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.