61. I say, in the first place, that habit reduces the painful sensations, which take place on mucous membranes, to indifference. The presence of the catheter, which is passed up the urethra for the first time, is cruel the first day, painful the second, inconvenient the third, scarcely felt the fourth; pessaries introduced into the vagina, bougies into the rectum, tents in the nasal fossæ, the canula in the nasal canal, produce, in different degrees, the same phenomena. It is upon this remark that is founded the possibility of introducing instruments into the trachea to aid respiration, and into the œsophagus to afford artificial deglutition. This law of habit may even transform a painful into a pleasant impression; of this fact the use of snuff, tobacco, and various kinds of food, furnish us with remarkable examples. 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.