67. I refer to the second class those sympathies in which the irritation of one point on mucous surfaces produces irritability in a different structure; thus, too lively an impression on the pituitary membrane occasions sneezing; the irritation of the bronchi coughing; biliary concretions produce spasmodic vomiting; stones in the bladder occasion retraction of the testicle towards the ring. In all these cases there is contraction of the muscles produced by the irritation of the mucous surface, distant from the place in which that contraction occurs.
68. The last class of the sympathies of mucous membranes embraces those in which the irritation of any part of their extension determines elsewhere the exercise of their tonicity. Here we must refer to what we have said upon glandular action being augmented by the irritation of the extremities of the excretory ducts. Thus it is evident, that the increase of the tonic power of the parotid for the secretion of the saliva, and of its excretory duct in order to transmit it, when the extremity of this duct is irritated by food, sialogogue medicines, &c.,—it is evident, I say, that this augmentation is a phenomenon purely sympathetic. We may designate each of these three classes by the name of the vital power which they bring into action, calling the first sympathy of sensibility; the second, sympathy of irritability; and the third, sympathy of tonicity.
69. This manner of classing the sympathies is entirely borrowed from the state of the vital powers, of which they are but irregular modifications, and only aberrations, still unknown in their nature. Nevertheless it is subject to very great inconveniences: yet it appears to me to be preferable to that of Whytt, who simply follows the order of the regions; and even to that of Barthy, who, more methodical, examines them successively in the organs connected by systems, in those which are insulated, and in those situated in symmetrical halves of the body.
[SECTION IX.]
OF THE FUNCTIONS OF MUCOUS MEMBRANES.
70. I have already examined many of the functions of mucous membranes. I have considered them (1) As one of the grand emunctories of the animal economy. (2) As performing the same functions with respect to heterogeneous bodies, which may be within our organs, as the skin does with regard to the bodies with which it may be in contact. (3) As facilitating the passage of foreign bodies by means of the mucous fluid by which they are lubricated. It remains for me to examine three questions much agitated at this time. (1) If the mucous membranes have any influence over the redness of the blood. (2) If they exhale. (3) If the absorbents arise from them; and if absorption consequently takes place there.
71. The remarkable redness of these membranes, the analogy of respiration, during which the blood becomes changed in colour through the mucous surface of the bronchi, the well-known experiment of a bladder filled with blood and placed in oxygen gas, by which this fluid becomes also changed in colour,—have led to the belief, that the blood, being separated from the atmospheric air merely by a very fine pellicle on certain mucous surfaces, as the pituitary membrane, the palatine, the glans, &c., would there also take a brighter red colour, either by parting with a portion of carbonic acid gas, or by combining with the oxygen of the atmosphere, and that these membranes thus fulfilled functions accessory to those of the lungs. The experiments of Jurine upon the cutaneous organ, experiments adopted by many celebrated physicians, appear also to favour the reality of that conjecture.