If to render my ideas on this head more clear I were to use a vulgar expression, I should say that distributed in such a dose to an organ, sensibility is animal: in such another dose, organic.[23]--Now that, which varies the dose of sensibility, is sometimes the order of nature, (in which way the skin and the nerves are more sensible than the tendons, and cartilages;) at other times, disease; thus in doubling the dose of sensibility to the cartilages inflammation renders them equal in this respect, and even superior to the former, and as a thousand causes may at every moment exalt or diminish this power in any part of the body it may be changed at every moment from the animal to the organic type. Hence the reason, why authors, who have made it the object of their experiments, have come to results so different; and why some of them have observed the periosteum and dura mater to be insensible, while others have put them down on the contrary as endowed with an extreme sensibility. IV. _Of the relation which exists between the sensibility of each organ, and foreign bodies._ Although the sensibility of each organ be subject to continual variations, it is nevertheless distributed to each by nature in a determined quantity; in a quantity to which it ever returns after its alternations of augmentation or decrease. In this respect it resembles the pendulum, which in each of its different oscillations resumes the place to which it is brought down by gravitation. It is this determined sum of sensibility, which especially composes the life of each organ, and fixes the nature of its relations with foreign bodies; in this way the ordinary sum of sensibility in the urethra fits it for the passage of the urine, but if this sum be augmented, as in strong erection of the penis, the above relation ceases: the canal refuses passage to the urine, and suffers itself to be traversed by the semen only, which in its turn has no relation with the sensibility of the urethra when the penis is not erected.[24] From hence proceeds the reason of the puckering up and spasm of the parotid, the cystic, and pancreatic ducts, as well as of the excreting tubes in general, when the molecules of any other fluid than that, which they are destined to convey are presented to them. The sum of their sensibility corresponds exactly with the nature of their respective fluids, but is disproportioned to that of any other.[25]--The spasmodic contraction of the larynx when irritated by any foreign body is produced in the same manner; for the same reason the ducts, which open upon the mucous surfaces, though at all times in contact with a variety of different fluids, are never penetrated by them.[26] The mouths of the lacteals, however patulous within the alimentary canal, will take up the chyle only, they reject the fluids, which are mixed with it; for with these their sensibility has no relation.