, but that it is insensible, it must be allowed then, that it cannot perform the office which is attributed to it, viz. that of obliterating an opening often large enough to admit a quill. It is true, that if the orifice of one of these canals be irritated for a long time, a swelling of the membrane which lines it is sometimes produced, and the opening is then really lessened. But in these cases there is no occasion to be deceived; we see that this swelling is produced at that point by the afflux of the fluids, as it would be in any other part subjected to a similar excitement. Besides, it should be observed that the obliquity of insertion of the excretory ducts is alone sufficient to explain how the substances which pass in front of their orifices are not introduced into them. In fact these substances, at the moment of their passage, by the pressure which they exert, tend to obliterate the opening of the canal, by flattening its parietes against each other; it is thus that the pressure of the urine, upon the inferior extremity of the