62. In the second place I observe, that habit produces indifference to those sensations on the mucous membranes which were at first agreeable. The perfumer placed in a fragrant atmosphere, and the cook, whose palate is constantly affected by delicious flavours, do not experience, in their professions, the exquisite pleasures that they prepare for others. Habit may even change pleasant sensations to painful ones, as in the preceding paragraph we saw it changed painful to pleasing sensations. I observe, further, that this remarkable influence of habit is exercised only over sensations produced by simple contact, and not over those produced by real lesion of the mucous membranes: thus it does not ameliorate the pain produced by stone in the bladder, nor that which attends polypus in the uterus.
63. It is to this power of habit over the vital energies of the mucous membranes that we must, in part, refer the gradual diminution of their functions which accompanies advancing age. All is susceptibility in the infant: in old age all is dull. In the one the very active sensibility of the alimentary, biliary, urinary, and salivary mucous surfaces, is that which principally produces that rapidity with which the digestive and secretory phenomena succeed each other. In the other this sensibility, weakened by the habit of contact, does not so closely connect the same phenomena.
64. Does not the following remarkable modification of the sensibility of the mucous surfaces depend upon the same cause, viz. that at their origin, as on the pituitary membrane, the glans, the anus, &c., they give us the sensations of bodies with which they are in contact, and that they do not produce this sensation in the deeply seated organs which they line, as the intestines, &c.? In the interior of these organs this contact is always uniform; the bladder is in contact with the urine only, the gall bladder with the bile, the stomach with the aliments masticated and reduced to an homogeneous, pulpy paste, whatever may be their diversity. This uniformity of sensation prevents perception, because, in order to perceive, we must compare, and here two terms of comparison are wanting. Thus the fœtus has no sensation of the liquor amnii: the air is also very irritating at first to the new-born infant, but at length it is not felt. On the contrary, at the origins of mucous membranes exciting agents vary every instant: the mind can, therefore, perceive their presence, because it is able to establish relations between their various modes of action. What I say is so true, that if in the interior of the organs the mucous membranes be in contact with a foreign body differing from that which is habitual to them, they transmit the sensation of it to the mind; instruments introduced into bladder or stomach are examples of it. Fresh air, which in very hot weather is suddenly introduced into the trachea, causes an agreeable sensation over the surface of the bronchi; but from habit we soon become insensible to it, and the perception ceases.
65. It is very difficult to point out with precision the character of the tonic powers of mucous membranes, because, being almost in every part united to a muscular layer, we can hardly distinguish what belongs to the tonicity of the one from what depends upon the irritability of the other; or otherwise, if the mucous membranes be isolated, as in the nostrils, yet their attachment renders the phenomena of their tonic powers very obscure. Nevertheless, the action of the excretory ducts on their respective fluids, that of the gall bladder, and of the vesiculæ seminales, which are destitute of muscular attachments, and the spasmodic contraction of the urethra, which sometimes takes place when the sound is introduced, leave no doubt of the energy of this tonic power, doubtless similar in its various modifications to that which is observed in the cutaneous organ.
[SECTION VIII.]
OF THE SYMPATHIES OF MUCOUS MEMBRANES.
66. I distribute the sympathies of mucous membranes, like those of most of the other organs, into three general classes. In the first class are ranked the sympathies in which irritation, on one part of the mucous surface, produces a sensation in a distant part. A stone in the bladder occasions pain at the end of glans; worms in the intestines excite an itching at the nose. Whytt has seen a painful affection induced over the whole side of the head by a foreign body in the ear; an ulcer in the bladder produces a pain in the superior parts of the thighs every time that the patient passes his urine.